Malaria Parasite Density Estimation using Actual and Assumed White Blood Cells Count in Children in Eastern Sudan

Abstract

Background: Estimating malaria parasite count is needed for estimating the severity of the disease and during the follow-up.

Objective: This study was conducted to determine the malaria parasite density among children using actual white blood cell (WBC) and the assumed WBC counts (8.0 × 10⁹/l).

Methods: A cross-sectional study was conducted at New Halfa Hospital, Sudan. WBC count and count of asexual malaria parasite were performed on blood films.

Results: One hundred and three children were enrolled. The mean (SD) WBCs was 6.2 (2.9) cells × 10⁹/l. The geometric mean (SD) of the parasite count using the assumed WBCs (8.0 × 10⁹/l cells/μl) was significantly higher than that estimated using the actual WBC count [7345.76 (31 038.56) vs. 5965 (28 061.57) rings/μl, p = 0.042].

Conclusion: Malaria parasitemia based on assumed (8.0 × 10⁹/) WBCs is higher than parasitemia based on actual WBCs.

INTRODUCTION

Malaria should be confirmed by a parasitological test [1]. Microscopy is an important tool for parasite quantification / malaria parasite density, which is performed by counting parasites within a given number of microscopic fields, against counted white blood cells (WBCs) within the same fields [1, 2]. Owing to the scant resources in malaria-endemic countries for quantification of patients’ WBCs, an assumed WBC count of 8.0 × 10⁹/l is used to quantify malaria parasite densities [3]. However, WBCs vary greatly from individual to individual, are age dependent and are unreliable during febrile episodes [4]. With the advent of automated machines, researchers are evaluating the assumed WBCs method vs. the actual count of the WBCs [5–8]. This study was conducted to compare the malaria parasite density among children using actual WBC and the assumed WBC counts (8.0 × 10⁹/l).

METHODS

A cross-sectional study was conducted at New Halfa Hospital, Sudan, during the period from August to October 2013. After signing an informed consent form, febrile children (temperature ≥ 37.5°C) were examined and data sheet was used to collect demographic information.

Blood films for malaria and hematological analysis were performed as described in our previous work [8]. In summary, thick blood slides were 10% Giemsa stained and examined under the × 100 oil immersion objective lens of a light microscope. The asexual parasites density was counted against 200 WBCs. Parasite densities (parasite/μl of whole blood) were then calculated as follows:

(Number of parasites counted/WBC counted) × WBC count/µl of participant.

The complete blood count, including white cells, was analyzed using Sysmex KX-21N^TM automated hematology analyzer according to the manufacturer’s instruction manual.

Ethics

This study received ethical clearance from Board of the Faculty of Medicine, University of Khartoum, Sudan.

Statistics

Data were entered in computer using SPSS for Windows. Mann–Whitney U-test was used to compare the geometric means of the parasite count using the actual count and the assumed (8.0 × 10⁹/l) WBCs for each patient. Nonparametric correlation was used to detect the relation between continuous variables.

RESULTS

Of the 103 children enrolled, 53.4% were males; major characteristics and blood indices are illustrated in Table 1. The mean (SD) WBC count was 6.2 (2.9) cells/μl, with a range of 1.2–18.7 and a median of 6.1 cells × 10⁹/l.

Table 1.

Age and laboratory characteristics of children with malaria participating in the study

Variable	Mean (SD) [range]
Age in years	7.71 (5.06) [0.25–15]
Hemoglobin concentrationg/dl	9.87 (2.6) [3.6–15.8]
White cell count 109/l	6.2 (2.9) [9.8–18.7]
Platelet count 109/l	154.44 (74.07) [26–400]

The geometric mean (SD) of the parasite count using the assumed WBCs (8.0 × 10⁹/l) was significantly higher than that estimated using the actual WBC count [7345.76 (31 038.56) vs. 5965 (28 061.57) rings/µl, p = 0.042]; figure 1. The two parasitemias were positively correlated (r = 0.95, p < 0.001); figure 2. There was no significant difference in the mean (SD) WBCs count [6.4 (3.6) vs. 5.9 (2.2) cell × 10⁹/l, p = 0.558] and parasitemia (using both assumed and actual WBCs) in children when correlated by the age; figure 3.

Fig. 1.

Comparison of parasitemia in children using assumed and actual WBC counts of 8000 cell/μl.

Fig. 2.

Correlations between log of parasitemias using actual and assumed white cell count of 8000 cell/μl.

Fig. 3.

(A, B) Comparison between log parasitemia counted on assumed (8000 cell/μl) and actual white cell count in children correlated by age.

DISCUSSION

The main finding in this study was that parasite density calculated using the actual WBC count was significantly lower than that calculated using the assumed WBCs. This is in agreement with our previous results [8] and findings of Adu-Gyasi et al. [6] and Jeremiah and Uko [9].

CONCLUSION

Malaria parasitemia based on assumed (8.0 × 10⁹/l) WBCs is higher than parasitemia based on actual WBCs.