Standardised versus actual white cell counts in estimating thick film parasitaemia in African children under five


Corresponding Author Piero Olliaro, Special Programme for Research & Training in Tropical Diseases (TDR), World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland. E-mail:


In patients with malaria, parasitaemia is usually estimated by assuming 8000 white cell counts (WCC) per microlitre of blood. In a sample of 3044 African children under 5 years of age with uncomplicated falciparum malaria, parasitaemia estimated using standardised WCC was compared to parasitaemia calculated based on each child’s own WCC. The two methods produced comparable results. However, WCC were >8000 in under-fives with an inverse relationship with age, resulting in the standard approximation method significantly underestimating parasitaemia in the youngest age group and overestimating parasitaemia in the oldest age groups.

Communication brève: Parasitémie estimée et réelle dans la malaria aigüe non compliquée à Plasmodium falciparum chez les enfants africains de moins de cinq ans

Chez les patients malariques, la parasitémie est généralement estimée à partir du seuil de 8.000 globules blancs par microlitre de sang. Dans un échantillon de 3.044 enfants africains de moins de cinq ans avec une malaria non compliquée àP. falciparum, la parasitémie estimée et réelle (calculée sur base du taux de globules blancs de chaque enfant), n’étaient en général pas significativement différentes. Cependant, il y avait une association inverse entre le taux de globules blancs et l’âge, résultant ainsi au fait que la méthode d’approximation standard sous-estime nettement la parasitémie dans le groupe d’âge le plus jeune et surestime la parasitémie dans les groupes les plus âgés.

Comunicación corta: Parasitemia estimada versus real en niños Africanos menores de 5 años con malaria aguda, no complicada por Plasmodium falciparum

En pacientes con malaria, normalmente se estima la parasitemia asumiendo que hay 8000 leucocitos por microlitro de sangre. En una muestra de 3044 niños africanos menores de cinco años de edad, con malaria no complicada por falciparum, la parasitemia estimada y la parasitemia real (calculada basándose en los conteos leucocitarios de los propios niños) en general no eran estadísticamente diferentes. Sin embargo había una relación inversa entre los conteos leucocitarios y la edad, de forma que el método de aproximación estándar subestima significativamente la parasitemia en el grupo de edad de los más jóvenes y sobrestima la parasitemia entre los grupos de los mayores.

Plasmodium falciparum parasitaemia is normally estimated by counting on microscopic examination of Giemsa-stained blood smears the number of parasites after a pre-determined number of white blood cells (normally 100–500) has been counted and assuming a standard white cell count (WCC, normally 8000/μl). The following formula is generally applied: parasites/μl = [(number of parasites counted/number of leucocytes counted) × 8000 leucocytes/μl] [World Health Organization (WHO) 2010].

This approximation is expedient as WCC normally cannot be determined in most primary health care settings of the malaria-endemic areas. However, WCC vary greatly and are age-dependent. As opposed to ‘western’ standards (which have 5.0–11.9 WBC × 109/l for all children under 6 years of age), a study in Mozambique reported intervals of 2.2–16.6 in children 1–2 years old, 5.8–5.6 in 2- to 3-year-olds, 5.5–14.7 in 3- to 4-year-olds and 5.6–14.4 in 4- to 5-year-olds (Quintóet al. 2006). Several studies, predominantly in malaria-infected adults, concur that WCC increase in acute malaria (Erhart et al. 2004; McKenzie et al. 2005). Therefore, this estimation may generate systematic errors and lead to incorrect conclusions. The present analysis was conducted to assess the correlation between parasitaemia as generally estimated and parasitaemia calculated based on a patient’s WCC.

Children under 5 years of age were selected because they are the most vulnerable group and constitute the study population in the majority of clinical trials of antimalarial drugs in sub-Saharan Africa. Data on age, parasitaemia and white blood cell total and differential counts were extracted from a database of seven randomised controlled trials of artesunate plus amodiaquine vs. various comparator drugs for treating acute uncomplicated falciparum malaria at 14 sites in 10 sub-Saharan African countries (on average 217 patients per site or 304 patients per country). These studies were conducted between February 1999 in Kenya and December 2006 in Senegal. All were approved by local or national and WHO ethics committees. The designs of these clinical trials were similar and based on the WHO (2006) protocol, as described by Zwang et al. (2009). Here, analyses were restricted to the patients under 5 years of age with recorded WCC at baseline (n = 3044, i.e. 26% of the 11 700 patients enroled in the trials).

