Diagnostic Performance of Conventional and Ultrasensitive Rapid Diagnostic Tests for Malaria in Febrile Outpatients in Tanzania

[[[ Background: ]]] A novel ultrasensitive malaria rapid diagnostic test (us-RDT) has been developed for improved active Plasmodium falciparum infection detection. The usefulness of this us-RDT in clinical diagnosis and fever management has not been evaluated. [[[ Methods: ]]] Diagnostic performance of us-RDT was compared retrospectively to that of conventional RDT (co-RDT) in 3000 children and 515 adults presenting with fever to Tanzanian outpatient clinics. The parasite density was measured by an ultrasensitive qPCR (us-qPCR), and the HRP2 concentration was measured by an enzyme-linked immunosorbent assay. [[[ Results: ]]] us-RDT identified few additional P. falciparum-positive patients as compared to co-RDT (276 vs 265 parasite-positive patients detected), with only a marginally greater sensitivity (75% vs 73%), using us-qPCR as the gold standard (357 parasite-positive patients detected). The specificity of both RDTs was >99%. Five of 11 additional patients testing positive by us-RDT had negative results by us-qPCR. The HRP2 concentration was above the limit of detection for co-RDT (>3653 pg of HRP2 per mL of blood) in almost all infections (99% [236 of 239]) with a parasite density >100 parasites per µL of blood. At parasite densities <100 parasites/µL, the HRP2 concentration was above the limits of detection of us-RDT (>793 pg/mL) and co-RDT in 29 (25%) and 24 (20%) of 118 patients, respectively. [[[ Conclusion: ]]] There is neither an advantage nor a risk of using us-RDT, rather than co-RDT, for clinical malaria diagnosis. In febrile patients, only a small proportion of infections are characterized by a parasite density or an HRP2 concentration in the range where use of us-RDT would confer a meaningful advantage over co-RDT.