Internal dressings for healing perianal abscess cavities

  • Protocol
  • Intervention

Authors


Abstract

This is the protocol for a review and there is no abstract. The objectives are as follows:

To assess the effects of internal dressings in healing wound cavities resulting from drainage of perianal abscesses.

Background

Description of the condition

A perianal abscess is a collection of pus under the skin around the anus. Most perianal abscesses result from an infection in the anal glands (cryptoglandular infection), which can then spread via several different routes. The most common spread of infection is a downward extension to the perianal skin forming a perianal abscess (Parks 1961). Alternatively infection may spread alongside the rectum (ischiorectal), upwards above the muscles of the pelvic floor (supralevator), or between the muscles of the anal canal (intersphincteric) (Parks 1976). Extension across the midline of the body results in a horseshoe abscess. The cause of cryptoglandular infection is unique to perianal abscesses and therefore makes this condition pathologically distinct from abscesses in other locations. Approximately 10% of perianal abscesses are not caused by infected anal glands and result from conditions such as: skin appendage infection; Crohn’s disease; tuberculosis; trauma; chronic inflammation and infection of sweat glands (hidradenitis suppuritiva); human immunodeficiency virus (HIV) infection; sexually transmitted diseases; radiation therapy; malignancy or foreign bodies (Eisenhammer 1978; Goligher 1975; Hamadani 2009; Nelson 1985; Phillips 1997; Sangwan 1996; Whiteford 2007).

In the UK, the annual incidence of perianal abscess is 40 per 100,000 of the adult population, and acute perianal abscess is the seventh most common emergency condition in general surgery (www.hesonline.gov.uk). During 2012 to 2013, perianal abscesses resulted in 20,451 hospital episodes in England (diagnosis codes; anal abscess, anorectal abscess, ischiorectal abscess, intersphincteric abscess) (www.hesonline.gov.uk). This represents 3.4% (20,451 out of 605,107) of all general surgery emergency hospital episodes, and equates to 1.1% (20,451 out of 1,823,749) of total general surgery hospital episodes. The acute and on-going management of perianal abscess cavities accounts for significant hospital and community resource utilisation in England and probably elsewhere, although we have been unable to locate equivalent non-UK data to support this.

Depending on the exact anatomical location, perianal abscesses usually result in pain, swelling and redness of the anal area. Other symptoms may include fever and malaise. Over time, abscesses increase in size and, if untreated, will usually burst, which may or may not result in adequate drainage of the pus. It is thought that if the pus is not adequately drained, the skin may close prematurely and the abscess could recur. More rarely, untreated abscesses can also result in severe whole body (systemic) infection or devastating, rapidly spreading tissue infection and gangrene (necrotising fasciitis), therefore, early surgical drainage of the abscess is recommended. In the UK, standard surgical drainage involves incising the skin over the abscess in order to drain all the pus and allow wound irrigation. Following drainage, an internal dressing is applied ('packed') to the resulting cavity for haemostasis. Common practice in the UK is to continue packing until the abscess cavity has healed without being sewn shut (by secondary intention). The pack is changed every day or every few days by community nursing teams.

Alternative strategies for managing the abscess cavity have been tried. In the USA and Australia, one variation in the technique for incision and drainage is to use a small stab incision and place a latex catheter (e.g. a 10-14F de Pezzer catheter) into the cavity (Beck 1988; Isbister 1987; Kyle 1990). This is done under local anaesthetic, unlike practice in the UK where a general anaesthetic is routinely used. The catheter is cut short and drains into an external dressing. It is removed when it stops draining. Even when a catheter is not used, in the USA simple drainage is often performed in an outpatient setting under local anaesthetic (Whiteford 2007).

Sewing up (curettage) of the abscess cavity with primary sutured closure was first described in 1960 (Ellis 1960). Primary closure with antibiotic treatment may result in reduced healing times, but the increased incidence of recurrent sepsis negates any benefit (Mortensen 1995). This technique of primary closure is no longer used.