In the original studies, parasitaemia (number/μl) was quantified by a standard approximation method (40 × number of parasites per 200 WCCs on thick film, i.e. assuming 8000 WCCs/μl) in six of the seven studies included in this analysis (Adjuik et al. 2002; Mårtensson et al. 2005; Karema et al. 2006; Dorsey et al. 2007; Djimdéet al. 2008; Sirima et al. 2009). The 7th study counted P. falciparum, assuming an average WCC of 7500/μl (Ndiaye et al. 2009). For the purpose of coherence, we also recalculated the estimated parasitaemia based on 8000 WCCs for the present study.

Parasitaemia on admission was recalculated for all studies assuming 200 WCCs for all patients vs. each patients’ own actual WCC on admission. The Wilcoxon signed rank test was used to compare both methods of estimating parasitaemia on paired patient data; the Wilcoxon rank test was used for non-paired data. For multivariate analysis, we used a linear model between age (in years) and parasitaemia (log-transformed) with a random effect for study site. P-values under 0.05 were considered significant. The statistical programme used was STATA (version 10; Stata Corp.).

The geometric mean of the actual pre-treatment parasitaemia was 18 334 per μl overall (Table 1). Combining data from all sites over the entire 0–4 year age range, no significant difference (+2%, P = 0.277) was detected between estimated and actual parasitaemia. However, statistically significant differences were detected when data were stratified by age and site. Using paired analysis, parasitaemia estimated using standardised WCC was significantly lower than when computed using actual WCC in children under 1 year of age (−14%, P = 0.001) and slightly higher in 4-year-olds (+8%, P = 0.076). Differences were detected at 11 of the 14 sites between the two methods, with five sites overestimating (Zanzibar and Rwanda where WCC were lower) and six underestimating parasitaemia (no significant difference at three sites) (Table 2). In the sites with significant differences between the two methods, the overall percentage overestimation of the geometric mean was twice (+37%) as much as that of the underestimation (−19%) corresponding approximately to the difference in WCC (10.6 and 6.6 × 109/l, respectively, P = 0.001) and in age (2 vs. 3 years, P = 0.001). Overall in these sites, the difference in parasitaemia estimated based on actual (17 852/μl) and standardised WCC (18 210/μl) was not significant (+2%, P = 0.313, paired analysis).

Table 1.   Pre-treatment parasitaemia in African malaria patients under 5 years of age estimated (as reported in the publication) based on standardised and actual white cell counts (WCC) by age (geometric mean)
Age (year) N WBC (× 109/l)Parasitaemia calculated on actual WCCParasitaemia estimated on standardised 8000 WCCRelative difference (%) P
  1. P-values result from a Wilcoxon paired analysis.

<122010 16715 12513 031−160.001
1370963418 36517 822−30.195
2863909519 90519 988 00.238
3866850717 92018 749 40.395
4725807218 09019 603 80.076
Total3044882718 33418 681 20.277
Table 2.   Estimated (as reported in the publication) and actual [recalculated using actual white cell counts (WCC)] pre-treatment parasitaemia by site (geometric mean)
Site N Age (years)WCC (× 109/l)Parasitaemia calculated on actual WCCParasitaemia estimated on standardised 8000 WCC P
  1. P-values result from a Wilcoxon paired analysis.

