Accurate tests are available to doctors to definitively confirm a diagnosis of diabetes.
Before tests are conducted, a diagnosis may be suspected when patients report certain symptoms. Doctors will evaluate these symptoms by asking questions about the patient’s medical history.
Doctors may also carry out a physical examination, including checks for complications that could have already developed – examining the feet for changes in sensation, for example.
Testing can be part of routine screening for people at risk of the disease, who may show up as having prediabetes. The US Department of Health and Human Services recommends diabetes testing for anyone overweight at the age of 45 years and over, alongside anyone under the age of 45 with one or more of the following risk factors:
- Hypertension (high blood pressure)
- High cholesterol
- History of diabetes in the family
- African-American, Asian-American, Latino/Hispanic-American, Native American or Pacific Islander background
- History of gestational diabetes (diabetes during pregnancy) or delivering a baby over 9 lbs.
Blood tests for diabetes diagnosis
Diagnoses of diabetes are confirmed through 1 of 3 types of blood test.
One of three blood tests can be used to confirm a diagnosis of diabetes:
- 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.
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:
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).
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).
Unless the clinical picture is clear, a positive blood test should also be repeated to rule out laboratory error.
Urine tests for diabetes diagnosis
Urine tests are no longer used to make a diagnosis of diabetes, although they were once common. Blood tests are used instead because urine tests are not sensitive or specific enough and offer only a crude indication of high blood sugar levels.
A urine sample may be used, however, to test for ketones, particularly in people with type 1 diabetes who exhibit certain symptoms. Here, the test can pick up ketoacidosis, a complication of diabetes.