Sitz baths (immersing the area in warm water) have been used for treating perianal abscess cavities following incision and drainage. These have been employed both with packing (Read 1979), and without packing (Tang 1996; Vasilevsky 1984). Although a review of the literature found no benefit for this practice, it makes logical sense to irrigate an infected wound (Tejirian 2005).

Perianal fistulae (fistulae-in-ano) are a complication of perianal abscesses. A fistula may occur if a perianal abscess drains spontaneously through the perianal skin and if the infection becomes chronic (Parks 1961). The fistula becomes a tract between the anal canal and the perianal skin that is lined with granulation tissue (present in healing wounds) or skin cells (epithelium). It intermittently discharges pus and may result in recurrent abscesses. Fistulae usually require surgical intervention. This can be at the initial draining of the abscess (Malik 2010), but usually is performed later. Up to a third of patients with a perianal abscess will develop a fistula (Hamadani 2009; Lohsiriwat 2010; Ramanujam 1984; Vasilevsky 1984).

Following incision and drainage, management of the remaining wound cavity becomes the primary clinical focus to ensure optimal wound closure and prevent recurrence. In the UK, anecdotally, it is common for the cavity to be dressed with an internal dressing (packed) and covered with an external dressing or pad. However, in some other countries use of an internal dressing (packing) is not routinely recommended (Ommer 2012). We propose a review that focuses on the impact of internal dressings (packing) in the healing of cavities resulting from the surgical drainage of perianal abscesses.

Description of the intervention

Several dressing types are available to manage perianal abscess cavities; we have summarised the key categories below. There are limited audit-type data available on the types of dressings that are used for packing cavities resulting from the drainage of perianal abscesses. Where packing is not used, the wound may just be covered with an external dressing that is not in contact with the cavity itself.

Basic wound contact dressings

Low-adherence dressings and wound contact materials: usually cotton pads that are placed directly in contact with the wound. These can be non-medicated (e.g. paraffin gauze dressing) or medicated (e.g. containing povidone iodine or chlorhexidine).

Absorbent dressings: applied directly to the wound or used as secondary absorbent layers in the management of heavily-exuding wounds.

Advanced wound dressings

Alginate dressings: highly absorbent dressings made of calcium alginate, or calcium sodium alginate, that may be combined with collagen. The alginate forms a gel when in contact with moisture; this gel can be lifted off when the dressing is removed or rinsed away with sterile saline. Bonding the alginate to a secondary viscose pad increases absorbency.

Foam dressings: normally these contain hydrophilic polyurethane foam and are designed to absorb wound exudate and maintain a moist wound environment. There are various versions, and some include additional absorbent materials, such as viscose and acrylate fibres or particles of superabsorbent polyacrylate, which are silicone-coated for non-traumatic removal.

Hydrocolloid dressings: these are occlusive dressings usually composed of a hydrocolloid matrix bonded onto a vapour-permeable film or foam backing. When in contact with the moisture at the wound surface this matrix forms a gel to provide a moist environment. Fibrous alternatives have been developed that resemble alginates and are not occlusive, but which are more absorbant than standard hydrocolloid dressings.

Capillary-action dressings: these consist of an absorbent core of hydrophilic fibres held between two low-adherent contact layers.

Odour-absorbent dressings: these dressings contain charcoal and are used to absorb wound odour. Often these types of wound dressings are used in conjunction with a secondary dressing to improve absorbency.

How the intervention might work

It is thought that use of an internal dressing (packing) aids haemostasis, that is control of bleeding from the small blood vessels that line the abscess cavity, as well promoting healing by secondary intention by preventing the wound edges from closing. It is thought that healing in this way prevents the recurrence of the abscess.

Why it is important to do this review

It is unclear whether packing (compared with not packing) of the cavity following drainage of a perianal abscess is advantageous in promoting healing and preventing adverse events including fistula, abscess recurrence and pain. Anedoctally, the use of packing as a treatment approach in this context varies internationally and there is limited guidance for practice (Ommer 2012; Steele 2011; Williams 2007). A German guideline that reported on the diagnosis and treatment of anal abscess stated that regular packing is not required (Ommer 2012), and cited one small trial to support this (Tonkin 2004).

