The HRP2 antigen - practicalities
P.falciparum parasites produce “histidine-rich” proteins (HRP). The HRP2 antigen is highly expressed and stable, and is therefore very useful in the detection of P.falciparum infection. Additionally, antibodies that detect HRP2 also cross react with the closely related HRP3 protein which can improve their sensitivity, particularly where there is HPR2 gene-deletion is not expressed.
At a high parasitaemia the sensitivity of HRP2 (like LDH-based tests) is likely to exceed 90% detection for P.falciparum. However, at lower parasite levels (<1000 parasites/μL) the sensitivity will be significantly less (around 70%), although HP2-based assays may still perform better than LDH-based assays in these circumstances.
Characteristics of HRP2 to be aware of
(1) Half-life: HRP2 has a long half-life in vivo and the antigne may persist in blood for some time following successful treatment. HRP2 should not therefore be used to monitor disease resolution (see sections on RDT ntepretation).
(2) HRP2 may be affected by false positive results when malaria antigen levels are very high (the prozone-like or postzone phenomenon - see description in the RDT-interpretation sections
(3) HRP2 is increasingly subject to gene deletion in some geographical areas* which may cause false negative results (see sections on RDT interpretation).
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