This study included 190 lactating cows from eight dairy farms and was designed to investigate the sensitivity and specificity of a milk test (Ketolac) for detecting subclinical ketosis in dairy cattle. Blood and milk samples were collected between 38 and 50 days post partum. At a beta‐hydroxybutyric acid concentration in blood of 1.2 mmol/L, the prevalence of ketosis was 14%. A Response Operator Characteristic curve was drawn for two milk test values: 100 μmol/L and 200 μmol/L. Assessment of several beta‐hydroxybutyric acid cut‐off values in blood showed that when 100 μmol/L on the Ketolac milk test was defined positive, the most desirable sensitivity‐specificity combination was achieved. This milk test can be routinely used to screen a herd for subclinical ketosis.