Background: Rapid diagnostic test (RDT) usage is prevalent in endemic settings but their performance is highly variable. The principle antigens targeted by malaria RDTs are histidine-rich protein 2 (HRP-2) and plasmodial lactate dehydrogenase (pLDH). Current RDT evaluation uses patient-derived clinical isolates for quality control and calibration but no standardized single-source reference material is available.
Materials/methods: A candidate International Standard (IS) comprising lyophilized P. falciparum (strain W2) was prepared. NIBSC, in collaboration with FIND, performed an international collaborative study (13 laboratories, 11 countries) that evaluated the IS in P. falciparum antigen detection assays (ELISA and RDTs). Commutability of the candidate IS was assessed by comparison to P. falciparum clinical isolates. Accelerated thermal degradation was used to predict the long-term stability of the preparation.
Results: Similar levels of intra- and inter-laboratory variation in HRP2 and pLDH were observed for the IS and clinical isolates. Comparison to the IS reduced the variability of ELISA data between laboratories. In all 19 RDTs tested the candidate IS was detected but not all clinical isolates. No degradation of HRP2 or pLDH is predicted when the IS is stored at -70℃.
Conclusions: The first WHO International Standard for P. falciparum antigens, to support rapid diagnostic tests, was established by the WHO Expert Committee on Biological Standardization (2017)1. It is calibrated in International Units for the harmonization, evaluation of performance, and sensitivity of HRP2 and pLDH antigen detection assays. It complements the IS for P. falciparum Nucleic acid Amplification Technologies (NAT;2006)2 and reference reagent for serology (2014)3. Current plans are to develop a suite of WHO International standards for P. vivax (NAT, antigen and serology).
The importance, priorities and processes for development of International Standards for malaria will be discussed in the context of this recent P. falciparum antigen standard and other WHO malaria reference materials.