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Why Is There a 12-Hour Fast for a Blood & Glucose Test?

Oral glucose tolerance testing measures how the body metabolizes blood sugar. For accurate results, the patient must fast for 12 hours before testing. The glucose test is commonly used for diagnosing diabetes or gestational diabetes.

  1. Fasting

    • Fasting allows the test to measure a baseline glucose level. This measures your glucose levels without any food affecting your blood sugar. Your blood and urine will be collected to calculate these levels.

    Glucola

    • After the establishment of a glucose baseline, you will drink 75 g of glucose (called glucola) or 100 g if you are pregnant. Tests will monitor your glucose levels every 30 to 60 minutes for three hours.

    Results

    • Abnormal oral glucose tolerance test results indicate that you may have diabetes or gestational diabetes. However, other medical conditions can cause abnormal blood sugar, such as Cushing’s syndrome or cystic fibrosis.

    Warning

    • The glucola may make you feel sweaty, nauseous or faint because during the test you ingest a significant amount of glucose on an empty stomach. This causes your blood sugar to change rapidly.

    Preparation

    • Eat a normal, balanced diet for at least three days before the glucose test. Do not drink anything except water 12 hours before testing. Check with your doctor to make sure none of your medications will interfere with the results.

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