Evaluation of the rapid diagnostic test CareStart pLDH Malaria (Pf-pLDH/pan-pLDH) for the diagnosis of malaria in a reference setting.

BACKGROUND: The present study evaluated CareStart pLDH Malaria, a three-band rapid diagnostic test detecting Plasmodium falciparum-specific parasite lactate dehydrogenase (Pf-pLDH) and pan Plasmodium-specific pLDH (pan-pLDH) in a reference setting. METHODS: CareStart pLDH was retrospectively and prospectively assessed with a panel of stored (n=498) and fresh (n=77) blood samples obtained in international travelers suspected of malaria. Both panels comprised all four Plasmodium species; the retrospective panel comprised also Plasmodium negative samples. The reference method was microscopy corrected by PCR. The prospective panel was run side-to-side with OptiMAL (Pf-pLDH/pan-pLDH) and SDFK60 (histidine-rich protein-2 (HRP-2)/pan-pLDH). RESULTS: In the retrospective evaluation, overall sensitivity for P. falciparum samples (n=247) was 94.7%, reaching 98.7% for parasite densities>1,000/µl. Most false negative results occurred among samples with pure gametocytaemia (2/12, 16.7%) and at parasite densities ≤ 100/µl (7/12, 58.3%). None of the Plasmodium negative samples (n=96) showed visible test lines. Sensitivities for Plasmodium vivax (n=70), Plasmodium ovale (n=69) and Plasmodium malariae (n=16) were 74.3%, 31.9% and 25.0% respectively. Wrong species identification occurred in 10 (2.5%) samples and was mainly due to P. vivax samples reacting with the Pf-pLDH test line. Overall, Pf-pLDH test lines showed higher line intensities compared to the pan-pLDH lines (67.9% and 23.0% medium and strong line intensities for P. falciparum). In the prospective panel (77 Plasmodium-positive samples), CareStart pLDH showed higher sensitivities for P. falciparum compared to OptiMAL (p=0.008), lower sensitivities for P. falciparum as compare to SDFK60 (although not reaching statistical significance, p=0.08) and higher sensitivities for P. ovale compared to both OptiMAL (p=0.03) and SDFK60 (p=0.01). Inter-observer and test reproducibility were good to excellent. CONCLUSION: CareStart pLDH performed excellent for the detection of P. falciparum, well for P. vivax, but poor for P. ovale and P. malariae.

Prozone in malaria rapid diagnostics tests: how many cases are missed?

BACKGROUND: Prozone means false-negative or false-low results in antigen-antibody reactions, due to an excess of either antigen or antibody. The present study prospectively assessed its frequency for malaria rapid diagnostic tests (RDTs) and Plasmodium falciparum samples in an endemic field setting. METHODS: From January to April 2010, blood samples with P. falciparum high parasitaemia (≥ 4% red blood cells infected) were obtained from patients presenting at the Provincial Hospital of Tete (Mozambique). Samples were tested undiluted and 10-fold diluted in saline with a panel of RDTs and results were scored for line intensity (no line visible, faint, weak, medium and strong). Prozone was defined as a sample which showed no visible test line or a faint or weak test line when tested undiluted, and a visible test line of higher intensity when tested 10-fold diluted, as observed by two blinded observers and upon duplicate testing. RESULTS: A total of 873/7,543 (11.6%) samples showed P. falciparum, 92 (10.5%) had high parasitaemia and 76 were available for prozone testing. None of the two Pf-pLDH RDTs, but all six HRP-2 RDTs showed prozone, at frequencies between 6.7% and 38.2%. Negative and faint HRP-2 lines accounted for four (3.8%) and 15 (14.4%) of the 104 prozone results in two RDT brands. For the most affected brand, the proportions of prozone with no visible or faint HRP-2 lines were 10.9% (CI: 5.34-19.08), 1.2% (CI: 0.55-2.10) and 0.1% (CI: 0.06-0.24) among samples with high parasitaemia, all positive samples and all submitted samples respectively. Prozone occurred mainly, but not exclusively, among young children. CONCLUSION: Prozone occurs at different frequency and intensity in HRP-2 RDTs and may decrease diagnostic accuracy in the most affected RDTs.