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Keywords:

  • immune thrombocytopenia (ITP);
  • splenectomy;
  • registry;
  • 111In;
  • platelets

Summary

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

While splenectomy is an effective therapy for primary immune thrombocytopenia (ITP), possible complications and observed non-complete response (CR) in one-third of patients demonstrate the need for further research into potential pre-surgical predictors of outcomes. Past investigations into platelet sequestration studies, a hypothesized predictive test, have adopted heterogeneous methods and varied widely with regard to power. By studying patients with primary ITP who underwent autologous 111In-labelled platelet sequestration studies at Barts and The London NHS Trust between 1994 and 2008, we evaluated the effectiveness of sequestration site in predicting short, medium, and long-term CR (platelet count >100 × 109/l) to splenectomy through multivariate (gender, age at splenectomy, and mean platelet lifespan) logistic regression modelling. In total, 256 patients with primary ITP underwent scans; 91 (35·5%) proceeded to splenectomy. Logistic regression revealed significant adjusted odds ratios for CR of 7·47 (95% confidence interval [CI], 1·89–29·43) at 1–3 months post-splenectomy, 4·85 (95% CI, 1·04–22·54) at 6–12 months post-splenectomy, and 5·39 (95% CI, 1·34–21·65) at last follow-up (median: 3·8 years [range: 0·5–13·1 years]) in patients with purely or predominantly splenic versus mixed or hepatic sequestration. These findings demonstrate the utility of autologous 111In-labelled platelet sequestration studies as an adjunct predictive instrument prior to splenectomy.

Primary immune thrombocytopenia (ITP) is an autoimmune disease characterized by autoantibody-mediated platelet destruction, T-cell-mediated platelet lysis, and suboptimal platelet production (Ballem et al, 1987; Olsson et al, 2003; McMillan et al, 2004). Primarily an acute (< 1 year) condition in children, its course is predominantly chronic among adults (Provan, 2003). The degree of bleeding in patients with primary ITP is largely, but not solely, dependent on the platelet count, with patients possessing counts below 10 × 109/l at greatest risk of a major haemorrhage (Frederiksen & Schmidt, 1999).

While primary ITP specialists have increasingly adopted a minimally interventional paradigm, therapy is still indicated for patients with symptomatic thrombocytopenia, at high risk of bleeding, or undergoing procedures likely to induce blood loss (Provan et al, 2010). Standard treatment modalities include oral corticosteroids, intravenous immunoglobulin (IVIg), and splenectomy (George et al, 1996).

The spleen serves as a principal site of anti-platelet antibody production and opsonized platelet phagocytosis (Kuwana et al, 2002; Cines et al, 2003). Splenectomy has therefore proved an effective therapy in the management of primary ITP, with a complete response (CR) observed in approximately two-thirds of adults (Kojouri et al, 2004). However, the procedure carries a risk of peri- and post-operative complications, including intrabdominal haemorrhage (Wanachiwanawin et al, 1993), thromboembolic events (Robinette & Fraumeni, 1977), and opportunistic post-splenectomy infections (Bisharat et al, 2001). While the combined risk of these complications in patients with primary ITP is estimated to be low (Schwartz et al, 2003), it provides sufficient impetus to investigate potential pre-operative predictors of response when coupled with a known risk of non-CR in a third of patients and limited data on long-term relapse.

To date, a number of such predictors have been posited, including (i) response to corticosteroid and IVIg therapies, (ii) platelet turnover and lifespan, (iii) patient age, (iv) duration of disease, (v) platelet-bound immunoglobulin, and (vi) site of platelet destruction as determined by radionuclide labelling techniques. In a systematic review of the effectiveness of splenectomy among adult patients with primary ITP, Kojouri et al (2004) reported inconclusive evidence in support of this last variable. Among a series of 15 studies, the authors found the site of platelet destruction to be correlated with post-splenectomy outcome in 6 (Burger et al, 1978; Russo et al, 1987; Lamy et al, 1993; Winde et al, 1996; Najean et al, 1997; Bourgeois et al, 2003) uncorrelated in 8 (Ries, 1977; den Ottolander et al, 1984; Fenaux et al, 1989; Siegel et al, 1989; Naouri et al, 1993; Louwes et al, 1999; Radaelli et al, 2000; Rossi et al, 2002), and inconclusive in one (Ikkala et al, 1978).

