How this instrument works
Severe hypertriglyceridemia can make a routine sodium panel read lower than it actually is — not because sodium has left the blood, but because the lab's measurement leans on an assumption that stops holding. Indirect ion-selective-electrode and flame-photometry methods dilute a serum sample and then infer sodium concentration per liter of whole serum, quietly assuming that serum is about 93% water. Triglycerides occupy space in that serum without carrying sodium, so when they climb into the thousands they crowd out enough of the water fraction that the same amount of sodium reads as a lower concentration — a measurement artifact called pseudohyponatremia, not a true drop in circulating sodium. Direct ion-selective-electrode assays, common in blood-gas analyzers, measure sodium activity in undiluted serum and are immune to this artifact entirely; indirect ISE remains the majority approach — a 2006 survey cited by Aziz et al. found it in use at more than two-thirds of U.S. labs, still the most recent widely-cited figure — so the artifact is far from a historical curiosity, while flame photometry shares the same water-displacement vulnerability but has been largely retired to a reference-method role in routine U.S. testing today.
The equation behind this correction has two parts, and they trace back through the literature differently. Serum water content (SWC%) = 99.1 minus 0.73 times protein in g/dL minus 1.03 times lipid in g/dL was originally derived by W.H. Waugh in a 1969 paper in Metabolism; that original paper predates open digital archives and sits behind a paywall this project could not clear directly, so this equation was read from a 2023 open-access, peer-reviewed reproduction by Aziz, Sam, Lew and colleagues in the Journal of Clinical Medicine, who quote Waugh's SWC equation in full. The second step — corrected sodium equals measured sodium times 93 divided by SWC% — is not printed as a single formula anywhere in Aziz et al.; it follows from combining their SWC equation with the 93%-water-fraction convention their paper describes elsewhere, and it matches the standard clinical correction formula found in other secondary sources on this topic. A separate 1989 review by Weisberg in the American Journal of Medicine is the paper most often credited with popularizing pseudohyponatremia as a clinical concept; like the Waugh paper, it sits behind a paywall this project could not clear directly, so it was not accessed directly for this page, and the numeric formula above comes entirely from the Waugh-via-Aziz route rather than from Weisberg's review.
This is a distinct question from the site's other sodium tools. The sodium correction calculator predicts how a liter of IV fluid will move today's reading; the sodium correction rate calculator checks whether a planned rise is safe; the sodium deficit calculator totals the mEq needed to close a gap — all three assume the measured sodium is real and plan treatment around it. This calculator instead asks whether the measured number is real in the first place, and it typically only matters once triglycerides climb past roughly 1500 mg/dL; mild or moderate elevations rarely displace enough serum water to shift the reading meaningfully. Most importantly, a low reading explained by this artifact should never be treated as if it were true hyponatremia — correcting a falsely low number with hypertonic saline or aggressive fluid restriction has caused documented, fatal overcorrection in patients whose actual sodium and water balance were normal. It's also worth being plain about what the corrected number is: a population-derived estimate from 1969 data, not an individually validated measurement, offered as a fallback for when direct ISE testing isn't available — where a direct ISE reading can be obtained, it takes precedence over this calculated correction for any treatment decision.
- Enter Measured serum sodium (mEq/L) exactly as it appears on the lab report — this is the number the correction re-expresses, not a value to change.
- Enter Total serum protein (g/dL) from the same blood draw; it sets a baseline non-water share even before triglycerides are added.
- Enter Triglycerides (mg/dL) from the same draw — this is usually the term driving the correction once it runs into the thousands.
- Read Serum water content (%): the estimated fraction of the serum sample that is actually water, given that protein and triglyceride load.
- Read Corrected serum sodium (mEq/L): what the concentration would read if measured per liter of serum water instead of per liter of whole serum.
- Confirm with the lab whether the sodium assay is indirect ISE or flame photometry — direct ISE assays aren't affected by this artifact and don't need the correction.
Worked example — sodium 130, protein 7.0, triglycerides 3000
A measured sodium of 130 mEq/L comes back with total protein at 7.0 g/dL and triglycerides at 3000 mg/dL — severe hypertriglyceridemia. Converting triglycerides to g/dL gives 3.0, so SWC% = 99.1 − (0.73 × 7.0) − (1.03 × 3.0) = 99.1 − 5.11 − 3.09 = 90.9%. Only about 90.9% of this serum sample is actually water; the rest is protein and lipid taking up space without carrying sodium.
Corrected sodium = 130 × 93 / 90.9 = 133.0 mEq/L. Nearly all of the gap between 130 and a low-normal reading turns out to be the lab artifact rather than a true sodium deficit — a result that would look like meaningful hyponatremia on the report but isn't, once the water-content assumption behind the assay is accounted for.