Packing has implications for patients and healthcare services: regular changing of internal dressings (change of packing) requires a considerable amount of community nurse time and, anecdotally, is painful for patients. Additionally the associated healthcare appointments may require patients to take time off work. We believe a transparent and robust review is required to the benefits, harms and costs of packing (compared with not packing) post-operative perianal abscess cavities in order to identify, synthesise and report the current evidence base.

Objectives

To assess the effects of internal dressings in healing wound cavities resulting from drainage of perianal abscesses.

Methods

Criteria for considering studies for this review

Types of studies

We will include published and unpublished randomised controlled trials (RCTs; i.e. must be described as randomised in the report), reported in any language, conducted in any country or setting. RCTs reported as conference abstracts will only be included where there is sufficient information available for data extraction and risk of bias assessment (either from the abstract or from the trial authors). RCTs that use a cross-over design will be included provided that outcome data are available at the point of cross-over. We will also include any RCTs with cluster randomisation. Quasi-randomised studies will be excluded.

Types of participants

Trials recruiting people who are undergoing surgical drainage of a perianal abscess. We will include studies involving patients of any age with any underlying aetiological cause for abscess formation.

Types of interventions

Any type of internal dressing (packing) used in the post-operative management of perianal abscess cavities. We will include studies where the presence or absence of an internal dressing (or packing) is the only systematic difference between treatment groups. Comparisons may include internal dressings compared with no internal dressing; one type of internal dressing compared with another type of internal dressing; or internal dressings compared with non-dressing treatments (for example, topical applications, drainage with a drain rather than an incision).

Types of outcome measures

We will include studies that report one or more of the outcomes below.

Primary outcomes
1. Wound healing

The primary outcome for the review will be wound healing.Trialists measure and report wound healing in many different ways, including: time for complete wound healing, proportion of wounds healed during follow-up and rates of change of wound size. For this review we will regard trials that report one or more of the following, as providing the best measures of wound healing in terms of relevance and rigour:

  • Time to wound healing within a specific time period correctly analysed using survival, time-to-event, approaches - ideally with adjustment for relevant co-variants such as baseline size. We will assume that the period of time in which healing could occur would be the duration of the trial, unless otherwise stated. We will use study authors' definitions of a healed wound.

  • Number of wounds completely healed within a specific time period, i.e. frequency of complete wound healing (we will assume that the period of time in which healing occurs would be the duration of the trial unless otherwise stated). We will use study authors' definitions of a healed wound.

  • Change (and rate of change) in wound size, when adjusted for baseline size - ideally analysed using multi-level modelling or (multiple) linear regression.

We note that, since wound healing is a subjective outcome, it can be at high risk of measurement bias when the outcome assessment is not blinded. We will downgrade the quality of the evidence from trials that report mean or median time to healing without time-to-event analysis (i.e. regarding time to healing as a continuous measure without accounting for censoring), and those that measure and report change, or rate of change, in wound size without further between-group analysis that includes adjustment for baseline size, or that use limited analytical approaches (e.g. based on only two measures, first measure and last measure).

2. Wound pain
  • Changes or comparison of validated pain scores, including pain measured on a visual analogue scale, during treatment or at dressing change, or both.

Secondary outcomes
  • Participant health-related quality of life/health status (measured using a standardised generic questionnaire such as EQ-5D, SF-36, SF-12 or SF-6 at noted time points). We will not include ad hoc measures of quality of life that are likely not to be validated and will not be common to multiple trials.

  • Frequency of abscess recurrence and fistulae.

  • Incontinence.

  • Time to return to work/normal function after perianal abscess incision and drainage.

  • Resource use (including measurements of resource use such as number of dressing changes, nurse visits, length of hospital stay and re-operation/intervention).