However, the methodologies of these studies were notably heterogeneous with regard to the isotopic label used, sequestration classification scheme, patient inclusion criteria, and both the definition of and period for response (Tables SI–IV). Moreover, 6 (67·8%) uncorrelated or inconclusive studies had fewer than 50 evaluable patients (Ries, 1977; Ikkala et al, 1978; Siegel et al, 1989; Naouri et al, 1993; Louwes et al, 1999; Rossi et al, 2002) undergoing both a platelet sequestration study and a splenectomy. The power of these investigations to detect an association between sequestration site and response to splenectomy was therefore limited. These shortcomings underscore the need for an additional suitably powered study, which incorporates currently standardized isotopic labelling techniques and consensus criteria for response evaluation.

Barts and The London NHS Trust is the principal ITP referral centre for adults in the United Kingdom (UK), and autologous 111In-labelled platelet sequestration studies have been routinely practiced here as part of the planning process prior to splenectomy since the mid-1990s. Using the cohort of patients with primary ITP who have undergone such studies, the objectives of our investigation were two-fold: (i) to evaluate the prognostic utility of platelet sequestration site as a predictor of short, medium, and long-term response to splenectomy through multivariate logistic regression modelling and (ii) to compare descriptively the long-term outcomes of splenectomy with alternate therapeutic approaches in patients with primary ITP.

Methods

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

Study design and population

A retrospect cohort study was performed. Inclusion in the study cohort was restricted to patients with primary ITP as defined by 2003 British Committee for Standards in Haematology (BCSH) guidelines (Provan, 2003) who had undergone an autologous 111In-labelled platelet sequestration study at Barts and The London NHS Trust between March 1st, 1994 and December 31st, 2008. All patients were required to have platelet counts between 5 × 109/l and 100 × 109/l at the time of their scan, and no antibody therapies were permitted 4 weeks prior to this date.

Autologous 111In-labelled platelet sequestration study protocol

Autologous 111In platelet-labelling was performed in accordance with recommendations of The International Committee for Standardization in Haematology (ICSH) Panel on Diagnostic Applications of Radionuclides (ICSH, 1988). Briefly, whole blood (50 ml) was withdrawn from the patient into acid citrate glucose (ACG, 6:1) as anticoagulant and centrifuged at 90–100 g for 10 min. The supernatant (platelet rich plasma [PRP]) was adjusted to a pH of 6·5 by further addition of ACG and centrifuged at 650 g for 10 min. The resulting supernatant (platelet poor plasma [PPP]) was used (0·3–0·5 ml) to suspend the derived platelet pellet. Tropolone (50 μl, 0·054% [w/v] tropolone in HEPES-saline buffer [pH 7·6]) was then added to the suspension, followed immediately by 111In-chloride (4 MBq). The mixture was incubated at room temperature for 5 mins, washed with PPP (5 ml), and centrifuged at 650 g for 10 min. The supernatant (containing unbound 111In-tropolone) was discarded, leaving 111In-tropolone-labelled platelets, which were suspended in PPP (5 ml) and injected into a peripheral vein.

At 0·5, 3, 24, and 48 h post-injection, blood samples (5 ml) were taken and centrifuged in a two-stage process to yield PPP and a platelet pellet. Surface counting was performed on both products, with a gamma camera set to register 111In-photon peaks at 171 and 245 KeV. Counting, corrected for background and decay, was additionally performed over the heart, liver, and spleen at the same intervals using the geometric mean of anterior and posterior views. Regions of interest were drawn over these organs using the 0·5-h acquisition and transferred to subsequent acquisitions to ensure analysis of identical anatomical areas.

A semi-logarithmic graph of platelet-associated radioactivity versus time was constructed to derive the platelet half-clearance time (t50%), which in turn was used to compute the mean platelet lifespan (MPLS) via the algorithm, 1·44 × t50% (Fenaux et al, 1989; Najean et al, 1991). The classification system proposed by Najean et al (1991) was adopted to catergorize platelet sequestration. Under this scheme, the relative increase in the spleen:liver (S:L) ratio from 0·5 h to the time of 80% platelet destruction was calculated. An S:L ratio increase, X > 2·0 was characterized as a purely splenic pattern of sequestration, 1·4 < X ≤ 2·0 as predominantly splenic, 0·8 < X ≤ 1·4 as mixed, and X ≤ 0·8 as hepatic.

