Several blood tests can diagnose diabetes. Fasting plasma glucose measures sugar after not eating overnight. HbA1c reflects average glucose over two to three months. An oral glucose tolerance test checks levels before and after a sugary drink. A random glucose test may be used when symptoms are clearly present.
Clinicians usually require results in the diabetic range, and often confirm with a repeat test on another day unless symptoms are obvious, to avoid misdiagnosis. The type of diabetes is then determined from your age, history, symptoms, and sometimes additional tests such as antibodies.
Home meters and over-the-counter kits cannot formally diagnose diabetes, although high home readings are a good reason to seek testing. If you have symptoms or risk factors, ask your doctor which test is right for you.