• MALARIA
• Staining and Labeling
• Quality Assurance, Health Care
• PARASITES
• Morphological and Microscopic Findings

Rapid antigen tests, such as the BinaxNOW Malaria assay (Abbott Diagnostics, Scarborough, Maine, USA), provide an excellent means of detecting malaria and differentiating between Plasmodium falciparum and non-falciparum infections.1 In our laboratory, the BinaxNOW assay is used as a rapid screening tool, with positive results and speciation subsequently confirmed by formal review of Giemsa stained thick and thin peripheral blood smears.

However, such immunochromatographic assays can fall victim to the ‘postzone’ or high-dose hook effect (sometimes mistakenly deemed the ‘prozone’ effect), a phenomenon whereby reagent antibody binding sites become saturated with excess antigen, preventing optimal antibody-to-antigen crosslinking, resulting in the absence of a test band and a false-negative result.2 False-negativity due to antigen excess has been well documented in the context of Cryptococcal antigen testing in the USA,3 as well as in cases of high-burden P. falciparum infections in endemic areas.4 However, the postzone phenomenon has rarely been implicated as a cause of false-negative …