Sodium correction for hyperglycaemia

Clinical background

In hyperglycaemia, glucose, which is confined to the extracellular compartment, draws water out of the intracellular compartment and thereby dilutes the serum sodium concentration. This phenomenon is called translocational hyponatraemia and is not a reduction in total body sodium but a pure distribution disturbance. The clinical difficulty arises in two situations: first, in assessing a patient with diabetic ketoacidosis or a hyperosmolar hyperglycaemic state, where one needs to know whether the sodium is genuinely low or merely diluted; and second, during treatment monitoring, when the glucose falls and the sodium rises as a purely physical consequence, without the patient having been given any sodium. Without correction, one risks misreading a dilutional effect as a sodium deficit and giving hypotonic fluid that can drive the osmolality down too quickly, with a risk of cerebral oedema and osmotic demyelination.

Calculating the sodium correction in hyperglycaemia

The calculator uses Katz's formula:

$$\text{Corrected Na} = \text{Measured Na} + 1{.}6 \times \frac{\text{Plasma glucose (mg/dL)} - 100}{100}$$

Here measured sodium is the laboratory value in mmol/L and plasma glucose is given in mg/dL. The factor of 1.6 mmol/L per 100 mg/dL of glucose above 100 mg/dL was derived by Katz in 1973 from a theoretical calculation of the osmotic water shift in hyperglycaemia [1]. This was not an empirical study but a physical derivation based on the assumption that glucose is an effectively extracellular osmole and that water distributes according to known physiological volumes.

Hillier et al. tested the factor experimentally in 1999 in a physiological study of six healthy volunteers in whom endogenous insulin secretion was blocked with somatostatin and glucose was raised above 600 mg/dL by intravenous glucose infusion [2]. The measured fall in serum sodium averaged 2.4 mmol/L per 100 mg/dL rise in glucose, significantly greater than Katz's factor of 1.6 (P = 0.02). The relationship was moreover non-linear: up to 400 mg/dL the factor of 1.6 worked well, but above a glucose of 400 mg/dL a factor of 4.0 was more accurate. The calculator therefore also reports Hillier's factor of 2.4 as an alternative, particularly in marked hyperglycaemia.

Interpretation in practice

The corrected sodium value should be interpreted as a measure of serum tonicity, not as a measure of sodium deficit. It indicates what the sodium concentration would be if the glucose normalised without sodium or water being added or removed.

One important application is to anticipate the rise in sodium during treatment. As the glucose falls, water redistributes into the intracellular compartment and the serum sodium rises automatically. According to the JBDS guideline for the hyperosmolar hyperglycaemic state, a fall in glucose of 5.5 mmol/L (100 mg/dL) corresponds to a rise in sodium of approximately 2.4 mmol/L [3]. If the sodium rises faster than this, it suggests inadequate fluid replacement; if it rises more slowly, it may indicate fluid overload. The corrected sodium should therefore be followed serially, not calculated once.

Validation and performance

Katz's original factor rests on a theoretical derivation without empirical validation [1]. Hillier's experimental study in six healthy individuals is the only controlled investigation of the factor, and it showed that 1.6 underestimates the true fall, particularly at a glucose above 400 mg/dL [2]. A factor of 2.4 was a better average estimate across the whole glucose range, and at very high glucose values 4.0 was more accurate. The non-linear relationship means that no single factor is optimal across the whole range.

In a review of dysnatraemia in the intensive care unit, Overgaard-Steensen and Ring state that sodium falls by approximately 0.4 mmol/L per mmol/L rise in glucose, corresponding to Hillier's factor of 2.4 per 100 mg/dL, and recommend this for clinical use [4]. The British JBDS guideline for HHS also uses 2.4 in its calculations and examples [3]. Katz's factor of 1.6 nonetheless persists in many textbooks and laboratory reports, which creates a risk of systematically underestimating the true sodium concentration in marked hyperglycaemia.

Limitations

The correction applies only to hyperglycaemia as the cause of translocational hyponatraemia. Other extracellular osmoles, above all mannitol, can produce an analogous effect that the formula does not capture. Pseudohyponatraemia in hyperlipidaemia or hyperproteinaemia is an entirely different mechanism, in which the plasma is normally tonic but the measurement method errs, and the correction must not be applied there.

The greatest pitfall is to treat the corrected value as a treatment target rather than a diagnostic marker. The sodium should not be actively raised to the corrected value by giving sodium. The corrected value only indicates the direction in which the sodium will move as the glucose falls. The actual rise is governed by fluid balance, and if it occurs too rapidly there is a risk of osmotic demyelination, particularly in patients with malnutrition, alcoholism, liver failure or hypokalaemia [4]. In intensive care it is generally recommended that sodium correction should not exceed 10 to 12 mmol/L per 24 hours in hyponatraemia, whatever the cause [4].

A further limitation is that Hillier's study was conducted in six healthy individuals during acute, experimentally induced hyperglycaemia over one hour [2]. Clinical hyperglycaemia in DKA or HHS develops over hours to days and involves insulin deficiency, ketone production and osmotic diuresis, which may affect water and sodium balance in ways the experimental model does not capture. The factor is therefore an approximation, not an exact prediction.

Units and practical application

Many laboratories report plasma glucose in mmol/L rather than in mg/dL. The calculator handles the unit conversion internally, but for manual calculation the glucose must be converted: $\text{mg/dL} = \text{mmol/L} \times 18{.}018$. Intensive care practice for DKA and HHS broadly follows international guidelines, in which osmolality, not sodium alone, is the guiding measure. The target values for the fall in osmolality (3 to 8 mOsm/kg/hour) and in glucose according to the JBDS guideline [3] are applicable in this setting.

References

1. Katz MA. Hyperglycemia-induced hyponatremia: calculation of expected serum sodium depression. N Engl J Med. 1973;289(16):843–4. PMID: 4763428
2. Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399–403. PMID: 10225241
3. Mustafa OG, Haq M, Dashora U et al. Management of Hyperosmolar Hyperglycaemic State (HHS) in Adults: An updated guideline from the Joint British Diabetes Societies (JBDS) for Inpatient Care Group. Diabet Med. 2023;40(3):e15005. PMID: 36370077
4. Overgaard-Steensen C, Ring T. Clinical review: practical approach to hyponatraemia and hypernatraemia in critically ill patients. Crit Care. 2013;17(1):206. PMID: 23672688