Sodium Change Calculator - Hypertriglyceridemia
When triglyceride levels are severely elevated, standard indirect ion-selective electrode (ISE) measurements report a falsely low serum sodium - a phenomenon called pseudohyponatremia. This calculator estimates the artifactual sodium drop caused by hypertriglyceridemia and adds it back to reveal the corrected (true) plasma sodium. Enter the measured serum sodium and the serum triglyceride level; switch triglycerides between mg/dL and mmol/L. An optional Waugh-formula mode lets you include plasma protein and cholesterol for a more comprehensive correction.
Formula
Worked example
Patient with serum Na 128 mEq/L and TG 1500 mg/dL: sodium change = 1500 / 500 = 3.0 mEq/L. Corrected Na = 128 + 3.0 = 131 mEq/L. The lab shows moderate hyponatremia, but the corrected value of 131 mEq/L suggests only mild pseudohyponatremia - still below 135 mEq/L, so further evaluation is needed.
What is pseudohyponatremia in hypertriglyceridemia?
Pseudohyponatremia is a laboratory artifact in which the measured serum sodium is falsely low while the actual sodium concentration in plasma water remains normal. It occurs because standard sodium measurement methods - indirect ion-selective electrode (ISE) and flame photometry - report sodium as a concentration in total plasma volume rather than exclusively in the aqueous phase. Plasma normally consists of about 93% water and 7% proteins and lipids. When triglycerides are severely elevated (commonly above 1000 mg/dL), lipids displace a larger portion of the plasma volume, reducing the apparent aqueous fraction. Since sodium dissolves only in water, the same amount of sodium dissolved in less apparent water yields a lower measured concentration - even though the true sodium activity in plasma water is completely normal. Clinically, failure to recognise pseudohyponatremia and treat for true hyponatremia risks causing iatrogenic harm through unnecessary fluid restriction, hypertonic saline infusion, or vasopressin antagonists.
The correction formulas explained
Two correction approaches are widely used. The simple clinical rule estimates a sodium decrease of approximately 1 mEq/L for every 500 mg/dL rise in serum triglycerides above baseline: Sodium change (mEq/L) = Triglycerides (mg/dL) / 500. This rule is easy to apply at the bedside and is adequate for most clinical decisions when hypertriglyceridemia is the sole driver. The Waugh formula provides a more comprehensive correction that accounts for plasma protein and cholesterol as well as triglycerides: Corrected Na = Measured Na / [1 - 0.00073 x Protein (g/dL) - (0.00010 x TG mg/dL) / 1000 - (0.00004 x Cholesterol mg/dL) / 1000]. The denominator represents the estimated aqueous fraction of plasma; dividing the measured sodium by this fraction adjusts back to the concentration in plasma water. Both methods are estimates. The gold-standard confirmation is direct ISE measurement on a blood gas analyzer, which measures sodium activity in the plasma water phase alone and is therefore unaffected by lipid or protein load.
Common causes of pseudohyponatremia
Hypertriglyceridemia is the most common lipid cause of pseudohyponatremia. It typically becomes significant when triglycerides exceed 1000 mg/dL and can produce artifactual sodium drops of 3-6 mEq/L or more at very high levels. Other recognised causes include severe hypercholesterolemia, multiple myeloma or other paraproteinemias with markedly elevated total protein, intravenous immunoglobulin therapy, and certain radiocontrast agents. In all these settings the mechanism is the same: the non-aqueous fraction of plasma is enlarged by abnormal amounts of lipid or protein, diluting the apparent sodium concentration in total plasma volume while the concentration in plasma water remains unchanged. Clinicians should suspect pseudohyponatremia whenever hyponatremia is found in a patient with known severe hypertriglyceridemia or paraproteinemia, especially if the patient has no symptoms of true hyponatremia and the urine and serum osmolality findings are inconsistent with true hyponatremia.
