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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Diagnosis

Type 1 diabetes is diagnosed by a combination of symptoms, a person's age and blood tests. The blood tests include tests for sugar levels and for other substances.

Fasting plasma glucose (FPG) test. Blood is taken in the morning after fasting overnight. Normally, blood sugar levels remain between 70 and 100 milligrams per deciliter (mg/dL). Diabetes is diagnosed if a fasting blood sugar level is 126 mg/dL or higher.

Oral glucose tolerance test (OGTT). Blood sugar is measured two hours after drinking 75 grams of glucose. Diabetes is diagnosed if the 2-hour blood sugar level is 200 mg/dL or higher.

Random blood glucose test. A blood sugar of 200 mg/dL or greater at any time of day combined with symptoms of diabetes is sufficient to make the diagnosis.

Hemoglobin A1C (glycohemoglobin). This test measures the average glucose level over the prior two to three months. Diabetes is diagnosed if the hemoglobin A1C level is 6.5% percent or higher.

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