A 62-year-old woman with heart failure and preserved ejection fraction (HFpEF) is prescribed a thiazide diuretic for hypertension and fluid management. After 3 weeks, she reports polyuria, polydipsia, and fatigue. Fasting blood glucose is 180 mg/dL (previously 95 mg/dL). Which investigation would be most appropriate to confirm the metabolic complication and assess its severity?

Explanation

## Thiazide-Induced Hyperglycemia: Investigation of Choice ### Clinical Context Thiazide diuretics impair insulin secretion and increase insulin resistance, leading to hyperglycemia or unmasking latent diabetes. The patient's acute rise in fasting glucose (95 → 180 mg/dL) with polyuria and polydipsia suggests new-onset hyperglycemia. Comprehensive glucose assessment is needed to confirm and characterize the metabolic complication. ### Why HbA1c + Fasting Glucose + OGTT is the Best Choice **Key Point:** The combination of fasting blood glucose, HbA1c, and oral glucose tolerance test (OGTT) is the diagnostic gold standard for confirming thiazide-induced hyperglycemia and classifying it as impaired fasting glucose (IFG), impaired glucose tolerance (IGT), or frank diabetes mellitus. **High-Yield:** Thiazide-induced hyperglycemia typically presents as: - **Fasting glucose 100–125 mg/dL** → IFG - **2-hour OGTT glucose 140–199 mg/dL** → IGT - **Fasting glucose ≥126 mg/dL or 2-hour OGTT ≥200 mg/dL** → Diabetes **HbA1c** reflects average glucose over 2–3 months and helps distinguish acute stress hyperglycemia from true glucose dysregulation. ### Why This Matters Clinically **Clinical Pearl:** Thiazide-induced hyperglycemia is dose-dependent and often reversible with dose reduction or switch to alternative diuretic (e.g., potassium-sparing diuretic or loop diuretic). OGTT is essential because some patients have normal fasting glucose but abnormal postprandial glucose (IGT), which is missed by fasting glucose alone. ### Comparison of Investigations | Investigation | Utility | Limitation | |---|---|---| | **HbA1c + Fasting glucose + OGTT** | Comprehensive glucose assessment; classifies degree of dysregulation; gold standard for diagnosis | OGTT is time-consuming; HbA1c may be falsely low in hemolysis or anemia | | Serum osmolality & urine osmolality | Assesses for hyperosmolar hyperglycemic state; useful if severe | Does not diagnose hyperglycemia; used only if osmolality is suspected abnormal | | Serum Na+ & K+ | Identifies electrolyte abnormalities from diuretic | Does not assess glucose metabolism; thiazides cause hypokalemia and hyponatremia, not hyperglycemia | | Urine dipstick | Detects glycosuria (glucose >180 mg/dL) | Non-specific; does not quantify glucose or classify diabetes; cannot differentiate from other causes | **Mnemonic:** **OGTT for GLUCOSE CLASSIFICATION** = Oral Glucose Tolerance Test Identifies Fasting, Postprandial, and Diagnostic Thresholds (IFG vs. IGT vs. DM).