Clinical approach: distinguishing true from pseudo hyponatremia
The key diagnostic clue is plasma osmolality. In true hyponatremia, serum osmolality is low (typically below 280 mOsm/kg). In pseudohyponatremia, the serum osmolality measured by freezing-point depression is normal (280-295 mOsm/kg) despite the apparently low sodium, because the true sodium activity is unchanged. A visibly lipemic blood sample is another red flag. The osmolal gap - the difference between measured osmolality and the calculated osmolality (2 x Na + BUN/2.8 + Glucose/18) - is widened in pseudohyponatremia because the calculated value uses the artifactually low sodium. Confirming with a direct ISE blood gas sodium or requesting a repeat sodium on a cleared sample (after ultracentrifugation or after lipid-lowering treatment) is the definitive step. Management then focuses on treating the triglyceride elevation rather than the apparent hyponatremia.
Serum sodium classification
| Corrected sodium (mEq/L) | Classification | Clinical urgency |
|---|---|---|
| Below 120 | Severe hyponatremia | Emergency - immediate management |
| 120-129 | Moderate hyponatremia | Urgent evaluation needed |
| 130-134 | Mild hyponatremia | Evaluate and monitor closely |
| 135-145 | Normal (eunatremia) | No correction needed |
| 146-150 | Mild hypernatremia | Identify and address cause |
| Above 150 | Significant hypernatremia | Prompt intervention required |
Standard clinical thresholds for serum sodium in adults. These apply to the corrected (true) sodium, not the artifactually low measured value in pseudohyponatremia.
Frequently asked questions
What triglyceride level causes pseudohyponatremia?
Pseudohyponatremia from hypertriglyceridemia becomes clinically significant when triglycerides exceed approximately 1000 mg/dL (11.3 mmol/L). At that level the estimated sodium drop is around 2 mEq/L, which can push a borderline value below the 135 mEq/L threshold. At extreme levels above 5000 mg/dL, the artifactual drop can exceed 10 mEq/L and mimic severe hyponatremia.
Can I use this calculator for protein-related pseudohyponatremia?
Yes. Select the Waugh formula mode and enter plasma protein and cholesterol in addition to triglycerides. The Waugh formula was developed to correct for all three non-aqueous contributors to pseudohyponatremia and is more accurate when protein is also elevated, such as in multiple myeloma or severe paraproteinemia.
How do I confirm whether the hyponatremia is true or pseudo?
The most reliable method is direct ion-selective electrode (ISE) measurement on a blood gas analyzer, which measures sodium exclusively in the plasma water phase. A normal serum osmolality measured by freezing-point depression (280-295 mOsm/kg) with a visibly lipemic sample strongly supports pseudohyponatremia. Compare with the osmolal gap: a positive gap in the setting of low measured sodium suggests a falsely low value.
Does this calculator replace a blood gas sodium measurement?
No. This calculator provides an estimate based on a published clinical rule and is useful for rapid bedside assessment and decision support, not for replacing laboratory confirmation. Always correlate with clinical context, serum osmolality, and - when available - a direct ISE sodium measurement before making treatment decisions.
What is the difference between the simple rule and the Waugh formula?
The simple rule (1 mEq/L per 500 mg/dL of triglycerides) requires only the triglyceride concentration and is convenient for quick estimation when triglyceride elevation is the sole or dominant driver. The Waugh formula incorporates plasma protein and cholesterol as well, making it more accurate when multiple non-aqueous components are elevated simultaneously. In most cases of isolated hypertriglyceridemia the two approaches give similar results.
Why does the calculator show the corrected sodium on a gauge?
The corrected sodium gauge lets you see at a glance where the true sodium falls relative to the normal range (135-145 mEq/L) and clinical severity thresholds. A measured sodium of 128 mEq/L looks worrying, but if the corrected value is 134 mEq/L the gauge shows it barely below normal rather than in the moderate hyponatremia band - a clinically important distinction.