Follow-up

At least one-year following the sequestration study, haematologists were questioned as to whether their patients had undergone a splenectomy. For those who had, 1–3 and 6–12 months post-surgery platelet counts were requested in addition to treatment status and platelet count at last follow-up. Haematologists of patients who did not have a splenectomy were alternatively asked to supply a reason (open-field) why surgery was not performed in addition to providing the treatment status and platelet count of their patient at last follow-up. Death certificates were obtained for patients who died during follow-up.

Outcomes

The outcome of interest among the splenectomized population was CR, defined using recent consensus terminology guidelines (Rodeghiero et al, 2009) as a platelet count greater than 100 × 109/l at 1–3 (short-term) and 6–12 (medium-term) months post-surgery and both a platelet count greater than 100 × 109/l and non-reliance treatment at last follow-up (long-term).

Exposures and covariates

Platelet sequestration site, dichotomized as purely or predominantly splenic and mixed or hepatic, comprised the principal exposure in the study. Additional covariates included gender, age at surgery (continuous), and MPLS (continuous).

To create a suitable comparative cohort for splenectomized patients, a categorical variable of reason(s) for not undergoing surgery, was constructed. Open-field responses from haematologists were grouped into one of the following ten categories: (i) an asymptomatic or only mildly symptomatic patient; (ii) co-morbid condition(s); (iii) a spontaneous, first-line (corticosteroids or IVIg) treatment-based, or unspecified remission; (iv) a well-controlled patient on a first-line treatment; (v) physician, parent, or patient choice (vi) a non-first-line treatment-based remission; (vii) a well-controlled patient on a non-first-line treatment; (viii) platelet sequestration study test result; (ix) multiple reasons; (x) no reason provided. Patients listed as not having undergone surgery for reason 5, 6, 7, 8, or 9 (without their other reasons classifiable under 1, 2, 3, or 4 above) were selected to comprise the comparative, non-splenectomized cohort.

Statistical analyses

The investigation was framed with an aim to determine whether autologous 111In-labelled platelet sequestration studies could identify patients who would benefit from surgery. Unadjusted and adjusted (gender, age at splenectomy, and MPLS) odds ratios (ORs) and 95% confidence intervals (95% CIs) for CR were modelled at 1–3 and 6–12 months post-surgery and last follow-up using logistic regression (STATA 9.2; College Station, TX, USA).

For all secondary analyses, differences between groups were evaluated at α = 0·05 using Student’s t (two-tailed), Kruskal–Wallis, and Pearson’s chi-square tests for normally distributed, non-parametric, and dichotomous outcomes, respectively. Determination of normality for continuous variables was based on visual inspection of both box-plots and histograms.

Subgroup analyses

Date of primary ITP diagnosis was available for a subset of patients undergoing autologous 111In-labelled platelet sequestration studies. A pre-hoc decision was therefore made to perform subgroup analyses, incorporating disease duration at the time of splenectomy as an additional covariate in the multivariate logistic regression models outline above.

Ethics

The study was conducted under the auspices of the UK Adult ITP Registry, an active, linked-anonymized repository of hospital-based clinical data (demographics, bleeding events, ITP-specific treatments, laboratory results, and co-morbid conditions) on and biological samples (DNA extracted from whole blood or saliva) of adults with primary ITP. Launched in late-2007, the UK Adult ITP Registry has been approved for multi-centre operation by the London Research Ethics Committee (Reference: 07/H0718/57) until mid-2017 and is sponsored by Barts and The London NHS Trust. Informed consent was obtained from every patient prior to enrolment. All patients who were children (< 16 years) at the time of their autologous 111In-labelled platelet sequestration study had reached adulthood by the time of study initiation.

