Plasmodium falciparum parasite density and its association with malaria rapid diagnostic test positivity among school-aged children across diverse malaria transmission settings in mainland Tanzania

Malaria rapid diagnostic tests (mRDTs) are widely used for malaria diagnosis and surveillance; however, their performance is influenced by parasite density. This study assessed the relationship between parasite density and mRDT positivity among school-aged children (SAC) across diverse malaria transmission settings in mainland Tanzania. Data were obtained from 17,131 school-aged children participating in a nationwide malaria survey. Malaria infection was assessed using mRDT and quantitative polymerase chain reaction (qPCR) targeting the Plasmodium falciparum lactate dehydrogenase (PfLDH) gene. Parasite density distributions were examined across demographic, environmental, and epidemiological characteristics. Malaria prevalence was 19.5% by malaria rapid diagnostic test (mRDT) and 19.2% by PfLDH-PCR. Both methods detected a total of 2,489 infections, while 839 and 777 were detected by mRDT and PfLDH-PCR alone, respectively. Parasite density differed significantly across age groups (p < 0.0001), sex (p = 0.0208), elevation above sea level (p < 0.0001), and malaria epidemiological strata (p < 0.0001), but not by fever status (p = 0.899). School children aged 9-12 years, those living below 750 m elevation, and those residing in high-transmission strata exhibited the highest parasite densities. The probability of being malaria positive by mRDT increased sharply with increasing parasite density, exceeding 95% at moderate parasite densities and approaching 100% at higher parasite burdens. Parasite density is strongly associated with mRDT positivity among school-aged children in Tanzania. While mRDTs effectively detect moderate and high-density infections, low- density infections remain frequently undetected. Thus, incorporating molecular diagnostics into surveillance systems could improve the detection of hidden malaria reservoirs and enhance malaria control and elimination strategies.