CYP2C19*19 is assigned decreased function based on limited evidence. In vitro studies found CYP2C19*19 had decreased intrinsic clearance compared to wildtype with mephenytoin, omeprazole, and clopidogrel substrates (17455109, 21325430, 25001882). Although two in vitro studies found CYP2C19*19 demonstrated similar intrinsic clearance compared to wildtype with omeprazole and mephenytoin, the experts did not find these results convincing as CYP2C19*19 also demonstrated higher Km values compared to wildtype in these studies, suggesting CYP2C19*19 has decreased substrate affinity (18312490, 21325430). Additionally, decreased intrinsic clearance compared to wildtype with these two substrates was previously reported as mentioned above. Another study found CYP2C19*19 exhibited significantly reduced catalytic activities compared with CYP2C19*1 across multiple substrates (21692664).Therefore, consensus among experts was decreased function based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal function alleles.