Post treatment sensitivity studies with the parasight®-F test for malaria diagnosis in Zimbabwe

The post-treatment diagnostic performance of the Plasmodium falciparum histidine-rich protein (HRP-II) antigen detection test, ParaSight®-F test, was assessed on 55 falciparum malaria cases treated with chloroquine during in vivo drug sensitivity studies. The post-treatment sensitivity of the test remained high, except for an insignificant decline on day 1. However, specificity dropped sharply by day 1, subsequently increasing linearly with time to satisfactory values by day 10. As expected, from its inverse relationship to specificity, the false positive rate was high on day 1 and decreased linearly to low level by day 10. The temporary increase in false positive rate following treatment was due to persistent parasite antigen, rather than subpatent parasitaemia. Thus findings showed that positive readings by the test within 10 days post-treatment may occur in cured cases and will not necessarily imply treatment failure. Furthermore, it will be important to take patient antimalarial history into consideration during routine usage of the test for malaria diagnosis. The trend of Youdenʹs J-index for the ParaSight®-F test showed that from 10 days post-treatment, the test was generally reliable, with positive readings indicating active infection. It was concluded that the ParaSight®-F test was not only valuable at confirming malaria diagnosis on clinical cases in seasonal transmission areas, but had potential for application to detect recrudescent infections within 2 weeks of chloroquine treatment.