Egyptian Board Clinical Pathology: Sodium of 124 mmol/L in a Patient With a Triglyceride of 34 mmol/L

Sodium of 124 mmol/L in a Patient With a Triglyceride of 34 mmol/L

A 58-year-old man with type 2 diabetes is admitted with severe epigastric pain radiating to the back and repeated vomiting; serum lipase is twelve times the upper reference limit and CT shows pancreatic necrosis. The admission sample is visibly milky. On the main chemistry analyser (indirect ion-selective electrode, diluted sample) the results are: sodium 124 mmol/L, potassium 4.2 mmol/L, chloride 92 mmol/L, bicarbonate 21 mmol/L, urea 9.8 mmol/L, creatinine 96 µmol/L, glucose 11.4 mmol/L, triglycerides 34 mmol/L, total cholesterol 14 mmol/L. The lipaemic index is flagged as grossly abnormal. A blood gas sample taken at the same time on the ward analyser (direct ISE, whole blood) gives sodium 132 mmol/L and glucose 11.6 mmol/L; measured plasma osmolality by freezing point depression is 288 mOsm/kg. The admitting physician telephones asking whether to start hypertonic saline, and whether the laboratory will 'correct' the sodium before the result is entered in the electronic record.

1. Explain the discrepancy between the two sodium results. Which value represents the patient's true sodium concentration, and how would you confirm your interpretation with the data already available?

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The two analysers measure different things.

The main analyser uses an indirect ISE: the sample is prediluted and the result is expressed per litre of whole plasma, assuming a normal plasma-water fraction of about 93%. With triglycerides of 34 mmol/L (roughly 3000 mg/dL) the non-aqueous lipid phase occupies a large fraction of the plasma volume, so the amount of sodium per litre of plasma falls while the sodium concentration per litre of plasma water is normal.
The blood gas analyser uses a direct ISE on undiluted whole blood, which is not affected by volume displacement by lipid.

This is pseudohyponatraemia (isotonic/hyperlipaemic hyponatraemia), not true hyponatraemia. The true sodium is approximately 132 mmol/L.

Confirmatory reasoning from the data already in hand:

Osmolality check. Calculated osmolality using sodium 132 is 2(132) + 11.6 + 9.8 ≈ 285 mOsm/kg, which matches the measured 288. Using sodium 124 gives ≈ 269, a spurious osmolal gap of about 19 mOsm/kg.
Magnitude of the glucose effect. A glucose of 11.4 mmol/L translocational hyponatraemia would lower sodium by only about 2–3 mmol/L, not 8 mmol/L.
Repeat on a direct ISE (blood gas) and, if available, after ultracentrifugation of the sample; the value should stay near 132 once the lipid is removed.

A discrepancy that disappears when the sample is analysed undiluted or delipidated is diagnostic.

2. What should the laboratory report, and what immediate management advice should be given to the treating team?

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Reporting

Do not release 124 mmol/L as the sole sodium result. Report the direct ISE value (≈132 mmol/L) as the interpretable result and add a comment stating that the chemistry analyser result is falsely low because of gross lipaemia, with the measured value and method named.
Telephone the result with documentation of the read-back, the person contacted and the time, because a sodium of 124 mmol/L could otherwise trigger urgent hypotonic/major fluid decisions.
Do not apply a mathematical 'lipid correction' to the reported number; lipid interference is not a fixed linear effect across analysers and the direct measurement is available.

Management advice

Hypertonic saline is contraindicated. The patient is not hyponatraemic; giving 3% saline risks hypernatraemia and osmotic demyelination.
The clinical picture is pancreatitis with hypertriglyceridaemia. Sodium should be managed by treating the pancreatitis (nil by mouth, insulin infusion or apheresis if triglycerides remain very high, correction of volume status with isotonic crystalloid based on true tonicity).
Recheck sodium on the blood gas/direct ISE daily while the sample remains lipaemic, and again once triglycerides fall below about 10 mmol/L.
Flag the result in the record so future shifts do not repeat the same error; consider a standing comment on lipaemic samples.

3. Name three other analytes in this sample that are likely to be unreliable and describe how the laboratory should handle them.

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Severe lipaemia interferes with spectrophotometric (and to a lesser degree immunoturbidimetric) assays in three ways: light scattering, dilution of the aqueous phase, and partitioning of the analyte into the lipid phase.

Likely affected in this sample:

Enzymes measured optically - amylase, lipase, AST, ALT, GGT and ALP; the lipaemic index on the analyser should be checked against the manufacturer's threshold and, if exceeded, the result should be suppressed or measured on a diluted/ultracentrifuged aliquot with the dilution validated for that analyte.
Bilirubin and total protein/albumin - turbidity falsely raises bilirubin and can bias dye-binding albumin methods; a sample blank or a different method (e.g. capillary electrophoresis for protein) may be needed.
Potassium and other electrolytes on indirect ISE - pseudohyperkalaemia/pseudonormokalaemia by the same volume-displacement mechanism; report potassium from the blood gas (direct ISE) with a comment, after excluding haemolysis.

Practical handling:

Centrifuge at high speed (ultracentrifuge/airfuge) to clear the sample before re-analysis, and record that this was done.
Never report a value that falls outside the assay's declared lipaemic tolerance; state 'not reported - gross lipaemia' with the reason.
Where an alternative method exists (direct ISE, blood gas), use it and name the method in the report.
Document the incident in the quality system if results had already been released, and audit whether the analyser's lipaemic index thresholds are appropriate for each assay.