OBJECTIVE To compare screen-detected diabetes prevalence and the degree of diagnostic agreement by ethnicity with the current oral glucose tolerance test (OGTT)-based and newly proposed A1C-based diagnostic criteria. RESEARCH DESIGN AND METHODS Six studies (1999-2009) from Denmark, the U.K., Australia, Greenland, Kenya, and India were tested for the probability of an A1C > or =6.5% among diabetic case subjects based on an OGTT. The difference in probability between centers was analyzed by logistic regression adjusting for relevant confounders. RESULTS Diabetes prevalence was lower with the A1C-based diagnostic criteria in four of six studies. The probability of an A1C > or =6.5% among OGTT-diagnosed case subjects ranged widely (17.0-78.0%) by study center. Differences in diagnostic agreement between ethnic subgroups in the U.K. study were of the same magnitude as between-country comparisons. CONCLUSIONS A shift to an A1C-based diagnosis for diabetes will have substantially different consequences for diabetes prevalence across ethnic groups and populations.