Diabetes Screening: Blood Tests You Need

Diabetes screening revolves around three core measurements, each providing a different window into how your body handles sugar:

• Hemoglobin A1c (HbA1c) — measures the percentage of hemoglobin with glucose attached, reflecting your average blood sugar over the past 2–3 months. It's the gold standard for diabetes screening because it doesn't require fasting and captures real-world glucose control, not just a single snapshot.
• Fasting Glucose — a point-in-time measurement of blood sugar after 8–12 hours without food. Part of the Comprehensive Metabolic Panel (CMP). Normal is below 100 mg/dL, prediabetes is 100–125 mg/dL, and diabetes is 126 mg/dL or above (confirmed on two separate occasions).
• Fasting Insulin — the most sensitive early indicator of metabolic dysfunction. Your pancreas can compensate for insulin resistance by producing more insulin, keeping glucose normal for years. A fasting insulin above 10 µIU/mL suggests your body is working harder than it should; above 15 µIU/mL indicates significant insulin resistance.

The key insight: glucose-based tests (A1c and fasting glucose) detect problems after the pancreas has begun to fail. Fasting insulin detects the problem while compensation is still occurring — often 5–10 years before a diabetes diagnosis. This is why we include fasting insulin in our Diabetes Screening Panel.

The CMP also provides kidney function markers (creatinine, BUN, eGFR) that are important because the kidneys are one of the first organs damaged by chronically elevated blood sugar.

The Hemoglobin A1c test is expressed as a percentage and maps to an estimated average glucose (eAG):

• Below 5.7% (eAG below ~117 mg/dL) — Normal. Your blood sugar regulation is healthy.
• 5.7–6.4% (eAG 117–137 mg/dL) — Prediabetes. You have impaired glucose regulation but can reverse it with lifestyle changes. The risk of progressing to Type 2 diabetes is approximately 5–10% per year without intervention.
• 6.5% or above (eAG 140+ mg/dL) — Diabetes. This should be confirmed with a second A1c test or a fasting glucose ≥126 mg/dL.

Important nuances: A1c can be falsely low in people with hemoglobin variants (sickle cell trait, thalassemia), iron deficiency anemia, or significant blood loss/transfusion. In these cases, fasting glucose and fructosamine are better alternatives. A1c can be falsely high in iron deficiency or kidney disease.

Even within the "normal" range, trends matter. An A1c that has risen from 5.0% to 5.5% over two years signals worsening insulin resistance even though both values are technically normal. This is why longitudinal tracking is so valuable.

For people already diagnosed with diabetes, the ADA generally targets an A1c below 7.0% (eAG ~154 mg/dL), though individual goals vary based on age, complications, and hypoglycemia risk.

Approximately 98 million American adults — 1 in 3 — have prediabetes, and more than 80% don't know it. Prediabetes is not a benign condition: it carries increased risk of heart disease, stroke, kidney disease, and neuropathy even before progressing to full diabetes.

The good news: prediabetes is reversible. The landmark Diabetes Prevention Program (DPP) trial showed that lifestyle intervention (150 minutes per week of moderate exercise + 7% weight loss) reduced progression to Type 2 diabetes by 58% — more effective than metformin (31% reduction). These results have been replicated globally.

What lifestyle changes work?

• Weight loss: Even 5–7% of body weight (e.g., 10–14 lbs for a 200 lb person) significantly improves insulin sensitivity.
• Exercise: 150 minutes per week of brisk walking or equivalent. Both aerobic exercise and resistance training improve glucose uptake independently.
• Diet: Reducing refined carbohydrates and added sugars, increasing fiber (aim for 25–30g/day), and emphasizing whole foods over processed foods.
• Sleep: Short sleep duration (below 6 hours) and poor sleep quality independently worsen insulin resistance.

Blood work is your feedback mechanism. By retesting A1c and fasting insulin every 3–6 months, you can objectively measure whether your interventions are working. An A1c drop from 6.1% to 5.5% is concrete proof that you've improved your metabolic health.

The American Diabetes Association recommends screening for all adults beginning at age 35, and earlier if you have risk factors. Screen sooner (starting at age 25 or even younger) if you have:

• BMI ≥25 (≥23 for Asian Americans) with one or more additional risk factors
• First-degree relative with Type 2 diabetes
• History of gestational diabetes
• Polycystic Ovary Syndrome (PCOS)
• History of cardiovascular disease
• HDL below 35 mg/dL or triglycerides above 250 mg/dL
• Physical inactivity
• Belonging to a high-risk ethnic group (African American, Latino, Native American, Asian American, Pacific Islander)

Testing frequency:

• Normal results: Rescreen every 3 years
• Prediabetes: Retest A1c and fasting insulin every 3–6 months to track response to lifestyle changes
• High-risk with normal results: Annual screening
• On metformin for prediabetes: A1c every 6 months

Preparation: A 10–12 hour overnight fast is required for accurate fasting glucose and fasting insulin results. A1c does not require fasting. Water is fine during the fasting period. Avoid alcohol for 24 hours before the test.