DIAGNOSIS OF RESISTANCE TO CHLOROQUINE BY PLASMODIUM-VIVAX - TIMING OF RECURRENCE AND WHOLE-BLOOD CHLOROQUINE LEVELS

To develop criteria for the diagnosis of resistance to chloroquine usi ng an in vivo test, we examined published records of early clinical tr ials of quinine and chloroquine against Plasmodium vivax. These data e stablished the timing of relapse by tropical P. vivax relative to ther apy by these drugs. The first relapse occurred 17 days after initiatin g and three days after terminating quinine therapy. The median day of relapse was day 23, and 59% of the patients had relapsed by day 30 (n = 333). In contrast, no relapse occurred until day 36 following chloro quine treatment (n = 256). Data from our laboratory may help explain t his difference; among 91 Indonesian patients with malaria, the mean wh ole blood levels of chloroquine (CQ) and desethylchloroquine (DCQ) wer e 141 ng/ml (95% confidence interval = 115-167) on day 28 after initia ting standard therapy (25 mg base/kg in three doses over a 48-hr perio d). This exceeds the estimated minimal effective concentration of chlo roquine (100 ng/ml of whole blood). Thus, chloroquine lingering in the blood for at least 28 days after starting standard therapy was suffic ient to eliminate or at least suppress chloroquine-sensitive tropical P. vivax. We conclude that a parasitemia by P. vivax recurring in the 28 days after full compliance to standard chloroquine therapy demonstr ates resistance. If the recurrence appears before day 16, it is almost certainly a recrudescence and between days 17 and 28 it may be either a recrudescence or a relapse by chloroquine-resistant parasites. Recu rrences beyond day 28 could be relapse by chloroquine-sensitive P. viv ax.