Results

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

Autologous, 111In-labellled platelet sequestration studies

A total of 272 autologous 111In sequestration studies were conducted at Barts and The London NHS Trust between March 1st, 1994 and December 31st, 2008. As illustrated in Fig 1, scans were performed on 256 patients with primary ITP, 9 with secondary ITP (7 with SLE-induced thrombocytopenia and 2 with HIV-induced thrombocytopenia), 4 with congenital or familial thrombocytopenia, and 3 with Evans syndrome. The female-to-male ratio of patients with primary ITP was 1·7:1, and their median age was 38·0 years (range: 7·3–75·4 years).

image

Figure 1.  An overview of the study population. In total, 256 patients with primary ITP underwent autologous 111In-labelled platelet sequestration studies at Barts and The London NHS Trust between March 1994 and December 2008, with 91 (35·5%) of patients subsequently proceeding to splenectomy. SLE, systemic lupus erythematosus; HIV, human immunodeficiency virus; MPLS, mean platelet life-span.

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Platelet sequestration patterns among the primary ITP cohort were heterogeneous, with 68 (26·6%) patients exhibiting purely splenic sequestration, 76 (29·7%) predominantly splenic sequestration, 60 (23·4%) mixed sequestration, and 52 (20·3%) hepatic sequestration (Fig 2). The median MPLS of patients with primary ITP was 28·2 h (range: 6·8–407·0 h). A significant difference between the MPLS of patients with purely or predominantly splenic and mixed or hepatic platelet sequestration was not observed (median: 25·9 h [range: 6·8–407·0 h] vs. 31·1 h [range: 7·2–271·0 h]; P = 0·491).

image

Figure 2.  Sequestration patterns of patients who underwent underwent an autologous 111In-labelled platelet study and patients who subsequently underwent splenectomy. A statistically significant difference (P < 0·001) was observed between the proportions of purely or predominantly splenic versus mixed or hepatic patterned patients with primary ITP who proceeded to splenectomy.

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Follow-up

Over one-third of patients with primary ITP (N = 91 [35·5%]) elected to undergo a splenectomy following their autologous 111In sequestration study while slightly more than half (N = 131 [51·2%]) did not. Thirty-four patients (13·3%) were lost to follow-up (Fig 1). These patients did not differ significantly from patients not lost to follow-up with regard to age at scan (mean: 39·5 ± 18·7 years vs. 38·3 ± 17·5 years; P = 0·712), gender (female-to-male ratio: 2·8:1 vs. 1·6:1; P = 0·183), platelet sequestration pattern (purely and predominantly splenic: mixed and hepatic ratio: 1·4:1 vs. 1·3:1; P = 0·745), or MPLS (median: 36·3 h [range: 9·0–169·3 h] vs. 27·6 h [range: 6·8–407·0 h]; P = 0·314).

Haematologist-supplied reasons for patients not undergoing splenectomy (Table I), most commonly included (i) the platelet sequestration test result (i.e. a mixed or hepatic sequestration finding suggesting a lower probability of response to splenectomy, N = 39 [29·8%]); (ii) an asymptomatic or only mildly symptomatic patient (N = 26 [19·8%]); and (iii) patient, parent, or clinician choice (N = 23 [16·0%]).

Table I.   Reasons for decision not to proceed to splenectomy.
Reason (s)Number (%)
Test result39 (29·8)
Asymptomatic or only mildly symptomatic26 (19·8)
Patient, parent, or clinician choice23 (16·0)
Treatment-based remission11 (8·4)
 Corticosteroids5
 Rituximab2
 Intravenous immunoglobulin (IVIg)1
 Various treatments1
 Platelet transfusion and corticosteroids1
 Unspecified treatment1
Multiple reasons8 (6·1)
 Test result and patient choice5
 Test result and only mildly symptomatic patient1
 Test result and treatment-based remission (corticosteroids)1
 Co-morbid condition and only mildly symptomatic patient1
Co-morbid condition(s)5 (3·8)
Spontaneous remission5 (3·8)
Unspecified remission4 (3·1)
Well maintained on non-first line treatment3 (2·3)
 Azathioprine1
 Rituximab1
 Tranexamic acid1
No reason provided6 (4·6)
Total131

