Validity and reliability of methods to microscopically detect and quantify malaria parasitaemia

ObjectivesThe recommended microscopy method by WHO to quantify malaria parasitaemia yields inaccurate results when individual leucocyte (WBC) counts deviate from 8000leucocytes/l. A method avoiding WBC count assumptions is the Lambarene method (LAMBA). Thus, this study compared validity and reliability of the LAMBA and the WHO method. MethodsThree methods for counting parasitaemia were applied in parallel in a blinded assessment: the LAMBA, the WHO method using a standard factor of 8000leucocytes/l [simple WHO method' (sWHO)] and the WHO method using measured WBC counts [accurate WHO method' (aWHO)]. Validity was assessed by comparing LAMBA and sWHO to the gold standard measurement of aWHO. Reliability was ascertained by computation of intraclass correlation coefficients (ICCs). Results787 malaria-positive thick smears were analysed. Parasitaemia as determined by LAMBA and sWHO increasingly deviated from aWHO the more patients' WBCs diverged from 8000/l. Equations of linear regression models assessing method deviation in percent from gold standard as function of WBC count were y=-0.00608x (95% CI -0.00693 to -0.00524)+47.8 for LAMBA and y=-0.0125x (95% CI -0.01253 to -0.01247)+100.1 for sWHO. Comparison of regression slopes showed that the deviation was twice as high for sWHO as for LAMBA (P<0.001). ICCs were excellent (>90%) for both methods. ConclusionsThe LAMBA has higher validity than the sWHO and may therefore be preferable in resource-limited settings without access to routine WBC-evaluation. ObjectifsLa methode de microscopie recommandee par l'OMS pour quantifier la parasitemie du paludisme donne des resultats inexacts lorsque les comptages leucocytaires (GB) s'ecartent de 8000 leucocytes/L. Une methode qui evite la base du comptage des leucocytes est la methode de Lambarene. Cette etude a donc compare la validite et la fiabilite de la methode de Lambarene et de celle de l'OMS. MethodesTrois methodes de comptage de la parasitemie ont ete appliquees en parallele dans une evaluation en aveugle: la methode Lambarene (LAMBA), la methode OMS utilisant un facteur standard de 8000 leucocytes/L (<< methode simple de l'OMS >>, [sWHO]) et la methode OMS utilisant des comptages mesures des GB (methode precise de l'OMS, aWHO]). La validite a ete evaluee en comparant LAMBA et sWHO a la mesure de reference aWHO. La fiabilite a ete determinee par le calcul des coefficients de correlation intra-classe (CCI). Resultats787 frottis epais positifs pour le paludisme ont ete analyses. La parasitemie telle que determinee par LAMBA et sWHO deviait plus de celle de aWHO lorsque les GB des patients s'ecartaient plus de 8000/mu L. Les equations des modeles de regression lineaire evaluant l'ecart de la methode en pourcentage de la methode de reference en fonction de la numeration des GB etaient: y=-0.00608x(IC95%: -0.00693 a -0.00524)+47.8 pour LAMBA et y= -0.012x(IC95%: -0.1253 a -0.01247)+100.1 pour sWHO. La comparaison des pentes de regression a montre que l'ecart etait deux fois plus eleve pour sWHO que pour LAMBA (p<0.001). Les CCI etaient excellents (>90%) pour les deux methodes. ConclusionsLa methode de Lambarene a une validite superieure a celle de la methode simple de l'OMS et peut donc etre preferable dans les milieux a ressources limitees sans acces a l'evaluation de routine des GB.