Pseudohypernatremia and pseudohyponatremia: a linear correction.

Area of Science:

• Clinical Chemistry
• Laboratory Medicine
• Critical Care Medicine

Background:

• Serum sodium is measured by direct (DNa) and indirect (INa) potentiometry.
• Non-water fraction abnormalities cause pseudohyponatremia or pseudohypernatremia.
• Previous corrections for non-water bias using total protein (TP) or albumin (ALB) have been inconsistent.

Purpose of the Study:

• To quantify the effects of TP and ALB on the difference between INa and DNa (ΔNa).
• To develop a reliable correction for non-water bias in INa measurements.

Main Methods:

• Analyzed 774 critical care patients with paired metabolic and blood gas panels.
• Quantified the impact of TP and ALB on ΔNa, adjusting for other variables.
• Employed multiple linear regression to establish correction factors.

Main Results:

• ΔNa varied inversely with TP, ALB, and glucose difference (ΔGlu), and directly with pH and bicarbonate.
• The effect of TP on ΔNa was linear, decreasing by 0.64 mEq/L per 1 g/dL TP increase.
• A slightly steeper TP effect (0.69 mEq/L) was observed when adjusted for bicarbonate.

Conclusions:

• A simple linear correction using TP can adjust INa for non-water bias.
• Clinicians should adjust INa by 0.7 mEq/L in the same direction as TP changes.
• This correction improves the accuracy of INa in critical care settings.