Declining malaria transmission challenges rapid diagnostic test performance in Cambodia

Cambodia has made great progress in reducing malaria transmission and is targeting elimination. While this progress is particularly marked for Plasmodium falciparum , the situation is different for Plasmodium vivax . It is generally assumed that symptomatic patients are effectively diagnosed using rapid diagnostic tests (RDTs), regardless of transmission intensity. In 2023 we conducted a cross-sectional survey among 986 treatment-seeking patients in 6 provinces of Cambodia with varying reported malaria cases. Malaria RDT (Pf/Pv), microscopy and qPCR diagnostics of Plasmodium infections and species determination were performed. Using qPCR, Plasmodium infections were diagnosed in 156 patients (15.8%, 95% CI: 13.7–18.2%) from all 6 provinces. Positivity rate was markedly different between health centers (HCs) and ranged between 57.2% and 0.5%. Parasitemia of infected patients was different between HCs and was lower in HCs with the lowest positivity rate compared to those with higher rates. The majority of Plasmodium infections (75%) were caused by P. vivax , however all human malaria species were identified as well as the simian parasite P. knowlesi . Overall sensitivity of RDTs to detect Plasmodium infections was 39.7% (95% CI: 28.9–51.6%) and specificity was 100% (95% CI: 99.5–100%). The proportion of RDT true positives was significantly different between HCs, and a tendency for higher false negative rates in low transmission areas compared to higher ones was observed. While our results confirm that P. falciparum parasites are nearly eliminated from Cambodia, we show that current practice for diagnosis of Plasmodium infections among febrile patients is challenged, especially in very low transmission settings.