Blood sugar is one of the most consequential numbers in preventive medicine, and one of the most misread. Diabetes is common, prediabetes is more common still, and both raise cardiovascular risk long before symptoms appear. A handful of tests, understood clearly, can put you well ahead of the disease.
Most adults will encounter three measurements: fasting glucose, hemoglobin A1c, and the oral glucose tolerance test. Each answers a slightly different question, and the differences matter when a single result lands near a diagnostic threshold.
The three tests, translated
A fasting glucose is drawn after at least eight hours without calories. It captures how well your body maintains a baseline overnight, when insulin demand is low. A value under 100 mg/dL is normal, 100 to 125 is impaired fasting glucose, and 126 or higher on two occasions defines diabetes.
Hemoglobin A1c reflects the average amount of glucose stuck to red blood cells over roughly the prior three months. It requires no fasting, is convenient, and captures postprandial excursions that a single fasting draw misses. Below 5.7 percent is normal, 5.7 to 6.4 is prediabetes, and 6.5 or higher on two occasions defines diabetes.
An oral glucose tolerance test measures glucose two hours after a standardized 75-gram drink. It is the most sensitive of the three and is standard in pregnancy screening, but it is time-consuming and uncomfortable, so it is used selectively.
These tests do not always agree. A person can meet the A1c threshold for prediabetes with a normal fasting glucose, or the reverse. When results diverge, the one that crosses a threshold generally wins for screening purposes and prompts either confirmation or a broader evaluation.
Who should be tested, and how often
The USPSTF recommends screening for prediabetes and type 2 diabetes in adults aged 35 to 70 who are overweight or obese, and repeating every three years if normal. The American Diabetes Association casts a wider net, including anyone with additional risk factors such as a first-degree relative with diabetes, high-risk race or ethnicity, hypertension, low HDL or high triglycerides, polycystic ovary syndrome, physical inactivity, or a history of gestational diabetes.
If a screening result is abnormal, retesting sooner is reasonable. If you have prediabetes, an annual A1c is a common cadence.
What the numbers actually mean for risk
Diabetes is not a switch that flips at 6.5 percent. Risk of complications rises across the range, and prediabetes is itself a cardiovascular risk factor, not a warning of one to come.
- Roughly a third of adults with prediabetes progress to diabetes within five years without intervention.
- Structured lifestyle programs, including the CDC-recognized National Diabetes Prevention Program, cut that progression by more than half in randomized trials.
- Metformin reduces progression in high-risk groups, particularly younger adults with higher A1c or a history of gestational diabetes.
A single elevated A1c is a data point, not a verdict. What matters is the trend and what you do next.
Watch-outs when interpreting an A1c
A1c is a good test but not a perfect one. Conditions that shorten red blood cell lifespan, including hemolytic anemia, recent blood loss, and some hemoglobin variants, can produce a falsely low reading. Iron-deficiency anemia and chronic kidney disease can nudge it higher. Pregnancy alters red cell turnover and makes A1c less reliable, which is why glucose-based tests are used instead.
If your A1c seems inconsistent with symptoms or with home glucose readings, a fructosamine level or a repeat fasting test can help resolve the discrepancy.
What to do with a borderline result
A finding of prediabetes is one of the more actionable results in medicine. The evidence for the following is strong:
- Lose modest weight if overweight: 5 to 7 percent of body weight substantially cuts progression risk.
- Move more: at least 150 minutes a week of moderate activity, ideally with resistance work.
- Improve sleep: short and disrupted sleep worsen glucose regulation independent of diet.
- Cut ultra-processed carbohydrates and sweetened beverages first, before more ambitious dietary changes.
The bottom line
Blood sugar tests are cheap, widely available, and among the most decision-useful numbers in a preventive visit. If you are over 35, ask when you were last screened and what the numbers were. If you are in the prediabetes range, treat it as an opportunity, not a footnote.