# Unmeasured Osmoles: The Hidden Solutes Obscuring a Hyponatremia Evaluation

## TL;DR

This paper discusses how unmeasured solutes like ethanol can interfere with diagnosing hyponatremia, a condition involving low blood sodium.

## Contribution

The paper introduces a correction equation for ethanol's osmotic effect in hyponatremia evaluations.

## Key findings

- A patient's hyponatremia evaluation was complicated by unmeasured ethanol in the serum.
- Ethanol can falsely alter serum and urine osmolality measurements.
- A mathematical correction can improve diagnostic accuracy in such cases.

## Abstract

Hyponatremia is defined as serum sodium less than 135 mEq/L and is principally a result of water excess relative to total body sodium content. The evaluation of hyponatremia is incomplete without a careful assessment of the patient’s volume status, history, and acquisition of both serum and urine osmolality and sodium studies. Many of these studies can be affected by various clinical factors, and these nuances should be considered while interpreting the results. This is because these results guide the etiologic diagnosis of hyponatremia and consequently its management. In this report, we describe a 50-year-old male being evaluated for hyponatremia found to have unusual serum/urine osmolality studies but ultimately found to have an unmeasured serum osmole (ethanol) interfering with the interpretation of these results. Clinical scenarios that interfere with serum and urine studies commonly obtained in a hyponatremia evaluation are reviewed and an equation to correct for ethanol's osmotic contribution is described.