OBJECTIVE: To compare albumin-to-creatinine ratio (ACR) and albumin concentration (ALB) for detection of microalbuminuria using precise data on CIs for sensitivity and specificity and to determine sex- and age-specific discriminator values for ACR. RESEARCH DESIGN AND METHODS: All consecutive patients investigated for the presence of microalbuminuria with timed overnight urine collection were included in the study. Results from 1.171 men and 1,223 women were eligible for evaluation. Albumin excretion rate (the reference procedure), ALB, and ACR were compared by receiver operating characteristic (ROC) curve analysis. Age- and sex-specific discriminator values were established. Sensitivity and specificity were determined, and their CIs were compared with those calculated from data presented in the literature. RESULTS: Sensitivity and specificity were 89% (range, 85-92) and 90% (88-92), respectively, for women, and 90% (87-93) and 89% (86-91), respectively, for men, at a discriminator value of 15 mg/l for the ALB. This discriminator value did not show any relationship to sex or age. For the ACR, which depends on sex, the sensitivity and specificity were 94% (92-96) and 92% (91-94), respectively, at 2.5 g/mol for women and 94% (92-97) and 93% (91-95), respectively, at 1.8 g/mol for men. The discriminator value for the ACR appears to depend on age in both men and women. CONCLUSIONS: ROC curve analysis clearly shows that the ACR performs better than the ALB in screening for microalbuminuria. However, in contrast to the ALB, the ACR needs sex- and age-specific discriminator values.