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 ≥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 ≥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.