Improving the diagnosis of severe malaria in African children using platelet counts and plasma Pf HRP2 concentrations

Abstract

BackgroundSevere falciparum malaria is difficult to diagnose accurately in children in high transmission settings. Platelet counts and plasma concentrations of P. falciparum histidinerich protein-2 (Pf HRP2) are potential biomarkers to increase diagnostic accuracy.MethodsWe fitted Bayesian latent class models to platelet counts and Pf HRP2 concentrations in 2,649 patients enrolled in four studies of severe illness in three countries (Bangladesh, Kenya, and Uganda). We estimated receiver operating characteristic curves and compared parasite densities, haematocrits, total white blood cell counts, blood culture positivity rates, and haemoglobin S genotypes (HbAS and HbSS) across the subgroups defined by the probabilistic models.FindingsThe platelet count and the plasma Pf HRP2 concentration have substantial diagnostic value in severe malaria. In severely ill patients with clinical features consistent with severe malaria, a combined platelet count ≤ 150,000 per µL and a plasma Pf HRP2 concentration ≥ 1,000 ng/mL had an estimated sensitivity of 74% and specificity of 93% in identifying ‘true’ severe falciparum malaria. We estimate one third of African children enrolled in the two clinical studies of severe malaria had another cause of severe illness. Under the model, patients with severe malaria had higher parasite densities, lower haematocrits, lower rates of invasive bacterial disease, and a lower prevalence of both HbAS and HbSS than children misdiagnosed. Mortality in ‘true’ severe malaria was consistent across the African sites at ∼ 10%.InterpretationStudies of severe falciparum malaria in African children would be improved by including only patients with platelet counts ≤ 150,000 per µL and plasma Pf HRP2 concentrations ≥ 1,000 ng/mL.FundingWellcome

The impact of malaria-protective red blood cell polymorphisms on parasite biomass in children with severe Plasmodium falciparum malaria

“…After multiple imputation, we had full data on a total of 2,198 patients for our final analyses. The key predictors of the plasma Pf HRP2 concentration were the admission platelet count (log transformed as previously described) 13 , the admission haemoglobin concentration, and parasite density. The overall distributions of both parasite densities and plasma Pf HRP2 concentrations are summarized in Figure 1 panels A and B while the results of linear quantitative trait association models between each of these parameters as well as the plasma Pf HRP2 to parasite density ratio and the 14 severe malaria associated genetic polymorphisms are shown in Figure 1C.…”

“…We re-estimated case-control odds-ratios under a data-tilting framework for our five major red blood cell genetic polymorphisms using previously published probability weights based on the plasma Pf HRP2 and the platelet count to probabilistically downweigh patients who were unlikely to have true severe malaria 13,24 . This increased the evidence that HBB, ABO, HBA1-2 and FREM3/GYP are associated with protection against severe malaria when comparing against the non-weighted case-controls odds-ratios (for HBA1-2 : p=10 −5 versus p=10 −4 ; ABO : p=10 −13 versus p=10 −8 ; FREM3 : p=10 −12 versus p=10 −11 ).…”

“…Parameter estimates for each imputed dataset were combined using Rubin’s rules (R package mitools ). To demonstrate the phenotypic information contained in the plasma Pf HRP2 for the severe malaria cases, we estimated case-control odds-ratios for protection against severe malaria, using probabilistic data-tilting to down-weight patients who likely did not have severe malaria, using a previously reported model based on Pf HRP2 and platelet counts 13 . All analyses were done in R v4.0.2.…”

“…Total parasite biomass, which includes both the circulating and sequestered fractions, is better reflected by the plasma concentration of P. falciparum Histidine Rich Protein 2 ( Pf HRP2) 9 , a 30 kD molecule that is released from the cytoplasm of falciparum-infected red blood cells, mainly during schizont rupture 10-12 . Plasma levels of Pf HRP2 have diagnostic utility in differentiating between “true” severe malaria and other severe illnesses in the presence of incidental parasitaemia 13 .…”

Stopping prereferral rectal artesunate — a grave error

“…Even within prospective studies in hospital-based research centres, the diagnosis of severe malaria is incorrect in about one third of cases. 19 The misdiagnosed children commonly have sepsis, and they have a higher case specific mortality. Twenty years ago, when the ineffective chloroquine was still the main antimalarial drug in Africa, malaria was a much more important cause of childhood death.…”