Andrew Rashid

: Plasmodium lactate dehydrogenase (pLDH) is a common target in malaria rapid diagnostic tests (RDTs). These commercial antibody capture assays target either Plasmodium falciparum–specific pLDH (PfLDH), P. vivax–specific pLDH (PvLDH), or a conserved epitope in all human malaria pLDH (PanLDH). However, there are no assays specifically targeting P. ovale, P. malariae or zoonotic parasites such as P. knowlesi and P. cynomolgi. A malaria multiplex array, carrying the specific antibody spots for PfLDH, PvLDH, and PanLDH has been previously developed. This study aimed to assess potential cross-reactivity between pLDH from various Plasmodium species and this array. We tested recombinant pLDH proteins, clinical samples for P. vivax, P. falciparum, P. ovale curtisi, and P. malariae; and in vitro cultured P. knowlesi and P. cynomolgi. P. ovale-specific pLDH (PoLDH) and P. malariae-specific pLDH (PmLDH) cross-reacted with the PfLDH and PanLDH spots. Plasmodium Knowlesi-specific pLDH (PkLDH) and P. cynomolgi-specific pLDH (PcLDH) cross-reacted with the PvLDH spot, but only PkLDH was recognized by the PanLDH spot. Plasmodium ovale and P. malariae can be differentiated from P. falciparum by the concentration ratios of PanLDH/PfLDH, which had mean (range) values of 4.56 (4.07–5.16) and 4.56 (3.43–6.54), respectively, whereas P. falciparum had a lower ratio of 1.12 (0.56–2.61). Plasmodium knowlesi had a similar PanLDH/PvLDH ratio value, with P. vivax having a mean value of 2.24 (1.37–2.79). The cross-reactivity pattern of pLDH can be a useful predictor to differentiate certain Plasmodium species. Cross-reactivity of the pLDH bands in RDTs requires further investigation.

Malaria antigen detection through rapid diagnostic tests (RDTs) is widely used to diagnose malaria and estimate prevalence. To support more sensitive next-generation RDT development and screen asymptomatic malaria, we developed and evaluated the Q-Plex™ Human Malaria Array, which quantifies the antigens commonly used in RDTs—Plasmodium falciparum–specific histidine-rich protein 2 (HRP2), P. falciparum-specific lactate dehydrogenase (Pf LDH), P. vivax –specific LDH (Pv LDH), and Pan malaria lactate dehydrogenase (Pan LDH), and human C-reactive protein (CRP), a biomarker of severity in malaria. At threshold levels yielding 99.5% or more diagnostic specificity, diagnostic sensitivities against polymerase chain reaction–confirmed malaria for HRP2, Pf LDH, Pv LDH, and Pan LDH were 92.7%, 71.5%, 46.1%, and 83.8%, respectively. P. falciparum culture strains and samples from Peru indicated that HRP2 and Pf LDH combined improves detection of P. falciparum parasites with hrp2 and hrp3 deletions. This array can be used for antigen-based malaria screening and detecting hrp2/3 deletion mutants of P. falciparum.