Abstract
BackgroundProzone 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.MethodsFrom 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.ResultsA 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.ConclusionProzone occurs at different frequency and intensity in HRP-2 RDTs and may decrease diagnostic accuracy in the most affected RDTs.
Highlights
Prozone means false-negative or false-low results in antigen-antibody reactions, due to an excess of either antigen or antibody
Study period and patients included The study was conducted in the emergency ward of the Provincial Hospital of Tete (PHT), located in Central Mozambique
Invalid test results, reproducibility and inter-observer reliability Shipment and storage temperatures of rapid diagnostic tests (RDTs) ranged between 4.0°C and 23.5°C and 23.3° C and 32.3°C respectively
Summary
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. Malaria rapid diagnostic tests (RDTs) detect Plasmodium antigens in blood by antibody-antigen interactions on a nitrocellulose test strip. RDTs are being rolled out as an alternative to microscopic diagnosis in malaria endemic settings [1] and have demonstrated sensitivities close to 100% for the detection of P. falciparum at densities above 100 asexual parasites/μl or > 0.002% of parasitized red blood cells (RBC). False-negative results have been reported at high parasite densities Part of those may be ascribed to genetic variations of the HRP-2 [2,3,4,5,6], but the prozone phenomenon may be involved. Prozone is defined as false-negative or falselow results in antigen-antibody immunological reactions, due to an excess of either antigens or antibodies [7,8]
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