Pseudohyponatremia and Anion Gap in Diabetic Ketoacidosis
Mr. L, 31, is brought to the emergency department with 2 days of vomiting, abdominal pain, and confusion. He has type 1 diabetes and uses an insulin pump. Initial labs: glucose 34 mmol/L (612 mg/dL), arterial pH 7.09, pCO2 18 mmHg, bicarbonate 8 mmol/L, sodium 128 mmol/L, potassium 5.4 mmol/L, chloride 96 mmol/L, BUN 20 mg/dL, creatinine 1.4 mg/dL, measured osmolality 305 mOsm/kg, albumin 3.0 g/dL, and beta-hydroxybutyrate 6.8 mmol/L. The sample is lipemic, and triglycerides are 22 mmol/L (1,950 mg/dL). The clinician asks whether the sodium is real and whether the anion gap is reliable for guiding treatment. The laboratory uses an indirect ion-selective electrode for sodium on its chemistry analyzer. The patient has not received insulin in the emergency department yet.
1. Is the sodium of 128 mmol/L true hypotonic hyponatremia? Show the correction and explain the role of the lipemia.
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The sodium of 128 mmol/L is not true hypotonic hyponatremia. Correct for hyperglycemia: add 1.6 mmol/L per 100 mg/dL glucose above 100, so (612−100)/100 × 1.6 = 8.2 mmol/L; corrected sodium ≈136 mmol/L. The measured osmolality of 305 mOsm/kg is also not low, which rules out true hypotonic hyponatremia. However, the sample is lipemic with triglycerides 22 mmol/L, and the analyzer uses indirect ion-selective electrode (ISE), which dilutes the sample and can falsely lower sodium when the aqueous phase is reduced by large lipoprotein particles-pseudohyponatremia. The two effects point in opposite directions. The best next step is to measure sodium by direct ISE or after ultracentrifugation; in the meantime, treat the corrected value as the better estimate and do not give hypertonic saline for a low measured sodium.
2. Calculate the anion gap and delta ratio. What acid-base disorder do they indicate?
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Anion gap = Na − (Cl + HCO3) = 128 − (96 + 8) = 24 mmol/L, which is elevated (reference ~8–12). Correct for albumin: AG corrected = 24 + 2.5 × (4.0 − 3.0) = 26.5 mmol/L. The delta ratio = (AG − 12)/(24 − HCO3) = (26.5 − 12)/(24 − 8) = 14.5/16 ≈ 0.9. A ratio near 1.0 indicates a pure high-anion-gap metabolic acidosis, here from diabetic ketoacidosis; there is no superimposed metabolic alkalosis or non-gap acidosis. The pH 7.09 and pCO2 18 mmHg are consistent with appropriate respiratory compensation (Winter's formula predicts pCO2 ≈ 1.5 × 8 + 8 = 20 ± 2). Treatment should focus on insulin, fluids, and potassium replacement; the anion gap should close as ketones clear.
3. Which other laboratory results are unreliable because of the lipemia, and what should the laboratory do?
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Lipemia interferes with several spectrophotometric and indirect ISE measurements. Sodium, potassium, chloride, and total protein may be falsely low by indirect ISE or dye-binding methods; amylase and lipase can be affected; and the sample may be flagged for hemolysis or icterus. The laboratory should ultracentrifuge the sample (or use direct ISE) and re-run the critical analytes, then report the corrected results with a comment. Triglycerides should be reported because severe hypertriglyceridemia can cause pancreatitis and because it explains the pseudohyponatremia. Do not withhold treatment while waiting; communicate the corrected sodium and the anion gap to the clinician, and repeat electrolytes after initial therapy.