Search methods for identification of studies

Electronic searches

We will search the following databases to identify reports of RCTs:

  • Cochrane Wounds Group Specialised Register (latest issue);

  • The Cochrane Central Register of Controlled Trials (CENTRAL) (latest issue);

  • The Database of Abstracts of Reviews of Effects (DARE) (latest issue);

  • The NHS Economic Evaluation Database (latest issue);

  • Ovid MEDLINE (1946 to date);

  • Ovid MEDLINE (In-Process & Other Non-Indexed Citations, to date);

  • Ovid EMBASE (1974 to date);

  • EBSCO CINAHL (1982 to date).

The following provisional search strategy will be used in the Cochrane Central Register of Controlled Trials (CENTRAL):

#1 MeSH descriptor: [Soft Tissue Infections] explode all trees
#2 soft tissue infection* or skin infection*:ti,ab,kw
#3 ((wound* or site* or incision*) near/5 infect*):ti,ab,kw
#4 (Sepsis or sepses or suppurat*):ti,ab,kw
#5 MeSH descriptor: [Abscess] explode all trees
#6 ((Anorectal or anal or rectal or rectum or perianal or perirectal or ischiorectal or intersphincteric or supralevator) near abscess*):ti,ab,kw
#7 ((Anorectal or anal or rectal or rectum or perianal or perirectal or ischiorectal or intersphincteric or supralevator) near absess*):ti,ab,kw
#8 (cavity or cavities):ti,ab,kw
#9 #1 or #2 or #3 or #4 or #5 or #6 or #7 or #8
#10 MeSH descriptor: [Surgical Wound Infection] explode all trees
#11 MeSH descriptor: [Surgical Wound Dehiscence] explode all trees
#12 (surg* near/5 infect*):ti,ab,kw
#13 (surg* near/5 wound*):ti,ab,kw
#14 (surg* near/5 site*):ti,ab,kw
#15 (surg* near/5 incision*):ti,ab,kw
#16 (surg* near/5 dehisc*):ti,ab,kw
#17 (wound* near/5 dehisc*):ti,ab,kw
#18 (wound* near/5 infect*):ti,ab,kw
#19 (wound near/5 disruption*):ti,ab,kw
#20 (wound next complication*):ti,ab,kw
#21 #10 or #12 or #13 or #14 or #15 or #16 or #17 or #18 or #19 or #20
#22 (intent* or second* or heal* or complic*):ti,ab,kw
#23 ((open* or clos*) near/5 wound*):ti,ab,kw
#24 #22 or #23
#25 #21 and #24
#26 MeSH descriptor: [Occlusive Dressings] explode all trees
#27 MeSH descriptor: [Bandages, Hydrocolloid] explode all trees
#28 MeSH descriptor: [Biological Dressings] explode all trees
#29 MeSH descriptor: [Alginates] explode all trees
#30 MeSH descriptor: [Hydrogels] explode all trees
#31 MeSH descriptor: [Silver] explode all trees
#32 MeSH descriptor: [Silver Sulfadiazine] explode all trees
#33 MeSH descriptor: [Honey] explode all trees
#34 (dressing* or hydrocolloid* or alginate* or hydrogel* or "foam" or "bead" or "film" or "films" or tulle or gauze or non-adherent or "non adherent" or silver or honey or matrix):ti,ab,kw
#35 #26 or #27 or #28 or #29 or #30 or #31 or #32 or #33
#36 #9 and #25 and #35

We will adapt this strategy to search Ovid MEDLINE, Ovid EMBASE and EBSCO CINAHL. We will combine the Ovid MEDLINE search with the Cochrane Highly Sensitive Search Strategy for identifying randomised trials in MEDLINE: sensitivity- and precision-maximising version (2008 revision) (Lefebvre 2011). We will combine the EMBASE search with the Ovid EMBASE filter developed by the UK Cochrane Centre (Lefebvre 2011). We will combine the CINAHL searches with the trial filters developed by the Scottish Intercollegiate Guidelines Network (SIGN 2011). We will not restrict studies with respect to language, date of publication or study setting.

