- As part of standard clinical care, if a pregnant patient has a positive 1-hour GCT at any point in her pregnancy (defined by plasma glucose level of 140 mg/dl or greater) they are to receive a 3-hour 100g oral glucose tolerance test (OGTT) at a later date. In this test, the patients are advised to avoid smoking or alcohol for 3 days preceding the test date, although they may continue with their regular diet during this period. The patient is instructed to fast over-night for 8 -10 hours prior to the test. A 2.5 ml blood sample is drawn for the fasting plasma glucose level, then a 100 g glucose solution is given to the patient orally. A 2.5 ml blood sample is then drawn each at 1, 2 and 3 hours. A diagnosis of diabetes mellitus is made if there are two or more abnormal plasma glucose values using the Carpenter and Coustan criteria (Fasting plasma glucose > 90 mg/dl, 1 hour > 180 mg/dl, 2 hours > 155 mg/dl and 3 hours > 140 mg/dl) 21. A random plasma glucose or 1-hour 50 g GCT> 200 mg/dl will also be considered as diagnostic of diabetes mellitus and will no require the 3-hours OGTT.