Of the 71 patients with purely or predominantly splenic sequestration opting for splenectomy, 57 (80·3%) had evaluable results at 1–3 months post-surgery, 54 (76·1%) at 6–12 months post-surgery, and 69 (97·2%) at last follow-up (Table II). Complete responses were observed in 50 (87·7%), 44 (87·0%), and 60 (87·0%) of these patients, respectively. Among the 20 patients undergoing splenectomy with a mixed or hepatic pattern of platelet sequestration, 19 (95·0%) had evaluable results at 1–3 months post-surgery, 18 (90·0%) at 6–12 months post-surgery, and 20 (100%) at last follow-up, with 7 (36·8%), 5 (27·8%), and 7 (35·0%) experiencing a CR, respectively (Table II). Median post-surgical times at last follow-up were 3·8 years (range: 0·5–13·1 years) for the purely or predominantly splenic, splenectomized cohort, during which no deaths were reported, and 2·8 years (range: 0·5–10·8 years) for the mixed or hepatic, splencetomized cohort, during which the following 2 non-haemorrhagic fatalities were observed: (i) acute myocardial infarction and coronary atherosclerosis, (ii) cerebral infarction and type II diabetes. Histograms of platelet counts for the purely or predominantly splenic and mixed or hepatic, splenectomized cohorts at each of the three aforementioned intervals are illustrated in Figs 3 and 4.

Table II.   Short, medium, and long-term outcomes.
CohortEvaluable N (%)Complete response N (%)Platelet count median (range) (× 109/l)Post-scan time median (range) (years)On treatment N (%)
1–3 months post splenectomy
 Purely or predominantly splenic, splenectomized cohort57 (80·2)50 (87·7)325 (16–701)  
 Mixed or hepatic, splenectomized cohort19 (95·5)7 (36·8)151 (3–1637)  
6–12 months post splenectomy
 Purely or predominantly splenic, splenectomized cohort54 (76·1)44 (87·0)270 (20–701)  
 Mixed or hepatic, splenectomized cohort18 (90·0)5 (27·8)140 (9–514)  
Last follow-up
 Purely or predominantly splenic, splenectomized cohort69 (97·2)60 (87·0)292 (14–589)3·0 (0·7–14·4)3 (4·3)
 Mixed or hepatic, splenectomized cohort20 (100)7 (35·0)199 (2–577)3·0 (0·8–12·0)6 (30·0)
 Comparative, non-splenectomized cohortCount: 72 (100) Treatment: 68 (94·4)55 (80·9) 56 (1–358)2·8 (0·1–11·4)28 (41·2)
image

Figure 3.  Histograms of platelet counts (× 109/l) among the purely or predominantly splenic (0) and mixed or hepatic (1), splenectomized cohorts at 1–3 months (A) and 6–12 months (B) post-splenectomy.

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image

Figure 4.  Histograms of platelet counts (× 109/l) among the purely or predominantly splenic (0) and mixed or hepatic (1), splenectomized cohorts and the comparative, non-splenectomized cohort (2) at last follow-up.

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Multivariate modelling

Unadjusted and adjusted (gender, MPLS, and age at splenectomy) odds of short, medium, and long-term CR to splenectomy were increased in patients with purely or predominantly splenic versus mixed or hepatic sequestration. Logistic regression modelling yielded significant (α = 0·05) unadjusted ORs at 1–3 months post-splenectomy (OR = 4·96 [95% CI, 1·41–17·46]) and at last follow-up (OR = 3·59 [95% CI, 1·13–11·40]), but not at 6–12 months post-splenectomy (OR = 2·58 [95% CI, 0·70–9·49]). However, as shown in Table III, adjusted ORs were significant at each of these intervals: 1–3 months post-splenectomy (OR = 7·47 [95% CI, 1·89–29·43], 6–12 months post-splenectomy (OR = 4·85 [95% CI, 1·04–22·54], and last follow-up (OR = 5·39 [95% CI, 1·34–21·65]; median post-surgery time: 3·8 years [range: 0·5–13·1 years]). While no other variables achieved significance at 1–3 and 6–12 months post-splenectomy, patient age at surgery did emerge as an independent predictor of CR at last follow-up (OR = 0·95 [95% CI, 0·91–0·99]).

Table III.   Multivariate analyses.
Post-surgery assessment timeNPattern PS/PS vs. M/H* OR (95% CI)†Gender female vs.male OR (95% CI)(per hour)MPLS OR (95% CI) (per hour)Age at surgery OR (95% CI) (per year)Post-ITP duration OR (95% CI) (per month)
  1. *Purely or predominantly splenic versus mixed or hepatic.

  2. †Odds ratio and 95% confidence interval.

