OBJECTIVE: This study was designed to test the accuracy of capillary ketonemia for diagnosis of ketosis after interruption of insulin infusion. RESEARCH DESIGN AND METHODS: A total of 18 patients with type 1 diabetes treated by external pump were studied during pump stop for 5 h. Plasma and capillary ketonemia and ketonuria were determined every hour from 7:00 A.M. (time 0 min = T0) to 12:00 P.M. (time 300 min = T300). Plasma beta-hydroxybutyrate (beta-OHB) levels were measured by an enzymatic end point spectrophotometric method, and capillary beta-OHB levels were measured by an electrochemical method (MediSense Optium meter). Ketonuria was measured by a semiquantitative test (Ketodiastix). Positive ketosis was defined by a value of >/=0.5 mmol/l for ketonemia and >/=4 mmol/l (moderate) for ketonuria. RESULTS: After stopping the pump, concentrations of beta-OHB in both plasma and capillary blood increased significantly at time 60 min (T60) compared with T0 (P < 0.001), reaching maximum levels at T300 (1.30 +/- 0.49 and 1.23 +/- 0.78 mmol/l, respectively). Plasma and capillary beta-OHB values were highly correlated (r = 0.94, P < 0.0001). For diagnosis of ketosis, capillary ketonemia has a higher sensitivity and negative predictive value (80.4 and 82.5%, respectively) than ketonuria (63 and 71.8%, respectively). For plasma glucose levels >/=250 mg/dl, plasma and capillary ketonemia were found to be more frequently positive (85 and 78%, respectively) than ketonuria (59%) (P = 0.017). The time delay to diagnosis of ketosis was significantly higher for ketonuria than for plasma ketonemia (212 +/- 67 vs. 140 +/- 54 min, P = 0.0023), whereas no difference was noted between plasma and capillary ketonemia. CONCLUSIONS: The frequency of screening for ketosis and the efficiency of detection of ketosis definitely may be improved by the use of capillary blood ketone determination in clinical practice.