Corrected Sodium for Hyperglycemia

What corrected sodium is and when it is used

Corrected sodium is an estimate of serum sodium after discounting the effect of hyperglycemia. Excess glucose increases the osmolality of the extracellular fluid and draws water out of cells; this water dilutes sodium, which falls without any real loss of sodium.

In 1973, Katz calculated, from a model of water distribution between compartments, that sodium falls about 1.6 mEq/L for every 100 mg/dL increase in glucose. Adding this fall back helps answer whether the patient would have normal, low or high sodium with glucose under control.

The correction appears in the assessment of severe diabetic decompensations, such as ketoacidosis and the hyperglycemic hyperosmolar state, and in any report with hyponatremia and high glucose. It is usually read together with calculated serum osmolality, which also uses sodium and glucose, and with the anion gap, when acidosis is suspected.

How to calculate corrected sodium: inputs and units

The calculator asks for two values from the same blood test:

• Measured sodium, in mEq/L, between 100 and 200. Example: 135.
• Blood glucose, in mg/dL, greater than 0 and up to 3,000. Example: 250.

Values outside these ranges produce a message saying which field to correct. The tool's steps:

1. If glucose is greater than 100 mg/dL, it calculates how many hundreds it exceeds 100 by: (glucose − 100) ÷ 100.
2. It multiplies this number by 1.6 (Katz) and by 2.4 (Hillier).
3. It adds each product to the measured sodium.
4. If glucose is 100 mg/dL or less, it returns the measured sodium itself on both lines.

Both results appear in mEq/L with one decimal place, each with the factor and the source in the label. Above 400 mg/dL, a message reminds you that Hillier observed an even greater fall in that range. The tool does not classify the value: the reference interval is the one on the report. With the example values in the fields, the results are 137.4 and 138.6 mEq/L.

Corrected sodium formula (Katz, 1973)

The calculator applies the formula with a reference glucose of 100 mg/dL and two factors. The expression is:

Corrected Na = Measured Na + factor × (Glucose − 100) ÷ 100, if Glucose > 100 mg/dL
Corrected Na = Measured Na, if Glucose ≤ 100 mg/dL

• Measured Na: measured serum sodium, in mEq/L.
• Glucose: blood glucose from the same sample, in mg/dL.
• Factor: estimated fall in sodium, in mEq/L, per 100 mg/dL of glucose above normal: 1.6 (Katz) or 2.4 (Hillier).
• 100: glucose considered normal in the formula.

The 1.6 factor comes from a theoretical calculation. Hillier, Abbott and Barrett (1999) raised the blood glucose of healthy volunteers above 600 mg/dL and observed a mean fall of 2.4 mEq/L per 100 mg/dL. In the same study, the relationship was not linear: up to 400 mg/dL the 1.6 factor worked well, and above that a factor of 4.0 described the data better. That is why the tool shows both factors side by side.

Worked example of corrected sodium

With measured sodium of 130 mEq/L and glucose of 600 mg/dL, the tool shows 138.0 mEq/L by the Katz factor and 142.0 mEq/L by the Hillier factor. The steps:

1. Glucose above normal: 600 − 100 = 500 mg/dL.
2. In hundreds: 500 ÷ 100 = 5.
3. Katz correction: 5 × 1.6 = 8.0 mEq/L; corrected sodium 130 + 8.0 = 138.0 mEq/L.
4. Hillier correction: 5 × 2.4 = 12.0 mEq/L; corrected sodium 130 + 12.0 = 142.0 mEq/L.

Since glucose exceeds 400 mg/dL, the tool also warns that, in Hillier's study, the fall in sodium in that range was even greater. The 4 mEq/L gap between the two results shows why the factor chosen must be stated when the value is recorded.

The example is didactic. Reading a real case takes into account volume status, kidney function, measured osmolality and ongoing treatment, and is the responsibility of the professional caring for the patient.

How to understand the corrected sodium result

Corrected sodium tells whether the observed hyponatremia is explained by glucose or whether there is a sodium disorder beyond it. The tool does not classify the result: the valid reference interval is the one given on the report, which depends on the laboratory's method and population.

Comparing the corrected value with that interval, three general readings are possible:

• Low measured sodium and corrected sodium within the interval: the fall is compatible with the dilutional effect of glucose.
• Corrected sodium still low: there is hyponatremia beyond that caused by glucose.
• High corrected sodium: suggests a loss of free water greater than that of sodium, common in osmotic diuresis.

When the two factors lead to different readings, for example one within and one outside the interval, the result is in a zone of uncertainty and calls for other data, such as measured osmolality. Management, including the rate of sodium correction, is defined by the care team. The fractional excretion of sodium can complement the renal workup.

Common errors, units and verification checklist

The most common error is entering glucose in mmol/L in a field that asks for mg/dL. A glucose of 33.3 mmol/L entered as is becomes 33.3 mg/dL, below 100, and the tool applies no correction at all, because that value is still possible in mg/dL.

• Glucose in mg/dL: if the report is in mmol/L, multiply by 18 (molar mass of glucose about 180 g/mol). 33.3 mmol/L × 18 ≈ 600 mg/dL.
• Sodium: mEq/L and mmol/L give the same number.
• Same sample: sodium and glucose must come from the same collection; during insulin therapy, both change quickly.
• Factor recorded: note whether the value used was the 1.6 or the 2.4 per 100 mg/dL.
• Glucose above 400 mg/dL: according to Hillier and colleagues, the 1.6 factor tends to underestimate the correction in that range.
• Pseudohyponatremia: marked lipemia or very high proteins can lower sodium measured by an indirect method. This analytical error is not corrected by this formula.

If the question concerns effective osmolality, compare with the serum osmolality calculator.

Limitations and comparison between correction factors

Sodium correction is a model-based approximation, and the ideal factor is still debated. The table summarizes the two proposals cited on this page.

Other limitations to consider:

• Hillier's study had few participants, all healthy, and the response may differ in dehydrated patients or those with kidney disease.
• Solutes such as mannitol also shift water and lower sodium, but are not included in the formula.
• The correction does not replace osmolality measured by an osmometer when other solutes are in question.

In kidney disease, filtration by CKD-EPI helps understand the capacity to excrete water and sodium.

Frequently asked questions about sodium corrected for glucose

References

1. Katz MA. Hyperglycemia-induced hyponatremia: calculation of expected serum sodium depression. N Engl J Med. 1973;289(16):843-4. doi:10.1056/NEJM197310182891607. View
2. Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399-403. doi:10.1016/s0002-9343(99)00055-8. View