Primary analyses
1–3 months717·47 (1·89–29·43)0·76 (0·18–3·13)1·01 (0·98–1·03)0·99 (0·94–1·03) 
6–12 months674·85 (1·04–22·54)3·69 (0·85–15·91)1·01 (0·98–1·05)0·96 (0·91–1·00) 
Last follow-upmedian: 3·8 years(range: 0·5–13·1 years)845·39 (1·34–21·65)2·70 (0·75–9·68)1·01 (0·98–1·04)0·95 (0·91–0·99) 
Subgroup analyses
1–3 months438·01 (1·63–39·50)0·69 (0·12–3·85)1·00 (0·96–1·05)1·00 (0·95–1·06)1·10 (0·90–1·35)
6–12 months3923·40 (1·87–292·33)4·84 (0·56–41·63)1·04 (0·97–1·10)1·00 (0·93–1·08)0·93 (0·74–1·17)
Last follow-upmedian: 4·1 years(range: 0·5–13·1 years)529·20 (1·54–54·94)4·95 (0·90–27·28)1·03 (0·98–1·08)0·96 (0·91–1·02)1·32 (0·77–2·27)

Inclusion of the lapse of time between primary ITP onset and splenectomy as an additional covariate yielded different ORs but did not alter the significance of sequestration site as a predictor of CR: 1–3 months post-splenectomy (OR = 8·01 [95% CI, 1·63–39·50]), 6–12 months post-splenectomy (OR = 23·40 [95% CI, 1·87–292·33]), and last follow-up (OR = 9·20 [95% CI, 1·54–54·94]).

Splenectomized versus comparative, non-splenectomized cohorts

In total, 72 (55·0%) patients with primary ITP who did not undergo splenectomy met inclusion criteria for the comparative, non-splenectomized cohort. CRs were observed in 13 (19·1%) of these patients over a median post-scan follow-up time of 2·7 years (range: 0·1–11·4 years; Table II). Two non-haemorrhagic deaths were reported over this period: (i) parieto occiptal ependymoma, (ii) chronic myelomonocytic leukaemia and renal impairment/septicaemia. While the median platelet count of this cohort at last follow-up was significantly lower than those of the mixed or hepatic and purely or predominantly splenic, splenectomized cohorts (58 × 109/l [range: 2–358 × 109/l] vs. 199 × 109/l [range: 2–577 × 109/l] vs. 292 × 109/l [range: 14–589 × 109/l], respectively; P < 0·001), the proportion of patients on treatment within it was comparable with that of the mixed or hepatic, splenectomized cohort (28 [41·2%] vs. 6 [30·0%] patients, respectively; P = 0·47). Furthermore, 8 (28·6%) patients in the comparative, non-splenectomized cohort who were on treatment at last follow-up were solely taking low-dose corticosteroids (< 10 mg/day). Of note, only 3 (4·3%) splenectomized patients with purely or predominantly splenic platelet sequestration were on treatment at last follow-up (Table II).

Discussion

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

The results of our study revealed an increased likelihood of CR to splenectomy in patients exhibiting a purely or predominantly splenic pattern of platelet sequestration as shown through autologous 111In-labelled scanning. At last follow-up, patient age at splenectomy also emerged as an independent predictor of response, implicating a 5% increased odds of CR per year difference in younger candidates for surgery. This finding is consistent with a majority of past investigations (Kojouri et al, 2004).

The significance of platelet sequestration pattern as pre-surgical predictor of short, medium, and long-term CRs was not affected by the incorporation of primary ITP duration as an additional covariate in subgroup analyses (Table III). Of note, the association also did not appear to be tied to a difference in MPLS, which was similar between the cohorts (median- purely or predominantly splenic: 31·1 h [range: 7·2–271·0 h] vs. mixed or hepatic: 25·9 h [range: 6·8–407·0 h], P = 0·491). These data suggest that the rate of platelet destruction was independent of both the predominant location of platelet destruction and the likelihood of response to splenectomy in patients with primary ITP, a disparate conclusion than that reached by Siegel et al (1989).

