Kesinee ChotivanichRupam TripuraDebashish DasPoravuth YiNicholas P J DaySasithon PukrittayakameeChar Meng ChuorDuong SocheatArjen M. DondorpNicholas J. WhiteMahidol UniversityNational Center for Parasitology, Entomology and Malaria ControlChurchill Hospital2018-11-092018-11-092014-01-01Antimicrobial Agents and Chemotherapy. Vol.58, No.6 (2014), 3157-316110986596006648042-s2.0-84901283343https://repository.li.mahidol.ac.th/handle/20.500.14594/34729Conventional 48-h in vitro susceptibility tests have low sensitivity in identifying artemisinin-resistant Plasmodium falciparum, defined phenotypically by low in vivo parasite clearance rates. We hypothesized originally that this discrepancy was explained by a loss of ring-stage susceptibility and so developed a simple field-Adapted 24-h trophozoite maturation inhibition (TMI) assay focusing on the ring stage and compared it to the standard 48-h schizont maturation inhibition (WHO) test. In Pailin, western Cambodia, where artemisinin-resistant P. falciparum is prevalent, the TMI test mean (95% confidence interval) 50% inhibitory concentration (IC50) for artesunate was 6.8 (5.2 to 8.3) ng/ml compared with 1.5 (1.2 to 1.8) ng/ml for the standard 48-hWHO test (P=0.001). TMI IC50s correlated significantly with the in vivo responses to artesunate (parasite clearance time [r=0.44, P=0.001] and parasite clearance half-life [r=0.46, P=0.001]), whereas the standard 48-h test values did not. On continuous culture of two resistant isolates, the artemisinin-resistant phenotype was lost after 6 weeks (IC50s fell from 10 and 12 ng/ml to 2.7 and 3 ng/ml, respectively). Slow parasite clearance in falciparum malaria in western Cambodia results from reduced ring-stage susceptibility. © 2014, American Society for Microbiology.Mahidol UniversityMedicinePharmacology, Toxicology and PharmaceuticsArticleSCOPUS10.1128/AAC.01924-13