We will also search the following clinical trials registries:

Searching other resources

We are keen to explore sources of unpublished data. To maximise identification of unpublished studies not located during the search stage we will search the bibliographies of all retrieved and relevant publications identified by these search strategies for further studies. We will also: examine the content of conference proceedings and systematic reviews in the field that may refer to data we have not found; contact key manufacturers to ask about unpublished (as well as on-going) work and contact key authors and opinion leaders in the field. We will also search for key abstracts from the following meetings and request unpublished data from the authors as required:

  • The American Society of Colon and Rectal Surgeons

  • Association of Coloproctology of Great Britain & Ireland

  • Section of Coloproctology, Royal Society of Medicine

  • Section of Colon and Rectal Surgery, Royal Australasian College of Surgeons

  • Colorectal Surgical Society of Australia and New Zealand

  • European Society of Coloproctology (ESCP)

  • Tripartite meeting (a combined meeting of the above Associations)

Data collection and analysis

Selection of studies

Independently, two review authors will assess the titles and abstracts of retrieved studies for relevance. After this initial assessment, we will obtain full copies of all studies felt to be potentially relevant. Independently, two review authors will check the full papers for eligibility; disagreements will be resolved by discussion and, where required, the input of a third review author. We will record all reasons for exclusion of studies for which we obtained full copies. We intend to complete a PRISMA flowchart to summarize this process (Liberati 2009).

Data extraction and management

We will extract and summarize details of the eligible studies using a data extraction sheet. Two review authors will extract data independently and resolve disagreements by discussion. Where data are missing from reports, we will attempt to contact the study authors to obtain this information. We will include studies published in duplicate once, but extract the maximal amount of data. We will extract the following data, where possible:

  • country of origin;

  • participants' type and location of abscess – where recorded;

  • unit of randomisation and analysis

  • care setting;

  • number of participants randomised to each trial arm;

  • eligibility criteria and key baseline participant data;

  • details of the intervention regimen received by each group;

  • details of any co-interventions;

  • primary and secondary outcome(s) (with definitions);

  • outcome data for primary and secondary outcomes (by group);

  • duration of follow-up;

  • number of withdrawals (by group);

  • adverse events;

  • publication status of study; and,

  • sources of funding for the trial.

Assessment of risk of bias in included studies

Two review authors will independently assess each included study for bias using the Cochrane Collaboration tool for assessing risk of bias (Higgins 2011a). This tool addresses six specific domains, namely sequence generation, allocation concealment, blinding, incomplete outcome data, selective outcome reporting and other issues (e.g. extreme baseline imbalance, issues with unit of investigation). Please see Appendix 1 for details of the criteria upon which judgements will be made. We will assess blinding and completeness of outcome data for each outcome separately. We will complete a 'Risk of bias' table for each eligible study. We will present assessment of risk of bias using a 'Risk of bias' summary figure, which will present all of the judgements in a cross-tabulation of study by entry. This display of internal validity will indicate the weight the reader may give the results of each study. Disagreements about risk of bias assessment will be resolved by discussion. Where possible, when a lack of reported information results in a decision of 'unclear', trial authors will be contacted for clarification.

We will classify trials as being at high risk of bias overall if they are rated 'high risk' for any one of three key criteria, namely: randomisation sequence, allocation concealment, and blinded outcome assessment. We will also consider the potential for performance and measurement bias for each primary and secondary outcome we extract.

Measures of treatment effect

Where possible, we will present the outcome results for each trial with 95% confidence intervals (CI). We will report estimates for dichotomous outcomes (e.g. healed: yes/no) as risk ratios (RR). We will use the RR rather than odds ratio (OR), since, when event rates are high, as is the case for many trials reporting wound healing, ORs (when interpreted as RRs) can give an inflated impression of the effect size (Deeks 2002). We plan to report outcomes relating to continuous data (e.g. pain) as mean differences (MD) and overall effect size (with 95% CI). Where treatment outcomes are measured in different ways we will calculate standardised mean correcting for direction of scale as appropriate. Where a study reports time to healing data (the probability of healing over a consecutive time period) we plan to report and plot these data (where possible) using hazard ratio estimates.