Long-term outcomes, as measured by platelet count and ITP-specific treatment status, among patients indicated but not opting for splenectomy (the comparative, non-splenectomized cohort) were notably poor in comparison with splenectomized patients with purely or predominant platelet sequestration. However, they corresponded to those of the mixed or hepatic, splenectomized cohort with respect to ITP-specific treatment status. At last follow-up, 41·2% (28 of 68 patients) of the comparative, non-splenectomized cohort was on treatment as compared with 30·0% (6 of 20 patients) of splenectomized patients with mixed or hepatic sequestration (Table II). When excluding patients taking solely low-dose corticosteroids, the difference between these proportions narrowed: 30·0% (20 of 68 patients) vs. 25·0% (5 of 20 patients), respectively.

These results highlight splenectomy as an effective treatment for younger patients with primary ITP exhibiting a purely or predominantly splenic platelet sequestration. Short, medium, and long-term success was observed in 85–90% of all patients with such sequestration while reliance on treatment was limited to 4·3% (3 of 69 patients) a median of 3·8 years (range: 0·5–13·1 years) following surgery.

More controversially, these data further support the adoption of a cautious approach to splenectomy for mixed or hepatic patterned patients. While wide 95% CIs prevent precise classification of the ORs of non-CR in patients with mixed or hepatic versus purely or predominantly splenic sequestration, our findings illustrate the existence of a significant difference between the cohorts. Coupled with increased recognition of the physiological importance of the spleen, potential long-term vascular complications following splenectomy for haematological disorders (Crary & Buchanan, 2009), and arguably similar, clinically relevant long-term outcomes among patients indicated but not opting for splenectomy, this association raises the question as to whether patients with mixed or hepatic platelet sequestration may fare comparably without splenectomy. Undeniably, platelet counts in the comparative, control cohort were significantly lower than those of the mixed or hepatic, splenectomized cohort (median: 58 × 109/l [range: 2–358 × 109/l] vs. 199 × 109/l [range: 2–577 × 109/l], P < 0·001). However, most patients (57 of 68, 83·8%) in the former cohort had counts above 10 × 109/l (Fig 4), a conventional threshold for increased risk of major bleeding events (Provan & Newland, 2002). Furthermore, no fatal haemorrhages were observed in this group over a median of 2·7 years (range: 0·1–11·4 years).

Three methodological strengths make our study an important addition to the existing literature on the utility of platelet sequestration studies in patients with primary ITP. First, the use of 111In-tropolone may have contributed to more accurate test results than previous investigations using either 51Cr or 111In-oxine. 111In-tropolone has been reported by some research groups to exhibit better platelet labelling efficiency and imaging properties than 51Cr (Robertson et al, 1981). It is additionally more soluble than 111In-oxine in aqueous media, sparing possible platelet impairment from ethyl alcohol, used on occasion with the latter radionuclide label (Dewanjee et al, 1982).

Second, the labelling of autologous platelets enabled accurate adjustment for MPLS in our logistic regression models, preventing potentially spurious findings that may have resulted from alloimunization (Stratton et al, 1989). It was this substitution of homologous with autologous platelets by Ballem et al (1987) that helped illustrated the existence of suboptimal levels of platelet production among a segment of primary ITP population.

Third, the development of a cohort of patients indicated but not opting for splenectomy represented a novel construct by which to compare long-term outcomes in splenectomized and non-splenectomized patients. Although past investigations have aptly documented a high success rate for splenectomy in patients with primary ITP, they have left unanswered the question of whether equivalent outcomes were possible via an alternate treatment paradigm.

Four limitations to the study should similarly be noted. First, while data were collected on 91 patients undergoing splenectomy following an autologous 111In-labelled platelet sequestration study, the results of the scan were not blinded to patients or their haematologists. As a result, only 20 (20·4%) patients with mixed or hepatic sequestration elected to undergo surgery. The significant difference in the proportion of patients with mixed or hepatic versus purely or predominantly splenic sequestration proceeding to splenectomy (P < 0·001; Fig 2) raises the possibility of selection bias, namely that mixed or hepatic patterned patients who opted for splenectomy were worse off than those who did not. Though plausible, it is similarly possible for haematologists to have encouraged surgery among mixed or hepatic patterned patients deemed more likely to succeed. To address this issue, post-hoc analyses were conducted on a subset of patients for whom pre-surgery platelet counts and post-diagnosis duration were available. Neither variable differed significantly (P = 0·59 and P = 0·83, respectively). Furthermore, mixed or hepatic patterned patients who underwent splenectomy were on average eleven years (P = 0·01) younger than those who did not, suggesting that non-adjusted results would have likely been biassed toward, rather than away from, the null.

