A Sodium Result That Does Not Match the Patient
A 54-year-old man with alcohol-related pancreatitis is on total parenteral nutrition through a central line. The nighttime specimen is visibly milky. Results: sodium 113 mmol/L by the indirect ion-selective electrode on the main chemistry analyzer, potassium 4.1 mmol/L, chloride 84 mmol/L, bicarbonate 24 mmol/L, glucose 96 mg/dL, BUN 12 mg/dL, creatinine 0.9 mg/dL, triglycerides 2450 mg/dL. Measured osmolality by freezing point depression is 288 mOsm/kg. The patient is alert, oriented, has no seizures, no focal deficit, no edema, and a normal neurologic exam. The covering resident pages the laboratory saying that 3% saline has been ordered and asks the technologist to confirm the sodium is really 113.
1. Do the laboratory numbers support true hyponatremia? Show the arithmetic.
Read the worked answer
They do not. The internal inconsistency is the giveaway.
Calculated osmolality from the reported values: 2 x 113 = 226, plus glucose 96/18 = 5.3, plus BUN 12/2.8 = 4.3, for a total of about 236 mOsm/kg. The measured osmolality is 288 mOsm/kg. That is an apparent gap of roughly 52 mOsm/kg - far too large to be explained by an unmeasured osmole such as ethanol or mannitol, and this patient is not on either.
Now work backwards. If the sodium were about 140 mmol/L, the calculated osmolality would be 280 + 5.3 + 4.3 = about 290 mOsm/kg, which matches the measured 288 almost exactly. The measured osmolality agrees with a normal sodium, not with 113.
The explanation is pseudohyponatremia from marked hypertriglyceridemia. Indirect ion-selective electrodes measure sodium in the plasma water fraction after dilution, so a large nonaqueous lipid volume displaces water and drives the reported sodium down even though the sodium concentration in the water phase is normal. A normal measured osmolality plus a low calculated osmolality in a lipemic sample is the classic signature.
2. What should the laboratory do to resolve this before any treatment is given?
Read the worked answer
Measure sodium on an instrument that uses direct ion-selective electrode technology, such as a blood gas analyzer, which is not affected by the lipid displacement. A direct reading of approximately 140 mmol/L would confirm the artifact.
Practical steps:
- Note the lipemic index. Most modern analyzers flag a sample this lipemic automatically, and the result should never be verified silently.
- Repeat the sodium by direct ISE and, if the two methods disagree by more than a few mmol/L, report the direct value with a comment explaining that the indirect result is spuriously low due to hypertriglyceridemia.
- If no direct ISE is available, the sample can be ultracentrifuged or the patient can be redrawn after lipids are lowered, but neither should delay the call to the clinician.
- Hold the chemistry panel verification and telephone the result rather than letting a critical-value sodium of 113 trigger treatment, because a critical low sodium that is an artifact is itself a safety event.
3. If 3% saline had been given as ordered, what harm could result, and what does this case say about how the sodium should be handled for the rest of this admission?
Read the worked answer
The patient is not hyponatremic, so 3% saline would raise a normal sodium. Hypertonic saline in a normonatremic patient risks hypernatremia, osmotic injury, and volume overload, and rapid shifts in serum sodium in either direction risk osmotic demyelination. The potentially correct treatment, if he were truly at 113, would be a controlled rise of no more than about 8 mmol/L in 24 hours - the opposite of an unmonitored bolus.
For the rest of the admission:
- Flag the chart so that future sodium results on the indirect analyzer are interpreted with the lipid status in mind, and keep using direct ISE while triglycerides remain this high.
- Treat the hypertriglyceridemia; the pseudohyponatremia resolves as lipids fall.
- Do not order a lipid-containing parenteral formulation that keeps triglycerides at this level without monitoring.
- Remember the mirror-image pitfall: pseudohyperkalemia and pseudohypernatremia can also occur in extreme lipemia or hyperproteinemia, and the same direct-ISE cross-check applies.