Unit of analysis issues

We anticipate that most participants will only have one abscess being operated on. For abscess healing, unless otherwise stated, where the number of abscesses appears to equal the number of participants, we will treat the abscess as the unit of analysis. However, we will record occasions where multiple abscesses on a participant have been assessed and whether this has been accounted for in the analyses. Where data from cluster-randomised trials are included in the review we will adjust for cluster (as the unit of analysis) using a design effect (where possible) (Higgins 2011b). The design effect will use an estimate of the intra-cluster correlation coefficient (ICC) derived from the trial if possible, or from a similar trial. For adverse event outcomes, in order to facilitate further analyses, we will aim to establish whether data are presented at the level of the participant, because there is potential for data to refer to multiple events occurring to a single abscess (person), which means that the data cannot be analysed further without violating the assumption of independence.

Dealing with missing data

The problem of missing data is common in trial reports. Excluding participants from the analysis post-randomisation or ignoring those participants lost to follow-up can, in effect, compromise the process of randomisation, and thus, potentially introduce bias into the trial. In individual studies, where data on the proportion of abscesses healed are presented, we will assume that, if randomised participants are not included in an analysis, their abscess did not heal (i.e. they will be considered in the denominator but not the numerator). Where a trial does not specify participant group numbers prior to drop-out, we plan to present only complete case data. In a time-to-healing analysis using survival analysis methods, drop-outs should be accounted for as censored data. Hence, all participants should contribute to the analysis. We plan to present data for area change, and for all secondary outcomes, as a complete case analysis. Where there is no measure of variance for a continuous outcome, and no response from the authors, we anticipate that we will be unable to analyse or pool the relevant data.

Assessment of heterogeneity

We will consider both clinical and statistical heterogeneity. Wherever appropriate, that is where studies appear similar in terms of abscess type, intervention type, duration and outcome type, we will pool data using meta-analysis (conducted using RevMan 5.2) (RevMan 2012). We will assess statistical heterogeneity using the Chi² test (a significance level of P less than 0.1 will be considered to indicate heterogeneity) and the I² estimate (Higgins 2003). The I² estimate examines the percentage of total variation across studies due to heterogeneity rather than to chance. Values of I² over 50% indicate a high level of heterogeneity. In the absence of clinical heterogeneity and in the presence of statistical heterogeneity (I² over 50%), we will use a random-effects model, however, we will not pool studies at all where heterogeneity is very high (I² over 75%). Where there is no clinical or statistical heterogeneity we envisage using a fixed-effect model for statistical analysis.

Data synthesis

We will combine studies using a narrative overview, with meta-analyses of outcome data where appropriate (in RevMan 5.2). The decision to include studies in a meta-analysis will depend on the availability of treatment effect data and assessment of heterogeneity. For time-to-event data, we plan to use the inverse variance method on the estimated hazard ratio and standard error, when reported.

Subgroup analysis and investigation of heterogeneity

Should data be available, potential subgroup analyses will include:

  • underlying aetiological cause, e.g. cryptoglandular disease, Crohn’s disease, immunosuppression, trauma, sexually-transmitted infection;

  • anatomical classification of abscess e.g. horseshoe abscess (where abscess originates on one side of the perianal region and crosses the midline);

  • patient population age e.g. under 18 years of age and over 18 years of age.

Sensitivity analysis

If possible, we will undertake a sensitivity analysis to investigate the effects of removing studies at high risk of bias from the analysis.

'Summary of findings' table

We will present the main results of the review in 'Summary of findings' tables, which provide key information concerning the quality of evidence, the magnitude of effect of the interventions examined, and the sum of available data on the main outcomes, as recommended by the Cochrane Collaboration (Schunemann 2011a). Where possible we plan to include the following outcomes in the 'Summary of findings' tables:

  • complete wound healing;

  • pain.

