Carbohydrate-deficient transferrin (CDT) is used as a serum marker for heavy drinking. We compared a new Bio-Rad %CDT TIA assay with the CDTect assay; we also compared both to gamma-glutamyltransferase (GGT) as markers of heavy drinking.Serum samples of well-defined alcoholics (n = 404) and matched (age, race, and gender) social drinkers (204) from 10 clinical centers were assayed with both CDT assays. Both assays use microcolumn separation after iron saturation, followed by enzyme immunoassay (CDTect) or turbidimetric immunoassay (Bio-Rad %CDT). In the latter, CDT is expressed as a percentage of total transferrin.The slope and intercept [95% confidence intervals (CIs)] for linear regression of results obtained by the %CDT-TIA (as percentage) and CDTect (units/L) assays were 0.091 (0.088-0.097) and 0.70% (0.54-0.86%), respectively (S(y/x) =1.30%; r = 0.848). The areas under the ROC curves (95% CIs) for CDTect and Bio-Rad %CDT TIA were 0.89 (0.86-0.92) and 0.88 (0.85-0.91), respectively, for men (P, not significant) and 0.76 (0.72-0.80) and 0.72 (0.68-0.76) for women (P, not significant). When CDT (CDTect or Bio-Rad %CDT) was combined with GGT (either one positive), the clinical sensitivity in men was 90% for both assays, and specificities were 81% and 84%, respectively; sensitivities in women were 75% and 76%, respectively, and specificities were 87% and 91%.The new Bio-Rad %CDT TIA assay compares favorably to the widely studied CDTect assay in the detection of alcohol-use disorders.