Field evaluation of a rapid immunochromatographic test 'Parachek-F' in a post-monsoon Plasmodium falciparum malaria outbreak in villages of South India.

Abstract

An indigenous rapid immunochromatographic test Parachek-F for diagnosis of Plasmodium falciparum malaria was evaluated by the field staff in a post-monsoon P. falciparum malaria outbreak in villages of district Raichur, Karnataka, South India in November 1999. The test functions based on dipstick P. falciparum histidine rich protein-2 (PfHRP-2) antigen capture assay. Of the 232 uncomplicated clinically diagnosed malaria cases, 158 (68.1%) were positive for malaria by microscopy of JSB-stained thick blood smears. Of these, 13 were infected with P. vivax, 140 with P. falciparum and 5 had mixed infections of P. vivax and P. falciparum. Malaria patients were treated with age-specific oral doses of quinine followed by primaquine. Taking microscopy as gold standard, Parachek-F detected PfHRP-2 antigen in 136 samples (ratio 0.93) and was 93.1% sensitive and 98.8% specific. Positive predictive value, negative predictive value and efficacy were 99.2%, 89.6% and 95.2% respectively. No cross reactivity was observed with P. vivax infection. False negative interpretation was associated in 40% (10/25) lower-grade parasitaemias (parasitaemia <100/p/ blood) where sensitivity was only 60%. False positive result was associated in 1 case (1/74). Cases showing false negative results had taken presumptive treatment with chloroquine prior to the test. Careful microscopical examination on thin smears of such cases demonstrated that the morphology of the parasites was abnormal and distorted indicating the parasites were affected by chloroquine. The possible role of chloroquine resulting false negative results is suggested in this communication. Positive correlation between test bands intensity and parasite density was observed (r=0.137; P<0.05). The test is indigenously developed, rapid, simple in its application and was found suitable for field condition. Parameters like patients' conditions, history of drug intake, morphology of parasites at different developmental stages are to be considered for evaluation of such tests.