The 'Summary of findings' table includes an overall grading of the evidence related to each of the main outcomes, using the GRADE approach (Schunemann 2011b).

Acknowledgements

The authors would like to acknowledge the contribution of the peer referees and Wounds Group editors: Liz Bickerdike, Louise Hunt, Evan Kontopantelis, AG Radhika and Dirk Ubbink. In addition thanks to Elizabeth Royle who copy edited the protocol.

Appendices

Appendix 1. Assessment of risk of bias in included studies

1. Was the allocation sequence randomly generated?

Low risk of bias

The investigators describe a random component in the sequence generation process such as: referring to a random number table; using a computer random-number generator; coin tossing; shuffling cards or envelopes; throwing dice; drawing of lots.

High risk of bias

The investigators describe a non-random component in the sequence generation process. Usually, the description would involve some systematic, non-random approach, for example: sequence generated by odd or even date of birth; sequence generated by some rule based on date (or day) of admission; sequence generated by some rule based on hospital or clinic record number.

Unclear

Insufficient information about the sequence generation process provided to permit a judgement of low or high risk of bias.

2. Was the treatment allocation adequately concealed?

Low risk of bias

Participants and investigators enrolling participants could not foresee assignment because one of the following, or an equivalent method, was used to conceal allocation: central allocation (including telephone, web-based and pharmacy-controlled randomisation); sequentially-numbered drug containers of identical appearance; sequentially-numbered, opaque, sealed envelopes.

High risk of bias

Participants or investigators enrolling participants could possibly foresee assignments and thus introduce selection bias, such as allocation based on: using an open random allocation schedule (e.g. a list of random numbers); assignment envelopes were used without appropriate safeguards (e.g. if envelopes were unsealed or non opaque or not sequentially-numbered); alternation or rotation; date of birth; case record number; any other explicitly unconcealed procedure.

Unclear

Insufficient information provided to permit a judgement of low or high risk of bias. This is usually the case if the method of concealment is not described or not described in sufficient detail to allow a definite judgement, for example if the use of assignment envelopes is described, but it remains unclear whether envelopes were sequentially-numbered, opaque and sealed.

3. Blinding - was knowledge of the allocated interventions adequately prevented during the study?

Low risk of bias

Any one of the following.

  • No blinding, but the review authors judge that the outcome and the outcome measurement are not likely to be influenced by lack of blinding.

  • Blinding of participants and key study personnel ensured, and unlikely that the blinding could have been broken.

  • Either participants or some key study personnel were not blinded, but outcome assessment was blinded and the non-blinding of others was unlikely to introduce bias.

High risk of bias

Any one of the following.

  • No blinding or incomplete blinding, and the outcome or outcome measurement is likely to be influenced by lack of blinding.

  • Blinding of key study participants and personnel attempted, but likely that the blinding could have been broken.

  • Either participants or some key study personnel were not blinded, and the non-blinding of others was likely to introduce bias.

Unclear

Either of the following.

  • Insufficient information provided to permit a judgement of low or high risk of bias.

  • The study did not address this outcome.

4. Were incomplete outcome data adequately addressed?

Low risk of bias

Any one of the following.

  • No missing outcome data.

  • Reasons for missing outcome data unlikely to be related to true outcome (for survival data, censoring unlikely to be introducing bias).

  • Missing outcome data balanced in numbers across intervention groups, with similar reasons for missing data across groups.

  • For dichotomous outcome data, the proportion of missing outcomes compared with observed event risk was not enough to have a clinically relevant impact on the intervention effect estimate.

  • For continuous outcome data, plausible effect size (difference in means or standardised difference in means) among missing outcomes was not enough to have a clinically relevant impact on observed effect size.

  • Missing data have been imputed using appropriate methods.

High risk of bias

Any one of the following.

  • Reason for missing outcome data likely to be related to true outcome, with either imbalance in numbers or reasons for missing data across intervention groups.

