What is Pseudohyponatremia?
Pseudohyponatremia is an artifactually low sodium concentration measured in the laboratory when plasma contains abnormally high levels of lipids (hypertriglyceridemia) or proteins (hyperproteinemia such as in multiple myeloma). In this condition, the true sodium concentration in plasma water is actually normal, but the lab measurement is falsely low due to the methodology used.
Plasma is composed of approximately 93% water and 7% solids (proteins and lipids). Sodium is dissolved exclusively in the aqueous phase. When lipid or protein content increases significantly, the solid phase expands, the aqueous fraction decreases, and traditional laboratory methods that measure sodium per liter of plasma (rather than per liter of plasma water) report a falsely low value.
This distinction is critically important because pseudohyponatremia does not require treatment — the patient's true sodium status is normal. Treating pseudohyponatremia with fluid restriction or hypertonic saline could paradoxically cause harm.
Correction Formulas
Primary Correction Formula
The most commonly used correction accounts for the displacement of plasma water by excess triglycerides:
This formula assumes that normal triglycerides are approximately 150 mg/dL and that each mg/dL increase in triglycerides above this baseline displaces a proportional volume of plasma water.
Simple Rule of Thumb
For quick bedside estimation:
This simplified approach is useful for rapid mental calculations in clinical settings but is less precise than the formula above.
Katz Formula (Volume Displacement)
The Katz formula uses a more rigorous volume-displacement approach that accounts for both lipid and protein contributions to the solid phase of plasma:
Where the water fraction depends on lipid volume and protein content. The normal plasma water fraction is approximately 0.93 (93%). Severe hypertriglyceridemia can reduce this to 0.80 or lower.
Mechanism of Lipid Interference
To understand pseudohyponatremia, it helps to understand how sodium is measured in the laboratory:
- Indirect ion-selective electrode (ISE): The most common method in automated chemistry analyzers. Samples are prediluted before measurement, and the result is calculated based on the assumption of a normal plasma water fraction (0.93). When lipids or proteins are elevated, the actual water fraction is lower, and the calculated sodium is artifactually low. This method IS affected by pseudohyponatremia.
- Direct ISE: Measures sodium directly in undiluted plasma without predilution. Since it measures the sodium activity in the aqueous phase directly, it is NOT significantly affected by elevated lipids or proteins. This method is used in most point-of-care blood gas analyzers.
- Flame photometry: An older method that measures sodium per volume of total plasma. Like indirect ISE, it IS affected by pseudohyponatremia.
The key clinical pearl: if you suspect pseudohyponatremia, request a direct ISE measurement (typically from a blood gas analyzer) to confirm the true sodium level.
Lipemic Samples in the Lab
Severely lipemic (milky-appearing) samples cause numerous laboratory interferences beyond sodium measurement:
| Analyte | Direction of Interference | Mechanism |
|---|---|---|
| Sodium | Falsely decreased | Volume displacement (indirect ISE) |
| Chloride | Falsely decreased | Volume displacement |
| Hemoglobin | Falsely increased | Light scattering (spectrophotometric) |
| Bilirubin | Falsely increased | Light scattering |
| Glucose | Variable | Method-dependent |
| LDH | Falsely increased | Turbidity interference |
Most modern laboratories have lipemic indices that flag samples with excessive turbidity. Ultracentrifugation can remove lipids before re-analysis, or alternative methods (direct ISE, dilution protocols) can be employed.
When to Suspect Pseudohyponatremia
Consider pseudohyponatremia when:
- The patient has known severe hypertriglyceridemia (typically >1,000 mg/dL)
- The blood sample appears milky or lipemic
- The patient has conditions associated with severe hypertriglyceridemia: uncontrolled diabetes, pancreatitis, familial hypertriglyceridemia, nephrotic syndrome, or certain medications (retinoids, protease inhibitors)
- The patient is clinically euvolemic and asymptomatic despite apparently low sodium
- The measured serum osmolality is normal (280–295 mOsm/kg) despite low measured sodium — this is a hallmark of pseudohyponatremia
- The patient has known multiple myeloma or Waldenström's macroglobulinemia (protein-mediated pseudohyponatremia)
Clinical Management
The management approach depends on whether the hyponatremia is true or pseudo:
- If corrected Na is 135–145 mEq/L: This is pseudohyponatremia. No sodium correction is needed. Focus on treating the underlying cause of hypertriglyceridemia (e.g., insulin for diabetic hyperlipidemia, fibrates, dietary modification).
- If corrected Na is still <135 mEq/L: The patient has both hypertriglyceridemia AND true hyponatremia. Both conditions require independent management. Investigate the cause of true hyponatremia (SIADH, volume depletion, etc.).
- Confirm with direct ISE: Whenever possible, obtain a direct ISE sodium measurement from a blood gas analyzer to verify the corrected value calculated by formula.
Frequently Asked Questions
At what triglyceride level does pseudohyponatremia become clinically relevant?
Pseudohyponatremia from lipids generally becomes clinically significant when triglycerides exceed approximately 1,000 mg/dL. At 1,500 mg/dL, sodium may be artifactually reduced by approximately 2.7 mEq/L. At very high levels (>5,000 mg/dL), the reduction can exceed 5–10 mEq/L, potentially leading to misdiagnosis and inappropriate treatment.
Can I use a blood gas analyzer to check the true sodium?
Yes, and this is the recommended approach. Blood gas analyzers use direct ion-selective electrodes that measure sodium activity in undiluted plasma water, bypassing the volume displacement artifact. If the blood gas sodium is normal while the chemistry panel sodium is low, pseudohyponatremia is confirmed.
Does this calculator account for protein-mediated pseudohyponatremia?
The primary formula focuses on triglyceride-mediated effects. However, hyperproteinemia (>10 g/dL, as seen in multiple myeloma) can also cause pseudohyponatremia through the same volume displacement mechanism. The optional total protein input helps flag this consideration, though a separate protein-specific correction may be needed in severe cases.
Is pseudohyponatremia dangerous?
Pseudohyponatremia itself is not dangerous because the true plasma sodium is normal. However, misdiagnosing it as true hyponatremia and treating with hypertonic saline or aggressive fluid restriction could cause iatrogenic hypernatremia, which IS dangerous. This is why recognizing pseudohyponatremia is so clinically important.