Sodium is a crucial electrolyte within the human body, playing a significant role in maintaining proper fluid balance across cell membranes. Beyond fluid regulation, it is also involved in the normal functioning of nerves and muscles. Sodium levels in the blood are routinely measured as part of standard health assessments. While these measured levels provide valuable information, they sometimes require adjustment to accurately reflect the body’s true sodium status.
Understanding Measured Sodium
The directly measured serum sodium concentration may not always reflect the body’s true physiological sodium status. This discrepancy occurs when high levels of other substances, such as glucose, are present in the blood. When blood glucose levels become significantly elevated, a condition known as hyperglycemia, it increases the concentration of dissolved particles in the blood outside of cells. This draws water out of the body’s cells and into the bloodstream to equalize osmotic pressure.
This dilutes the sodium, making the measured level appear lower than it truly is, a phenomenon sometimes referred to as dilutional hyponatremia or pseudohyponatremia. The measured sodium value is artificially lowered because the laboratory test reports sodium concentration per total volume of plasma, which now contains a higher proportion of water. This effect is distinct from a true deficit of sodium in the body.
The Corrected Sodium Formula
To account for the dilutional effect of high blood glucose on measured sodium, a corrected sodium formula is used. The formula for correcting sodium in the presence of hyperglycemia is: Corrected Sodium = Measured Sodium + [0.016 (Glucose – 100)].
In this formula, “Measured Sodium” refers to the sodium concentration reported by the laboratory, typically in milliequivalents per liter (mEq/L) or millimoles per liter (mmol/L). “Glucose” represents the measured blood glucose level in milligrams per deciliter (mg/dL). The constant 0.016 reflects that for every 100 mg/dL increase in glucose above 100 mg/dL, the serum sodium concentration is expected to decrease by approximately 1.6 mEq/L due to dilution.
For example, if a patient has a measured sodium of 130 mEq/L and a glucose level of 500 mg/dL, the calculation would be: Corrected Sodium = 130 + [0.016 (500 – 100)]. This simplifies to Corrected Sodium = 130 + [0.016 400], which equals 130 + 6.4. Therefore, the corrected sodium would be 136.4 mEq/L.
When to Apply the Correction
The primary instance where this correction is applied is in cases of hyperglycemia, or high blood sugar. Beyond hyperglycemia, other conditions can also lead to a falsely low measured sodium, known as pseudohyponatremia.
These include severe hyperlipidemia, which is an unusually high concentration of fats in the blood, and hyperproteinemia, an excessive amount of protein in the blood. In these scenarios, the large volume occupied by lipids or proteins in the blood sample can displace the water component, where sodium is dissolved. This displacement leads to a lower measured sodium concentration, even though the actual concentration of sodium in the plasma water is normal.
Interpreting Corrected Sodium Levels
The calculated corrected sodium value offers a more precise understanding of the body’s true sodium status, guiding accurate clinical assessment and management. A normal range for serum sodium typically falls between 135 and 145 mmol/L or mEq/L, though this can vary slightly between laboratories.
Once corrected, a sodium level within this normal range, even if the initial measured sodium was low, suggests that the apparent low sodium was primarily due to the dilutional effect of high glucose or other substances. Conversely, if the corrected sodium level remains below 135 mEq/L, it indicates true hyponatremia, meaning a genuine deficit of sodium relative to water in the body. Similarly, a corrected sodium level above 145 mEq/L would suggest true hypernatremia.