The Plasmodium falciparum chloroquine in vivo test: extended follow-up is more important than parasite counting.

349 in vivo tests of the susceptibility of Plasmodium falciparum to chloroquine, 25 mg/kg, were analysed. In some surveys, standard in vitro tests were also carried out. The proportions of sensitive and resistant infections in different areas found by the 2 methods were similar, but, within a given area, correlation between the two methods was often poor. Two RI cases and one RII/RIII case were sensitive in vitro, and it is suggested that the extended in vivo test may sometimes be more sensitive than the in vitro test, and that even in endemic areas, where reinfection is possible, patency on day 14 will nearly always be due to resistance. Parasite density data were analysed by calculating the geometric mean of each day's parasite density as a percentage of the day 0 parasite density + 0.1. Most resistant and sensitive infections attained minimal values on day 4, and it is proposed that assessment of sensitivity based on parasite densities should use day 4 values. Contrasts between materials were more clearly defined statistically when comparisons were based on ranking in vivo test classifications, than when based on day 4 parasitaemia. It is therefore suggested that, for epidemiological purposes, extension of tests to at least 14 d is more important than parasite counting. Parasitaemia above 20-25% of the day 0 value on day 2 in a severely ill patient, or persistent patency on day 4 in a symptomatic patient, are both indications for a change of treatment.

Plasmodium falciparum in vitro schizont maturation tests in Mozambique are not improved by removing immune parasite carriers' plasma.

To study the effect of immune parasite carriers' plasma on Plasmodium falciparum schizont maturation, peripheral blood stages were incubated for 24-40 h in RPMI medium with either 5% carrier's plasma + 5% non-immune AB serum or 10% non-immune serum. The number of schizonts per 200 asexual P. falciparum was lower in non-immune serum than in the presence of carrier's plasma in 19 of 26 cases, due to increased frequency of schizont rupture when carrier's plasma was absent. It is concluded that, under these test conditions, the replacement of immune plasma by non-immune serum makes schizont maturation tests, which are based on the proportion of schizonts among asexual P. falciparum as a measure of growth, more difficult to interpret.