The aim of this study was to assess the performance of the transferrin (Tf) index in screening heavy drinkers and to compare its performance to that of gamma glutamyl transpeptidase (GGT). Tf index, GGT, and transaminase activities were determined in a group of 173 subjects (49% males) recruited in a family doctor's practice (age: 54 +/- 1.5 yr; daily alcohol intake: 32 +/- 3.5 g; mean +/- SEM). Tf subfractions were quantified by isoelectric focusing and immunofixation on polyacrylamide gel. The Tf pl 5.7 to 5.4 ratio, or Tf index, was used as a marker of excessive drinking, with a cut-off point at 7%. Alcohol consumption was assessed through a face-to-face interview. Excessive drinkers were defined as those with a daily alcohol intake greater than 80 g over at least 2 years; 20 were classified as excessive drinkers (alcohol consumption: 92-232 g/day). All but four had normal transaminase activities indicating the low prevalence of hepatic impairments in this sample. The Tf index was found to have a sensitivity of 45%, specificity 89%, positive predictive value 35%, negative predictive value 92%. The corresponding results for GGT were 52%, 80%, 27%, and 92%, respectively. Concordance between the Tf index and GGT was assessed by the kappa coefficient (kappa) which was 0.22 indicating poor agreement between the two markers in selecting excessive drinkers (perfect association: kappa = 1, no association: kappa = 0).