A fasting blood sugar level of 126 mg/dL (7.0 mmol/L) or higher indicates diabetes. That’s the most commonly used threshold, but it’s not the only one. Doctors use several different blood tests to diagnose diabetes, each with its own cutoff numbers, and the results also reveal whether you’re in the normal range or in the prediabetes zone between normal and diabetic.
Fasting Blood Sugar Levels
The fasting plasma glucose test is one of the most straightforward ways to check for diabetes. You fast for at least eight hours (water is fine), then have your blood drawn. Here’s how the numbers break down:
- Normal: below 100 mg/dL (5.6 mmol/L)
- Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes: 126 mg/dL (7.0 mmol/L) or higher
A single high reading isn’t usually enough for a diagnosis. The test typically needs to be repeated on a separate day to confirm the result, unless your level is very high or you already have classic symptoms like excessive thirst, frequent urination, and unexplained weight loss.
The A1C Test
The A1C test takes a different approach. Instead of capturing your blood sugar at one moment, it measures your average blood sugar over the past two to three months. It works by looking at how much sugar has attached to your red blood cells. The result is expressed as a percentage rather than mg/dL:
- Normal: below 5.7%
- Prediabetes: 5.7% to 6.4%
- Diabetes: 6.5% or higher
Within the prediabetes range, the higher your A1C, the greater your risk of progressing to diabetes. Someone at 6.3% is at significantly higher risk than someone at 5.8%, even though both fall in the same category. The A1C test doesn’t require fasting, which makes it convenient, and it’s less affected by day-to-day fluctuations in what you eat or how stressed you are.
Oral Glucose Tolerance Test
The oral glucose tolerance test measures how well your body handles a sugar load. After fasting overnight, you drink a sugary solution, then have your blood drawn two hours later. The two-hour reading is what determines your diagnosis:
- Normal: below 140 mg/dL (7.8 mmol/L)
- Prediabetes (impaired glucose tolerance): 140 to 199 mg/dL (7.8 to 11.0 mmol/L)
- Diabetes: 200 mg/dL (11.1 mmol/L) or higher
This test is particularly good at catching people whose fasting numbers look fine but whose bodies struggle to clear sugar after eating. It’s less commonly used than fasting glucose or A1C testing because it takes longer and requires you to sit in a clinic for at least two hours.
Random Blood Sugar Test
A random blood sugar test can be taken at any time of day, regardless of when you last ate. A result of 200 mg/dL (11.1 mmol/L) or higher, combined with classic symptoms of high blood sugar, is enough to diagnose diabetes. This test is typically used when someone shows up with obvious symptoms and the doctor needs a quick answer rather than scheduling a fasting test.
When the random result is very high or symptoms are severe, a second confirmatory test on another day may not be required.
Gestational Diabetes Thresholds
Pregnancy uses a different set of numbers because blood sugar control needs to be tighter to protect the developing baby. Screening usually happens between weeks 24 and 28 of pregnancy.
In the most common approach, you first take a one-hour screening test. If that result is elevated, you return for a three-hour glucose tolerance test. Gestational diabetes is diagnosed if your results exceed any of these cutoffs during the three-hour test:
- Fasting: 95 mg/dL (5.3 mmol/L) or higher
- One hour: 180 mg/dL (10.0 mmol/L) or higher
- Two hours: 155 mg/dL (8.6 mmol/L) or higher
- Three hours: 140 mg/dL (7.8 mmol/L) or higher
Some clinics use a standalone two-hour test instead, with slightly different thresholds: a fasting level of 92 mg/dL or higher, a one-hour level of 180 mg/dL or higher, or a two-hour level of 153 mg/dL or higher. Meeting just one of these values is enough for a gestational diabetes diagnosis.
What Prediabetes Numbers Mean
Prediabetes is the range where your blood sugar is higher than normal but hasn’t crossed into diabetes territory. It shows up as a fasting glucose between 100 and 125 mg/dL, an A1C between 5.7% and 6.4%, or a two-hour glucose tolerance result between 140 and 199 mg/dL. Not everyone with prediabetes will develop diabetes. Losing 5% to 7% of body weight and getting regular physical activity can significantly lower the risk of progression.
It’s worth noting that these tests don’t always agree. You might have a normal fasting glucose but an A1C in the prediabetes range, or vice versa. That’s because each test measures something slightly different: a snapshot versus an average, or how you process sugar at rest versus after eating.
Target Levels After Diagnosis
Once someone is diagnosed with diabetes, the goal shifts from diagnosis to management. The targets for day-to-day blood sugar control are different from the diagnostic thresholds. The general recommended ranges are:
- Before meals: 80 to 130 mg/dL
- Two hours after starting a meal: below 180 mg/dL
These are starting points. Your individual targets may be different depending on your age, how long you’ve had diabetes, whether you have other health conditions, and how prone you are to low blood sugar episodes. Older adults or people with heart disease, for example, often have slightly higher targets to reduce the risk of blood sugar dropping too low.
Converting Between Units
Blood sugar is measured in mg/dL in the United States and in mmol/L in most other countries. If you need to convert between the two, divide mg/dL by 18 to get mmol/L, or multiply mmol/L by 18 to get mg/dL. So a reading of 126 mg/dL equals 7.0 mmol/L, and a reading of 200 mg/dL equals 11.1 mmol/L. If your meter or lab report uses one unit and your doctor’s instructions use the other, this simple conversion keeps you on the same page.