Situation: A 76-year-old woman with type 2 diabetes mellitus is brought from home after three days of increasing drowsiness and poor oral intake during a febrile illness. She weighs 60 kg. The provider diagnoses hyperosmolar hyperglycemic state (HHS). Her results are sodium 141 mEq/L, glucose 1,026 mg/dL (57.0 mmol/L), and blood urea nitrogen (BUN) 42 mg/dL (15.0 mmol/L). Total serum osmolality = 2 × sodium + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. Effective osmolality leaves out any solute that crosses cell membranes freely. What is her effective serum osmolality? Round off to the nearest whole number.

Urea crosses cell membranes freely, so it does not pull water out of cells and is left out of effective osmolality: 2 × 141 + 1,026 ÷ 18 = 282 + 57 = 339 mOsm/kg. This is well above the HHS criterion of more than 300 mOsm/kg.

Understanding effective osmolality in HHS

The key to this calculation is distinguishing total serum osmolality from effective serum osmolality. Both are measures of solute concentration, but they differ in one critical way: whether the solute can cross cell membranes.

Urea crosses cell membranes freely, so it distributes evenly between the intracellular and extracellular compartments and does not create an osmotic gradient that pulls water out of cells. Because urea does not contribute to water shifts, it is excluded when calculating effective osmolality. Sodium and glucose, in contrast, are effective osmoles because they remain primarily in the extracellular space and do create osmotic gradients.

The formula for effective osmolality is:

Effective osmolality = 2 × sodium + glucose (mg/dL) ÷ 18

Plugging in the patient's values:

- Sodium = 141 mEq/L, so 2 × 141 = 282
- Glucose = 1,026 mg/dL, so 1,026 ÷ 18 = 57
- Effective osmolality = 282 + 57 = 339 mOsm/kg

The BUN of 42 mg/dL is intentionally omitted. If it were included, the total osmolality would be 339 + (42 ÷ 2.8) = 339 + 15 = 354 mOsm/kg, which corresponds to option 1. Watch out! Option 1 is the total osmolality, not the effective osmolality. The question specifically asks for effective osmolality, so urea must be left out.

An effective osmolality of 339 mOsm/kg is well above the HHS diagnostic threshold of greater than 300 mOsm/kg, confirming the hyperosmolar state. This elevation is driven almost entirely by the marked hyperglycemia, which pulls water from the intracellular space into the extracellular compartment, diluting the sodium concentration. The measured sodium of 141 mEq/L may appear normal or even low-normal, but it does not reflect the true total body sodium deficit because of the dilutional effect of glucose-driven water shifts.

Key point! In hyperglycemic emergencies, the presenting sodium concentration underestimates the actual sodium deficit. The effective osmolality calculation helps quantify the severity of the hyperosmolar state and guides the rate and composition of fluid replacement. The tonicity disorder in HHS is hypertonic, meaning water moves out of cells, causing intracellular dehydration despite the patient's overall volume depletion.

The distinction between total and effective osmolality also matters when interpreting sodium values in other settings. A sodium concentration that appears normal or low in the presence of severe hyperglycemia or marked azotemia may not indicate true hypotonicity, because glucose and urea affect osmolality differently. Glucose is an effective osmole and raises tonicity, while urea is an ineffective osmole and does not alter tonicity even when BUN is elevated.