Compare a milder case: measured sodium 135 mEq/L, protein 7.5 g/dL, triglycerides only 800 mg/dL — below the roughly 1500 mg/dL threshold where this artifact usually becomes meaningful. SWC% = 99.1 − (0.73 × 7.5) − (1.03 × 0.8) = 92.8%, and the correction moves the reading only slightly, to about 135.3 mEq/L, confirming that mild triglyceride elevations don't produce a clinically relevant artifact.
Questions
Why does a high triglyceride level make sodium read artificially low?
Certain lab methods measure sodium concentration per liter of whole serum and assume serum is about 93% water. Triglycerides take up physical space in serum without containing sodium, so when they climb into the thousands of mg/dL they crowd out enough of that water fraction that the same true amount of sodium gets spread across what the assay treats as a smaller water volume, producing a lower reported concentration even though nothing about the patient's actual sodium or water balance has changed.
Does every sodium test have this problem?
No. Direct ion-selective-electrode (ISE) assays, common in blood-gas analyzers, measure sodium activity in undiluted serum and are not affected by this artifact at all. Indirect ISE and flame photometry both dilute the sample first and assume a fixed water fraction, so both can be fooled — but the more-than-two-thirds-of-U.S.-labs figure describes indirect ISE specifically, from a 2006 survey (still the most recent widely-cited number); flame photometry shares the vulnerability but has been largely retired to a reference-method role in routine U.S. testing today. Checking which method a given lab used is worth doing before assuming a correction is needed.
How severe do triglycerides need to be before this correction matters?
This artifact typically only becomes clinically meaningful once triglycerides climb past roughly 1500 mg/dL. Mild or moderate elevations don't displace enough serum water to move a sodium reading in any way that changes its interpretation, so a triglyceride level in the hundreds is unlikely to explain a low sodium result on its own — the cause is worth looking for elsewhere.
How is this different from the site's other sodium calculators?
This tool corrects for a measurement artifact; the others plan treatment around a reading assumed to be real. The sodium correction calculator predicts how a liter of a chosen IV fluid will move today's sodium; the sodium correction rate calculator checks whether a planned rise is safe; the sodium deficit calculator totals the mEq of sodium needed to close a gap. None of those three question whether the starting number itself is accurate — this one does, and only for the specific case of lab-measured, not true, hyponatremia.
Should a low reading explained by this correction be treated with saline or fluid restriction?
No — and this is the most important caution on this page. Pseudohyponatremia is not real hyponatremia; the corrected figure explains why the lab number looks low, it does not indicate an actual sodium or water deficit exists. Treating a falsely low reading as if it were genuine — with hypertonic saline in particular — has caused documented, fatal cases of inappropriate correction in patients whose real sodium and water balance never needed adjusting. When a reading is genuinely uncertain, addressing the underlying triglyceride level or requesting a direct ISE measurement settles the question more safely than treating the number itself.
Where does the 93 in the formula come from?
It's the water fraction indirect sodium assays assume by default when they convert a diluted measurement back into a serum sodium concentration — serum in a typical, healthy person is close to 93% water. When protein and triglycerides are both normal, the calculated SWC% lands near that same figure and the correction barely moves the reading; it only diverges meaningfully when protein or triglycerides push the true water fraction well below 93%.
Where does this specific formula come from, and how solid is the sourcing?
The SWC% equation was derived by W.H. Waugh in a 1969 paper in Metabolism, which sits behind a paywall this project could not access directly. It was read instead from a 2023 open-access reproduction by Aziz, Sam, Lew and colleagues in the Journal of Clinical Medicine, who quote Waugh's SWC equation in full. The times-93-divided-by-SWC step is not printed as one formula in Aziz et al.; it is assembled from their SWC equation plus the 93%-water convention described elsewhere in that paper. A separate, also-paywalled 1989 review by Weisberg in the American Journal of Medicine popularized pseudohyponatremia clinically but was not accessed directly and contributed no numbers to the formula.
Does this correction also apply to high protein levels, not just high triglycerides?
Yes — the same formula accounts for both. The protein term subtracts a smaller water-displacement effect per g/dL of total serum protein, which matters most in conditions like multiple myeloma with very high paraproteins. In practice, triglycerides are the more common and dramatic driver, since severe hypertriglyceridemia can push levels far higher, relative to normal, than protein typically does. One gap: this formula ignores cholesterol. Severe hypercholesterolemia can displace serum water through the same mechanism, but Waugh's equation only takes triglycerides and protein as inputs, so cholesterol-driven pseudohyponatremia may show little correction here despite being real.
References
- Aziz F et al. Pseudohyponatremia: Mechanism, Diagnosis... J Clin Med 2023 (open access)
- Weisberg LS. Pseudohyponatremia: a reappraisal. Am J Med 1989 (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.