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

Area of Science:

• Malariology
• Immunodiagnostics
• Public Health

Background:

• Prozone effect leads to false-negative or false-low results in antigen-antibody reactions due to antigen or antibody excess.
• Assessed the frequency of prozone in malaria rapid diagnostic tests (RDTs) for Plasmodium falciparum in an endemic setting.

Purpose of the Study:

• To prospectively evaluate the occurrence and impact of the prozone phenomenon on malaria RDT performance.
• To determine the frequency of prozone in different types of malaria RDTs using Plasmodium falciparum samples.

Main Methods:

• Collected blood samples from patients with high Plasmodium falciparum parasitaemia (≥ 4% red blood cells infected) in Mozambique.
• Tested samples undiluted and 10-fold diluted with various RDTs, scoring line intensity.
• Defined prozone as a weaker or absent test line in undiluted samples compared to diluted samples, confirmed by blinded observers and duplicate testing.

Main Results:

• Of 7,543 samples, 873 (11.6%) showed P. falciparum, with 92 (10.5%) having high parasitaemia.
• All six histidine-rich protein 2 (HRP-2) RDTs exhibited prozone, unlike the two Pf-pLDH RDTs.
• Prozone frequencies in HRP-2 RDTs ranged from 6.7% to 38.2%, with negative/faint lines observed in 3.8% and 14.4% of prozone cases, respectively. Prozone was more common in young children.

Conclusions:

• The prozone effect occurs with varying frequency and intensity in HRP-2 RDTs.
• Prozone phenomenon can decrease the diagnostic accuracy of malaria RDTs, particularly HRP-2 based tests.
• Findings highlight the need to consider prozone in interpreting RDT results in high-parasitaemia malaria cases.