A normal HbA1c does not rule out prediabetes. It rules out one phase of it. The earlier phase, where the pancreas is working overtime to keep blood sugar in range, is invisible to the A1c. The oral glucose tolerance test (OGTT), paired with fasting insulin, is how you see it.
This is the case for treating the metabolic panel as a three-legged stool, not a single number.
What HbA1c measures
HbA1c (glycated hemoglobin) reflects the average blood glucose concentration over the prior 90 to 120 days, which is the lifespan of a red blood cell. Glucose binds to hemoglobin in proportion to how much glucose is circulating, and the proportion of glycated hemoglobin in the blood gives an integrated read of glycemic exposure.
The reference framework:
- Under 5.7 percent: normal.
- 5.7 to 6.4 percent: prediabetes.
- 6.5 percent or higher: diabetes.
HbA1c is cheap, requires no fasting, and is reproducible across labs. It is the right screening test for most adults.
What HbA1c misses
The limitation is built into the mechanism. HbA1c only moves when glucose actually elevates. In the earliest phase of insulin resistance, the pancreas compensates by producing more insulin, and blood glucose stays in normal range. The HbA1c reads normal. The patient is metabolically sick.
This is the phase where lifestyle changes are most effective and most reversible. It is also the phase that the standard screening test misses.
Other limitations:
- Hemoglobinopathies (sickle trait, thalassemia) can produce falsely low or high A1c values.
- Iron deficiency anemia falsely elevates A1c.
- Recent blood loss falsely lowers A1c.
- Chronic kidney disease can distort the relationship between A1c and average glucose.
What the OGTT does differently
The oral glucose tolerance test is a stress test for glucose handling. You fast overnight, drink a 75-gram glucose solution, and have blood drawn at fasting, 1 hour, and 2 hours.
The 2-hour glucose value classifies metabolic status:
- Under 140 mg/dL: normal.
- 140 to 199 mg/dL: impaired glucose tolerance (prediabetes).
- 200 mg/dL or higher: diabetes.
The OGTT catches the patient whose pancreas keeps fasting glucose normal but cannot handle a real glucose load. That is the early insulin resistance phenotype.
Roughly 20 to 30 percent of adults with a normal HbA1c will have an abnormal OGTT. That is not a rare miss. That is a routine miss.
Fasting insulin: the third leg
The OGTT shows you what happens when glucose is challenged. Fasting insulin shows you what the pancreas is doing at rest. Together they describe the metabolic state with much more resolution than either does alone.
The reference framework for fasting insulin in adults:
- Optimal: under 6 microIU/mL.
- Acceptable: 6 to 10 microIU/mL.
- Elevated: 10 to 15 microIU/mL.
- High: over 15 microIU/mL.
Lab "normal" ranges often extend to 25 microIU/mL because the reference population includes large numbers of metabolically unwell adults. Population average is not the same as clinically optimal.
The HOMA-IR calculation combines fasting insulin and fasting glucose into a single insulin resistance score:
HOMA-IR = (fasting insulin × fasting glucose) ÷ 405
Under 1.0 is excellent. Over 2.0 is meaningful insulin resistance even when A1c and fasting glucose are normal.
When to order which test
The decision tree we use:
- HbA1c every year for screening. It is the right starting point for most men.
- Fasting insulin every year as a parallel marker. Low cost, high information.
- OGTT when the picture is mixed. A normal HbA1c with an elevated fasting insulin, a family history of diabetes, central adiposity, or a borderline A1c (5.5 to 5.9) earns the OGTT.
- OGTT after a GLP-1 taper. The drug masks glucose handling. The post-taper OGTT is the honest read.
The men we see most often who benefit from an OGTT are 38 to 48, normal-weight by BMI but with central adiposity, with a normal A1c and a fasting insulin in the 11 to 18 range. Half of that group has an abnormal OGTT. None of them would be flagged by a standard physical.
What changes when the OGTT is abnormal
The protocol moves earlier. The interventions are the same as for any insulin-resistant patient, but the threshold to act is lower:
- Resistance training, three to four sessions a week, prioritized over cardio for insulin sensitivity.
- Protein anchor at each meal, 30 to 50 grams. Smooths postprandial glucose meaningfully.
- Carbohydrate quality and timing. Refined carbohydrates on an empty stomach produce the biggest glycemic excursions.
- Sleep. Six hours of sleep produces measurable insulin resistance the next day.
- Visceral fat reduction. The single most effective lever for restoring insulin sensitivity.
- ApoB management. Insulin resistance increases atherogenic particle production. The lipid picture changes when the metabolic picture changes.
In selected cases, a GLP-1 agonist or metformin is added. The clinical bar is lower when the patient is 40 than when the patient is 60, because the runway for prevention is longer.
How Vane reads the metabolic panel
The annual Vane panel includes fasting insulin and HbA1c. The OGTT is added when the rest of the picture suggests it is needed, which is more often than most men expect.
The general principle: HbA1c tells you whether you have crossed the line into diabetes. Fasting insulin tells you whether you are walking toward it. The OGTT tells you whether you can still handle a glucose load. You need all three to write the protocol correctly.
The bottom line
A normal HbA1c is a starting point, not an all-clear. The men who catch insulin resistance early are the ones who never become the men who catch type 2 diabetes late. The cheapest way to catch it early is to look in the places the standard screen does not.
