Blood tests for diabetes diagnosis

One of three blood tests can be used to confirm a diagnosis of diabetes:^2-5

• Fasting plasma glucose (FPG) levels - a blood test after 8 hours of no eating
• Glycosylated hemoglobin (HbA1c) - to measure a marker of the average blood glucose level over the past 2-3 months
• Oral glucose tolerance testing (OGTT) - a test used less frequently that measures levels before and 2 hours after consuming a sweet drink (concentrated glucose solution).

Glycosylated hemoglobin is often abbreviated to A1C, and this blood test is also used in the monitoring of diabetes management.^2,5

To make an initial diagnosis, an HbA1c reading must be 6.5% or higher. An A1C result between 5.7% and 6.4% indicates prediabetes and a risk of type 2 diabetes.^2,5,6

The HbA1c is the preferred blood test for diagnosis because - while it is more expensive than the FPG test - it has advantages, including:⁵

Urine tests for diabetes were once common but are no longer considered reliable.

• Greater convenience (no need for fasting)
• Less day-to-day variation during stress and illness.

When the fasting plasma glucose test is used to confirm symptoms, diabetes is diagnosed at levels equal to or above 126 mg/dL (7.0 mmol/L).⁷

For oral glucose tolerance testing, the plasma glucose levels after 2 hours need to be equal to or above 200 mg/dL (11.1 mmol/L) for a diabetes diagnosis.⁷

Another blood test is the random plasma glucose test - taken regardless of time and eating - which diagnoses diabetes if the level is at least 200 mg/dL (11.1 mmol/L).⁷

Unless the clinical picture is clear, a positive blood test should also be repeated to rule out laboratory error.