Burkina Faso-Pouytenga5822. 79714 1510.001
Cameroon-Mendong722. 34554 3530.146
Gabon-Lambéréné713. 49227 5970.920
Kenya-Migori282. 86427 4020.007
Madagascar-Tsiroanomandidy553. 14510 4560.409
Mali-Bancoumana943.00.910.63.631 78723 8130.001
Mali-Bougoula6692. 16415 9350.001
Rwanda-Kicukiro1742. 99935 2950.001
Rwanda-Mashesha2182. 33121 9420.001
Rwanda-Rukara2502. 64137 6760.001
Senegal-Mlomp2043.10.911.24.625 84620 1290.001
Uganda-Kampala2493. 68211 0350.020
Zanzibar-Kivunge2782. 77716 4790.001
Zanzibar-Micheweni1002. 43217 0260.001
Total30442. 33418 6810.277

Using a linear random effects model, there was a trend for older children having higher log-transformed parasitaemia when using standardised WCC (r = 0.019, P = 0.058) but when using the actual WCC, age was not related to parasitaemia (r = −0.001, P = 0.954) (Figure 1).

Figure 1.

 Relationship between age and parasitaemia (calculated on actual white cell counts) (a) and white cells counts (b) on admission. The red dotted horizontal line represents the geometric mean of the estimated parasitaemia for the entire age range. WCC, white cells counts.

This study shows that, in children under 5 years of age with acute uncomplicated malaria living in sub-Saharan African countries with moderate to intense malaria transmission, the parasite biomass needed for malaria to be clinically manifest is not significantly different across the age range 1–4 years when calculated using the patient’s actual WCC. Overall, parasitaemia on presentation did not change significantly with age. One must not forget although that these, like any other trials, had set limits of parasitaemia to qualify for inclusion. Here, the upper limit of parasitaemia was 200 000/μl in all studies but the lowest limit varied from 500/μl (Dorsey et al. 2007) to 1000/μl (Adjuik et al. 2002; Ndiaye et al. 2009; Sirima et al. 2009) to 2000/μl (Mårtensson et al. 2005; Karema et al. 2006; Djimdéet al. 2008). Therefore, it is not possible to categorically set the pyrogenic threshold of P. falciparum parasitaemia in African children under 5 years of age from this dataset.

The admission parasitaemia as customarily estimated in the published series assuming 8000 WBCs/μl increased with age, which would be interpreted as a sign of mounting immunity with older children being able to tolerate higher parasitaemia. However, this appears to be an artefact, as it does no longer hold true when each subject’s own WCC are used to calculate parasitaemia – no significant association between age and parasitaemia was observed. This discrepancy occurs because although the two methods agree overall for all under-fives, WCC are higher in younger children.

These changes in WCC with age cause the standard approximation method to significantly underestimate parasitaemia in infants. The equipoise between the two methods of estimation in our study was at 2 years of age; thereafter, parasitaemia became (non-statistically significant) higher with standardised than with actual WCC. It should also be noted that here the average WCC were approximately 8800 and >8000 (the figure assumed in the standard method) at all ages.

A cross-sectional study in 240 Nigerian children aged 1–8 years found that the assumed parasite counts with the standard method overestimated parasitaemia but did not provide details as to possible age-related differences (Jeremiah & Uko 2007). Also, children in that study were asymptomatic, and parasite counts were considerably lower than those recorded in the symptomatic children of our study.

Most laboratories in malaria-endemic rural areas would not have the capacity to measure WCC; our data indicate that in routine practice using standard WCC counts seems altogether acceptable. Moreover, with WCC declining with age, the standardised 8000 WCC generates adequate estimates in older children and adults.

However, clinical trials of uncomplicated falciparum malaria should express parasitaemia calculated on the basis of the patients’ own baseline WCC. The differences observed between the two methods, although overall minimal, are important especially in the younger children. Using actual WCC will increase precision in estimating parasitaemia and improve our understanding of malaria and response to treatment, including haematological changes in acute and convalescent malaria. A larger sample size is desirable to confirm that this observation can be widely generalised.


We thank the patients and staff of all trial sites who participated in these trials. JZ received a grant from DNDi (Drugs for Neglected Diseases initiative) to do this analysis. The sponsor did not have any role in the design and conduct of the analysis, interpretation of results or write-up of the manuscript. We would like to thank Ric Price for advice. PO is a staff member of the WHO; the authors alone are responsible for the views expressed in this publication, and they do not necessarily represent the decisions, policy or views of the WHO.