A Low Sodium with a Normal Serum Osmolality
Osmolality splits a low sodium into three situations that are indistinguishable on the report and managed completely differently. Low osmolality means the blood really is too dilute. High osmolality means something is pulling water out of cells and diluting the sodium as it goes. Normal osmolality means the sodium is not actually low at all — the measurement was distorted by something else in the plasma.
The pattern on your report
- Sodium Low Key
- Serum osmolality Normal Key
- Triglycerides High Key
- Total protein High Key
- Glucose Normal Key
Printed as: Glucose in mmol/Lor mg/dL— A high value produces the third version, where the sodium has genuinely fallen and correcting the glucose corrects it.Serum osmolality in mOsm/kg— Only the measured value helps. The calculated version is derived from the sodium and inherits the same error.Sodium in mmol/Lor mEq/L— Measured after dilution in most laboratories, which is the step that goes wrong when plasma contains less water than the method assumes.Total protein in g/Lor g/dL— A raised value with a normal albumin should prompt a paraprotein screen.Triglycerides in mmol/Lor mg/dL
Why the numbers look like this
Sodium is measured in most laboratories by diluting the sample first and then reading the concentration. That method assumes plasma is mostly water, which it normally is.
When the non-water part of plasma is expanded — by very high triglycerides, or by a large excess of protein such as a paraprotein — the sample contains less water per unit volume than the method assumes. The sodium dissolved in that water is at a perfectly normal concentration, but the diluted measurement reports a lower figure. Osmolality is measured by a different principle that does not depend on this assumption, so it comes back normal, and that mismatch is the giveaway.
The high-osmolality version is a different problem altogether. Glucose, and occasionally mannitol, stay outside cells and draw water out into the bloodstream. That water dilutes the sodium, so the level really has fallen, but the total concentration of dissolved particles is high rather than low. Correcting the glucose corrects the sodium.
The low-osmolality version is ordinary dilutional hyponatremia, the subject of most of the sodium pages on this panel.
Direct measurement on a blood gas analyzer avoids the dilution step entirely, which is why a gas sample and a laboratory sample can disagree in exactly this situation.
Not being flagged is not the same as normal
Osmolality can be measured or calculated, and only the measured value is useful here. The calculated version is derived from the sodium itself, so it inherits the same error and will look low when the sodium looks low. Requesting a measured serum osmolality is therefore a specific request. Laboratories also differ in method: those using direct measurement without dilution are not affected by this at all, so whether the problem can arise depends on which analyzer produced the number.
What else on the report can hide this
A measured serum osmolality sorts this, and it has to be asked for; a calculated one will not do.
Glucose belongs in the same request, because a high value explains the high-osmolality version immediately and it is the commonest of the three to be missed.
Triglycerides and total protein cover the two things that expand the non-water fraction. A very high triglyceride level or a raised total protein with a normal albumin points straight at the measurement.
A sodium measured directly on a blood gas analyzer gives the true value in minutes and is often already available in someone who has had one taken.
Where the osmolality is genuinely low, the assessment moves to the usual causes of dilutional hyponatremia: urine osmolality, urine sodium, thyroid function and cortisol, taken before any treatment starts.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Very high triglycerides
Expands the non-water fraction of plasma, so the diluted measurement under-reports. Osmolality normal, and a direct measurement gives the true value.
- Very common
A high glucose diluting the sodium
The high-osmolality version. Water is pulled out of cells, so the sodium genuinely falls, and correcting the glucose corrects it.
- Common
A paraprotein or very high total protein
The same measurement effect as high triglycerides. A raised total protein with a normal albumin should prompt a paraprotein screen.
- Common
The osmolality was calculated rather than measured
Not a cause but the reason the test failed to sort it. The calculated value uses the sodium, so it inherits the same error.
- Common
Genuine dilutional hyponatremia
The low-osmolality version, and the commonest overall. The assessment then follows the usual path for a low sodium.
- Uncommon
Mannitol or another infused solute — in people who have received it in hospital
Behaves like glucose, staying outside cells and drawing water in. From the treatment record.
- Uncommon
Absorbed irrigation fluid during surgery
Fluid used during some operations is absorbed and lowers the sodium, sometimes markedly. From the operation record.
- Uncommon
A laboratory using direct measurement
Not affected by this at all, so whether the problem can arise depends on which analyzer produced the number.
- Rare
Severe hypercholesterolemia
Rarely reaches the level needed to produce this on its own, but it contributes alongside raised triglycerides.
What is usually checked next
- A measured serum osmolality Sorts the three situations, and it has to be requested rather than calculated from the sodium.
- Glucose Explains the high-osmolality version immediately, and it is the commonest of the three to be overlooked.
- Triglycerides and total protein Cover the two things that expand the non-water fraction and distort the measurement.
- A sodium measured directly on a blood gas analyzer Skips the dilution step and gives the true value in minutes, often from a sample already taken.
- Urine osmolality, urine sodium, TSH and cortisol if osmolality is low The usual assessment for genuine dilutional hyponatremia, taken before treatment starts.
When to seek care sooner
- Emergency Confusion, drowsiness, or difficulty waking
- Emergency A seizure
- Emergency Severe abdominal pain radiating to the back
- Same day Excessive thirst with passing large volumes of urine
- Same day A sodium that has dropped quickly from a previous value
- Soon Bone pain with a raised calcium and total protein
Questions worth bringing to your appointment
- Was my osmolality measured or calculated?
- Were my triglycerides or total protein raised?
- Was my glucose high at the same time?
- Has a sodium been done on a blood gas sample?
- Is my sodium actually low, or is this a measurement effect?