Second, though advocated by the International Working Group on ITP (Rodeghiero et al, 2009), the use of 100 × 109/l as a platelet count threshold for CR may have discounted clinically relevant responses to splenectomy. Ideally, effectiveness would have been assessed by therapeutic impact on not only platelet count but also the bleeding events and health-related qualify of life. However, sufficient data were not available to gauge the effect of splenectomy on the latter two variables, an inherent limitation of retrospective investigations. Despite this shortcoming, the relative distributions of platelet counts among the purely or predominantly and mixed or hepatic, splenectomized cohorts at each of the assessment intervals suggest that a greater proportion of patients within the latter cohort continued to have platelet counts placing them at increased risk of major haemorrhage (i.e. < 10 × 109/l) following surgery (Figs 3 and 4).

Third, the algorithm used to capture MPLS yielded skewed results for patients with near-normal platelet lifespans, as it has been shown to perform poorly in situations of uniform destruction (i.e. normal MPLS) (Maisey et al, 1998). These errant results at the normal end of the MPLS spectrum may have been compounded by the limited time points at which blood samples were taken (0·5, 3, 24, and 48 h post-injection), which may have hindered accurate regression of platelet associated radioactivity on time. However, as the vast majority of study patients, 94·5% (225 of 238 patients for whom MPLS was evaluable), exhibited a markedly reduced MPLS of < 4 d, it is unlikely that the multivariate models were appreciably biassed.

Fourth, reasons, supplied by haematologists, for patients not undergoing splenectomy were subjective and susceptible to possible recall bias. Use of these reasons nevertheless likely resulted in an improved picture of probable long-term outcomes had patients in the splenectomized cohort not opted for surgery than evaluation of results from the non-splenectomized cohort as a whole.

In conclusion, while this investigation demonstrates utility in including autologous 111In-labelled platelet sequestration studies as an adjunct predictive tool prior to splenectomy in primary ITP, the aforementioned limitations illustrate that considerable work is still needed on this topic. Potentially useful next steps include a meta-analysis of observational studies and the development of an international prospective cohort study, the latter of which would permit critical appraisal of proposed refinements to the scanning protocol and the incorporation data from autologous 111In-labelled platelet sequestration studies, which were beyond the scope of this investigation (e.g. platelet production levels).

Authorship

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

A.S., D.P., R.S. and A.N. designed the study protocol; A.S. and S.E. performed the statistical programming; A.S., D.P., and F.W.D.T.; A.S., D.P., S.E., R.S., and A.N. analysed the results and helped draft the manuscript.

Acknowledgements

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

We would like to thank Dr Jamie Cavenagh for his efforts to help establish the platelet sequestration service at our centre and Nish Fernando, Professor Keith Britton, and Drs Ana Canizales, Salah Elnaas, Maggie Cooper, Neil W. Garvie, and Margaret Newell for conducting the studies. We would also like to acknowledge Tie Sing Fong, Helen Ngu, and Deborah Kenny for providing invaluable assistance in securing data from collaborating centres and Dr Dimitri Bennett for offering critical feedback on the manuscript. Lastly, this work would not have been possible without the collaboration of over 175 haematologists across 100 centres throughout the United Kingdom. A.S. is a Ph.D. candidate at the University of Cambridge, and this work is submitted in partial fulfilment of the requirement for the Ph.D.

Conflict of Interest Disclosure

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

The investigation was supported in part by the UK ITP Support Association and GlaxoSmithKline. A.S. reports having received research funding from Amgen and GSK, consultancy payments from GSK, and an honorarium for participation in an advisory board for Baxter. D.P. reports having received research funding, and honoraria for participation in advisory boards for Amgen and GSK. S.E. reports having received consultancy payments from GSK. A.C.N. reports having received research funding and honoraria for participation in advisory boards for Amgen and GSK.

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  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Summary
  3. Methods
  4. Results
  5. Discussion
  6. Authorship
  7. Acknowledgements
  8. Conflict of Interest Disclosure
  9. References
  10. Supporting Information

Tables SI–IV. Past investigations of radioisotope labelled platelet sequestration studies in primary ITP (1976–1984).

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