An isoform of transferrin, carbohydrate-deficient transferrin (CDT) is increased in a high percentage of abusing alcoholics and has been found superior in its specificity compared with other biological markers. We used serum CDT as a screening parameter in 502 patients consecutively admitted to our medical department during a 4-week period. The intake of ethanol during the last 4 weeks was registrated by personal interviews and the mean daily consumption calculated. Serum CDT was measured at admission (CDTect) and compared with gamma-glutamyltranspeptidase (GGT), AST, ALT, and mean corpuscular volume (MCV). Serum CDT detected 18 of 26 (69%) patients who consumed > 50 g ethanol daily. The clinical sensitivity of CDT of detection ethanol consumption > 50 g daily was 69%, compared with 73%, 50%, 35%, and 52% for increased values of GGT, AST, ALT, and MCV, respectively. Altogether, 38 of 476 patients (8%) with a daily ethanol consumption < 50 g also had increased serum CDT levels. The specificity of CDT was 92%, compared with 75%, 82%, 86%, and 85% for GGT, AST, ALT, and MCV, respectively. In the 60 patients who consumed > 10 g ethanol daily, we found a significantly positive correlation between CDT and ethanol consumption (r = 0.52, p < 0.001). A positive correlation was also found between serum transferrin and CDT (r = 0.51, p < 0.001). In conclusion, the specificity of CDT is much higher compared with GGT in detecting alcohol abuse. Some acute and chronic illnesses may increase the serum level of CDT. False-positive CDT levels may be caused by changes in serum transferrin concentration.