Which laboratory finding best distinguishes hyperosmolar hyperglycaemic state (HHS) from diabetic ketoacidosis?

• A Serum glucose greater than 600 mg/dL
• B Arterial pH greater than 7.3 with minimal ketonaemia
• C Serum sodium greater than 145 mEq/L
• D Effective serum osmolality greater than 320 mOsm/kg ✓

Explanation

The defining feature of HHS is marked hyperosmolality, with an effective osmolality above 320 mOsm/kg producing the obtundation characteristic of the syndrome. Glucose above 600 mg/dL can occur in severe DKA, and pH above 7.3 occurs in mild DKA, so neither discriminates reliably. Hypernatraemia is common in both disorders and is not a defining criterion.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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