Diabetes is diagnosed through blood tests that measure how much sugar is in your blood, either at a single point in time or averaged over several months. Four main tests are used, each with specific cutoff numbers that separate normal blood sugar from prediabetes and diabetes. In most cases, an abnormal result needs to be confirmed with a second test before a formal diagnosis is made.
The Four Diagnostic Tests
Any of the following tests can be used to diagnose diabetes, and each measures blood sugar in a slightly different way.
A1C (hemoglobin A1C). This blood test reflects your average blood sugar over the past two to three months. It works by measuring the percentage of a protein in your red blood cells that has sugar attached to it. An A1C of 6.5% or higher means diabetes. The prediabetes range is 5.7% to 6.4%. You don’t need to fast before this test, and it can be drawn at any time of day, which makes it one of the most convenient options.
Fasting plasma glucose (FPG). This test measures your blood sugar after you’ve had nothing to eat or drink (except water) for at least 8 hours. It’s typically done first thing in the morning. A result of 126 mg/dL or higher indicates diabetes. The prediabetes range is 100 to 125 mg/dL.
Oral glucose tolerance test (OGTT). This is a two-part test. You fast for at least 8 hours, then drink a solution containing 75 grams of glucose. Your blood is drawn two hours later. A two-hour result of 200 mg/dL or higher means diabetes. This test is more time-consuming but is especially useful for detecting diabetes that a fasting test might miss.
Random plasma glucose. If you’re already showing classic symptoms of high blood sugar, like excessive thirst, frequent urination, or unexplained weight loss, a blood sugar reading of 200 mg/dL or higher at any time of day is enough for a diagnosis. No fasting is required, and no confirmatory test is needed when symptoms are present.
Why You Usually Need Two Abnormal Results
Unless you have obvious symptoms of high blood sugar along with a random glucose of 200 mg/dL or above, a single abnormal test result isn’t enough for a diagnosis. You’ll need either two abnormal results from different types of tests (for example, an elevated A1C and an elevated fasting glucose, which can be drawn on the same day) or the same test repeated on a separate occasion. This requirement exists because blood sugar can fluctuate from factors like stress, illness, or medication, and a single reading might not reflect your true baseline.
If two different tests give conflicting results, say your A1C comes back at 6.6% but your fasting glucose is normal, the test that came back abnormal is typically repeated. If the repeat confirms the abnormality, the diagnosis stands.
How Type 1 and Type 2 Are Told Apart
The tests above confirm that diabetes is present, but they don’t tell you which type you have. That distinction matters because the two types have different causes and different treatment paths. Type 1 is an autoimmune condition where the immune system destroys the cells in the pancreas that make insulin. Type 2 involves insulin resistance, where your body still produces insulin but can’t use it effectively.
To check for type 1, doctors test for specific antibodies in your blood that signal an autoimmune attack on the pancreas. There are four major targets these antibodies go after: insulin itself, an enzyme called GAD65, a protein called IA-2, and a zinc transporter called ZnT8. About 94% of people with type 1 diabetes have antibodies against at least one of these at the time of diagnosis. If autoantibody tests come back negative, type 2 is far more likely.
Another useful test is the C-peptide test. When your pancreas makes insulin, it releases a byproduct called C-peptide in equal amounts. Measuring C-peptide tells your doctor how much insulin your body is still producing on its own. People with type 1 diabetes typically have very low C-peptide levels (below 0.2 nmol/L), reflecting near-total loss of insulin production. People with type 2 generally have normal or even elevated levels, since their pancreas is still working but their body isn’t responding well to the insulin it makes. C-peptide values above 0.6 nmol/L point toward type 2.
Latent Autoimmune Diabetes in Adults
Some adults are initially diagnosed with type 2 diabetes but actually have a slow-developing form of autoimmune diabetes sometimes called LADA. These individuals test positive for autoantibodies like people with type 1, but they may still produce meaningful amounts of insulin for months or years. C-peptide testing helps guide treatment in these cases. Those with C-peptide levels above 0.7 nmol/L can often be managed similarly to type 2 diabetes at first, while those with levels below 0.3 nmol/L typically need insulin right away.
Screening During Pregnancy
Gestational diabetes, which develops during pregnancy in women who didn’t previously have diabetes, is screened for separately. Most pregnant women are tested between 24 and 28 weeks of gestation. The most common approach in the U.S. is a two-step process: first a glucose challenge test where you drink a sugary solution without fasting, and if that result is elevated, you return for a full oral glucose tolerance test. Some providers use a one-step approach with a 75-gram OGTT instead. The specific cutoff numbers differ from those used outside of pregnancy.
Preparing for Your Test
What you need to do before testing depends on which test your doctor orders. The A1C test requires no preparation at all. You can eat, drink, and go about your day normally before the blood draw. The fasting plasma glucose and oral glucose tolerance tests both require fasting for at least 8 hours beforehand. Water is fine, but skip food, coffee, and anything else with calories. These tests are usually scheduled in the morning so most of your fasting happens overnight.
If you’re taking the oral glucose tolerance test, expect to be at the lab for about two and a half hours total. You’ll have your fasting blood drawn, drink the glucose solution (which is very sweet and can cause mild nausea), then sit and wait for the two-hour mark before your second blood draw. Bring something to read or do, since physical activity during the wait can affect your results.
What the Numbers Mean at a Glance
- Normal A1C: below 5.7%
- Prediabetes A1C: 5.7% to 6.4%
- Diabetes A1C: 6.5% or higher
- Normal fasting glucose: below 100 mg/dL
- Prediabetes fasting glucose: 100 to 125 mg/dL
- Diabetes fasting glucose: 126 mg/dL or higher
- Diabetes (OGTT, 2-hour): 200 mg/dL or higher
- Diabetes (random glucose with symptoms): 200 mg/dL or higher
Certain conditions can affect the accuracy of specific tests. The A1C can give misleading results if you have certain blood disorders, significant blood loss, or are pregnant. Iron deficiency can falsely raise A1C readings. In these situations, your doctor will rely on glucose-based tests instead. Similarly, medications like corticosteroids can temporarily raise blood sugar, so your provider may want to retest once you’ve finished a course of treatment.