The American Diabetes Association (ADA) uses the formula **eAG (mg/dL) = 28.7 × A1C - 46.7**.

This is based on the ADAG (A1C-Derived Average Glucose) study of 507 patients with Type 1 and Type 2 diabetes. **Example conversions**: A1C 6.0% → eAG = 126 mg/dL (7.0 mmol/L), A1C 7.0% → eAG = 154 mg/dL (8.6 mmol/L), A1C 8.0% → eAG = 183 mg/dL (10.2 mmol/L), A1C 10.0% → eAG = 240 mg/dL (13.4 mmol/L). **To convert to mmol/L**: Divide mg/dL by 18.0. **Clinical significance**: Each 1% A1C increase = ~29 mg/dL higher average glucose. **Accuracy**: ±15-20 mg/dL individual variation due to red blood cell lifespan (90-120 days), hemoglobin glycation rate, and glucose measurement timing differences.

**ADA/IDF 2024 diagnostic criteria**: **Normal**: A1C <5.7% (eAG <117 mg/dL / 6.5 mmol/L)—healthy pancreatic function, low diabetes risk. **Prediabetes**: A1C 5.7-6.4% (eAG 117-137 mg/dL / 6.5-7.6 mmol/L)—5-10% annual Type 2 diabetes risk, reversible with lifestyle changes (7% weight loss + 150 min/week exercise reduces risk 58%). **Diabetes**: A1C ≥6.5% (eAG ≥137 mg/dL / 7.6 mmol/L)—confirmed on 2 separate tests, requires medication + lifestyle management. **Treatment targets** (individualized): General adult <7.0% (eAG <154 mg/dL), Pregnant <6.0% (eAG <126 mg/dL), Elderly/hypoglycemia risk 7.5-8.0%, Children <7.5%. **Complications threshold**: A1C >7.0% significantly increases microvascular risk (retinopathy, nephropathy, neuropathy), every 1% reduction lowers complications 25-40%.

A1C reflects **90-day average glucose** (red blood cell lifespan ~120 days, but 50% of A1C value comes from past 30 days). **Realistic reduction timeline**: **Aggressive management** (0.5-1.0%/month): 8.0% → 6.5% in **2-3 months** with strict diet (low-carb <50g/day), daily exercise (30-60 min), medication optimization (metformin + SGLT2i/GLP-1), CGM monitoring. **Moderate management** (0.3-0.5%/month): **3-5 months** with moderate carb restriction (100-150g/day), 3-4x/week exercise, standard medication. **Conservative** (0.2-0.3%/month): **5-8 months** with gradual lifestyle changes. **Example**: 8.0% → 7.5% (month 1) → 7.0% (month 2) → 6.5% (month 3). **Factors affecting speed**: Baseline insulin resistance, medication adherence, weight loss (5-10% bodyweight reduces A1C 0.5-1.0%), stress/sleep quality, comorbidities. **Caution**: Rapid A1C drops (>2% in 3 months) can temporarily worsen retinopathy—ADA recommends gradual reduction in long-standing diabetes.

A1C can be **inaccurate in specific conditions affecting red blood cells or hemoglobin**: **Falsely LOW A1C**: Iron deficiency anemia (shortened RBC lifespan, recent transfusion), hemolytic anemia (sickle cell, G6PD deficiency), chronic kidney disease Stage 4-5 (erythropoietin deficiency), pregnancy (increased RBC turnover), hemoglobin variants (HbS, HbC, HbE interfere with assay). **Falsely HIGH A1C**: Vitamin B12/folate deficiency (longer RBC lifespan), iron supplementation, uremia (carbamylation of hemoglobin), alcoholism, high-dose aspirin, vitamin C/E (glycation interference). **Alternative tests when A1C unreliable**: **Fructosamine** (reflects 2-3 week average glucose, unaffected by RBC lifespan, normal 200-285 μmol/L), **Glycated Albumin** (2-week average, better for CKD patients), **Continuous Glucose Monitoring (CGM)** (gold standard—14-day average glucose + time-in-range). **CKD-specific**: Use fructosamine or CGM if eGFR <30 mL/min. **Anemia**: Treat iron deficiency first (A1C can drop 0.5-1.0% after correction), then retest A1C 3 months post-treatment.

A1C represents 2-3 month average, so true month-to-month variation should be minimal (<0.3% in stable diabetes). **Legitimate A1C changes**: Medication adjustments (metformin/insulin titration), significant diet changes (keto/low-carb), weight loss/gain (5+ lbs), illness/infection (stress hyperglycemia), steroid use (prednisone increases glucose), seasonal variation (winter holidays, summer activity). **Measurement error/variation**: Lab assay differences (NGSP vs IFCC methods ±0.2%), POC (point-of-care) vs lab tests (±0.3%), hemoglobin variants interference, sample timing (fasting not required but consistency helps). **Red blood cell turnover**: Faster in prediabetes (3.5-month vs 3.0-month lifespan), slower in untreated B12 deficiency. **Glucose control patterns**: A1C 7.0% can mean stable 154 mg/dL all day OR dangerous swings (40-300 mg/dL averages to 154). **Best practice**: Check A1C every 3 months if not at goal (ADA guideline), every 6 months when stable, use CGM for daily glucose patterns (Time-in-Range 70-180 mg/dL >70% correlates with A1C <7%).

**Lab tests (gold standard)**: NGSP-certified labs (Quest, LabCorp) use HPLC or immunoassay—accuracy ±0.2%, detects hemoglobin variants, provides hemoglobin A1c + total hemoglobin.

Cost: $20-50 without insurance, covered 2-4x/year with diabetes diagnosis. **Home A1C kits** (CVS, Walgreens, A1CNow+): Fingerstick blood → immunoassay cartridge—accuracy ±0.5%, results in 5 minutes. **Pros**: Convenient, immediate feedback, quarterly monitoring. **Cons**: Higher error margin (can differ 0.3-0.8% from lab), no hemoglobin variant detection, more expensive ($25-40/test). **When to use home kits**: Stable diabetes (A1C <7.5% for >6 months), no anemia/kidney disease, quarterly monitoring between annual labs. **When to use lab**: Initial diagnosis (need 2 confirmatory tests), A1C >8.0% (medication changes), anemia/CKD/pregnancy, insurance coverage. **Validation strategy**: Run home kit and lab test simultaneously (initial use), if within 0.3% → reliable for home monitoring, if >0.5% difference → stick to lab. **Alternative**: Continuous Glucose Monitor (Dexcom, Libre)—real-time glucose + GMI (Glucose Management Indicator, CGM-derived A1C estimate, ±0.5%)—provides more actionable data than quarterly A1C alone.