  • For dichotomous outcome data, the proportion of missing outcomes compared with observed event risk was enough to induce clinically relevant bias in intervention effect estimate.

  • For continuous outcome data, plausible effect size (difference in means or standardised difference in means) among missing outcomes was enough to induce clinically relevant bias in observed effect size.

  • ‘As-treated’ analysis done with substantial departure of the intervention received from that assigned at randomisation.

  • Potentially inappropriate application of simple imputation.

Unclear

Either of the following.

  • Insufficient reporting of attrition/exclusions to permit a judgement of low or high risk of bias (e.g. number randomised not stated, no reasons for missing data provided).

  • The study did not address this outcome.

5. Are reports of the study free of suggestion of selective outcome reporting?

Low risk of bias

Either of the following.

  • The study protocol is available and all of the study’s pre-specified (primary and secondary) outcomes that are of interest in the review have been reported in the pre-specified way.

  • The study protocol is not available but it is clear that the published reports include all expected outcomes, including those that were pre-specified (convincing text of this nature may be uncommon).

High risk of bias

Any one of the following.

  • Not all of the study’s pre-specified primary outcomes have been reported.

  • One or more primary outcomes are reported using measurements, analysis methods or subsets of the data (e.g. subscales) that were not pre-specified.

  • One or more reported primary outcomes were not pre-specified (unless clear justification for their reporting is provided, such as an unexpected adverse effect).

  • One or more outcomes of interest in the review are reported incompletely so that they cannot be entered in a meta-analysis.

  • The study report fails to include results for a key outcome that would be expected to have been reported for such a study.

Unclear

Insufficient information provided to permit judgement of low or high risk of bias. It is likely that the majority of studies will fall into this category.

6. Other sources of potential bias

Low risk of bias

The study appears to be free of other sources of bias.

High risk of bias

There is at least one important risk of bias. For example, the study:

  • had a potential source of bias related to the specific study design used; or

  • has been claimed to have been fraudulent; or

  • had some other problem.

Unclear

There may be a risk of bias, but there is either:

  • insufficient information to assess whether an important risk of bias exists; or

  • insufficient rationale or evidence that an identified problem will introduce bias.

Contributions of authors

Stella Smith conceived the review question, developed the protocol, co-ordinated development, completed the first draft, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.
Lyndsay Pearce conceived the review question, developed the protocol, co-ordinated development, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.
Katherine Newton conceived the review question, developed the protocol, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.
Jo Dumville conceived the review question, developed the protocol, co-ordinated development, completed the first draft, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.
Jennifer Smith conceived the review question, developed the protocol, co-ordinated development, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.
Laura Hancock conceived the review question, developed the protocol, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.
Paul Barrow conceived the review question, developed the protocol, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.
James Hill conceived the review question, developed the protocol, performed part of the writing and editing of the protocol, made an intellectual contribution, advised on the protocol and approved the final draft prior to submission.

Contributions of editorial base:

Nicky Cullum: edited the protocol; advised on methodology, interpretation and protocol content. Approved the final protocol prior to submission.
Sally Bell-Syer: coordinated the editorial process. Advised on methodology, interpretation and content. Edited the protocol.
Amanda Briant: designed the search strategy and edited the search methods section.

Declarations of interest

The authors S Smith, L Pearce, K Newton, L Hancock, J Smith, P Barrow and J Hill are members of the Packing of Perianal Abscesses Steering Committee - a multi-centre observational study of outcomes following incision and drainage of perianal abscesses with the results intended to inform the design of a trial. This study is independent research and not sponsored by industry.

Jo Dumville is funded as part of the National Institute for Health Research Collaboration for Leadership in Applied Health Research and Care (NIHR CLAHRC) Greater Manchester. The views expressed in this article are those of the author and not necessarily those of the NHS, NIHR or the Department of Health.

Sources of support

Internal sources

  • No sources of support supplied

External sources

  • The National Institute for Health Research (NIHR) is the sole funder of the Cochrane Wounds Review Group, UK.

Ancillary