CYP2C9*38 is assigned decreased function based on limited evidence. Multiple in vitro studies found CYP2C9*38 demonstrated decreased clearance compared to wildtype for multiple substrates (24077631, 24118918, 25144335, 25951663, 25476996, 25832633, 25884291, 25924705, 23844998). Although two in vitro studies showed CYP2C9*38 had similar activity compared to wildtype, the experts did not find these results convincing as in these studies CYP2C9*2 which definitively has decreased function, demonstrated increased activity and similar activity compared to wildtype in these studies (25142737, 27163851). Additionally, the experts did not deem an in vitro study where CYP2C9*38 demonstrated activity ranging from decreased to increased compared to wildtype with 7-ethoxycoumarin, flavanone, and steroid substrates (28758225) as convincing evidence because it could not be ruled out that these results which differed from the majority of in vitro studies were due to the specific system used in the study. Therefore, consensus among experts was decreased function with an activity value of 0.5 based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal function alleles.