Step by Step
1
Fasting plasma glucose
Normal is below 100 mg/dL. Prediabetes is 100-125 mg/dL. Diabetes is 126 mg/dL or above, confirmed on two separate occasions. A random glucose of 200 mg/dL or above, combined with symptoms, is also diagnostic of diabetes.
2
2-hour oral glucose tolerance test (OGTT)
Normal is below 140 mg/dL. Prediabetes is 140-199 mg/dL. Diabetes is 200 mg/dL or above.
3
HbA1c — the 3-month average
HbA1c (glycated hemoglobin) reflects average blood glucose over about 3 months, corresponding to the lifespan of red blood cells. Normal is below 5.7%. Prediabetes is 5.7-6.4%. Diabetes is 6.5% or above. The target for diabetic patients (per the American Diabetes Association) is below 7%. As a useful conversion, each 1% change in HbA1c corresponds to roughly a 28 mg/dL change in average glucose.
4
Insulin and C-peptide — distinguishing diabetes types
Elevated insulin and elevated C-peptide together suggest Type 2 diabetes or an insulinoma. Decreased levels of both suggest Type 1 diabetes (no insulin production at all). Elevated insulin combined with decreased C-peptide suggests exogenous insulin injection (since injected insulin doesn't come with the C-peptide byproduct that the body's own insulin production creates) — a pattern sometimes seen in factitious hypoglycemia.
Applied Walkthrough
1
A patient's fasting glucose returns at 130 mg/dL on two separate occasions, confirming a diagnosis of diabetes (since this exceeds the 126 mg/dL threshold on repeated testing).
2
This same patient's HbA1c comes back at 8%, reflecting an average blood glucose over the past 3 months well above target — using the rough conversion (each 1% ≈ 28 mg/dL), this corresponds to an average glucose considerably higher than the diabetic treatment target of under 7% HbA1c.
3
To determine which type of diabetes this patient has, insulin and C-peptide levels are checked: both come back low, consistent with Type 1 diabetes, where the pancreas simply isn't producing insulin.
4
In a separate, unusual case, a patient with hypoglycemia shows elevated insulin but LOW C-peptide — this specific mismatch points toward exogenous insulin administration (injected insulin) rather than the body's own overproduction, since injected insulin doesn't come with the C-peptide byproduct naturally produced alongside endogenous insulin.
Exam Application
Exams test whether you know the diagnostic thresholds for fasting glucose, OGTT, and HbA1c, and whether you can use insulin and C-peptide levels together to distinguish Type 1 diabetes, Type 2 diabetes/insulinoma, and exogenous insulin administration.
⚠ Common Trap
The most common trap is forgetting that insulin and C-peptide must be interpreted TOGETHER, not separately — elevated insulin alone doesn't distinguish between the body's own overproduction (Type 2/insulinoma, where C-peptide would also be elevated) and injected insulin (where C-peptide would be low, since it's a byproduct only of endogenous insulin production).
✓ Quick Self-Check
1. What fasting glucose level confirms diabetes, and how many times must it be measured?
126 mg/dL or above, confirmed on two separate occasions.
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2. What HbA1c range indicates prediabetes, and what range indicates diabetes?
5.7-6.4% is prediabetes; 6.5% or above is diabetes.
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3. What does HbA1c actually reflect, and why does it correspond to a 3-month timeframe?
Average blood glucose; the 3-month timeframe corresponds to the lifespan of red blood cells.
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4. What insulin/C-peptide pattern suggests Type 1 diabetes?
Both decreased (low insulin, low C-peptide).
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5. What insulin/C-peptide pattern suggests exogenous insulin injection?
Elevated insulin combined with decreased C-peptide.
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