The prognostic and diagnostic value of intraleukocytic malaria pigment: an individual patient data pooled meta-analysis of 32,000 patients with severe falciparum malaria in Africa and Asia

BackgroundSevere falciparum malaria is a major cause of death in tropical countries, particularly in African children. Accurate diagnosis and prognostic assessment are critical to clinical management.MethodsThe prognostic value of the malaria parasite count, and the proportions of polymorphonuclear leukocytes (PMNs) and monocytes (PMMs) containing malaria pigment in peripheral blood films were assessed in three randomized controlled trials conducted in severe malaria patients; two in Southeast Asia (AQ Vietnam; n=483 and SEAQUAMAT; n=1,330) and one in Africa (AQUAMAT; n=4,211). Following a systematic review of the literature, we incorporated these data into an individual patient data meta-analysis including published data from the Severe Malaria in African children (SMAC) network (n=25,845) and a study from Mali (n=166).FindingsThe proportion of pigment containing PMNs on peripheral blood films was strongly positively correlated with prognosis (odds-ratio for in-hospital mortality for a tenfold increase: 2.53 [95% CI: 2.13-3.00], p = 10−26). The meta-analytic odds-ratio estimate for in-hospital death in patients with >5% pigment containing PMNs compared with lower values was 2.67 (95% CI: 2.08-3.42; p = 10−14). Particularly in African children, the proportion of pigment containing PMNs added substantially to the prognostic assessment from simple bedside examination, and also to the conventional parasite count. In all analyses, the proportion of pigment containing monocytes had a lower prognostic value.InterpretationMicroscopy assessment of the proportion of pigment containing PMNs in a blood film is simple and rapid, and should be performed in all patients hospitalised with suspected severe malaria. Patients with >5% pigment containing PMNs have more than double the risk of death.OtherFunded by Wellcome. The systematic review was registered prospectively on PROS-PERO, number CRD42021284527Research in contextEvidence before this studySevere falciparum malaria remains a major cause of preventable childhood mortality in sub-Saharan Africa. In 2019 there were an estimated 274,000 deaths in children under 5 years. Rapidly identifying patients at the greatest risk of death and providing effective treatment is essential to saving lives. Based on data from our prospective studies of strictly defined severe falciparum malaria in Vietnamese adults, the proportions of peripheral blood neutrophils and monocytes containing malaria pigment (haemozoin) was proposed as a prognostic factor for mortality. We carried out a systematic review on PubMed of all articles published between database inception and October 11, 2021, using search terms “intraleukocytic pigment” and “severe malaria”. In addition to papers published by our research group, we found two other studies that reported the prognostic value of intraleukocytic pigment counts in severe malaria cohorts of at least 100 patients: the SMAC network study, the largest published cohort study conducted in over 25,000 African children with suspected severe malaria, and a cohort of 172 children from Mali. The SMAC study reported that intraleukocytic malaria pigment counts were not a useful predictor of outcome in African children diagnosed with severe malaria. This differed from the results from the Malian study and our original study in Vietnamese adults.Added value of this studyWe provide new data on the prognostic value of intraleukocytic malaria pigment counts in over 6,000 adults and children with a strict diagnosis of severe falciparum malaria studied prospectively in Asia and Africa. These patients were enrolled in three of the largest randomised controlled trials in severe malaria. These randomised trials have provided the main evidence base for current global therapeutic recommendations. Our data show that there is substantial prognostic value in counting intraleukocytic malaria pigment. This was significantly greater for neutrophil rather than monocyte associated pigment. Pooling all the individual patient data showed that the prognostic value was consistent across studies and countries, despite the substantial differences in study populations and study designs. Having more than 5% pigment containing neutrophils was associated with over double the risk of death from severe falciparum malaria.Implications of all the available evidenceIntraleukocytic malaria pigment counts have sub-stantial prognostic value in severe falciparum malaria. The proportion of neutrophils containing malaria pigment should be counted in thin blood films in all patients with suspected severe malaria. Patients with over 5% of pigment containing neutrophils have a high risk of death.