Integrating Physiology Into Hyponatremia Algorithms: Moving Beyond Volume Status.

Hyponatremia is a prevalent electrolyte disorder with a broad differential diagnosis, which can pose a challenge for determining the underlying etiology. Academic resources teach two diagnostic frameworks designed to guide a physician to the underlying etiology. One approach uses volume status and serum osmolality as earlier branch points, with later consideration of urine osmolality and urine sodium for diagnosis within certain pathways in the branching logic. The other uses serum osmolality, urine osmolality, and spot urine sodium to guide the entire diagnostic reasoning process. These two approaches can ultimately guide clinicians to different diagnoses for the same patient. In this case series, we present an adapted physiology-based (European) algorithm and describe three patients who illustrate key shortcomings of the volume status-based approach to identifying the etiology of hyponatremia. The first case emphasizes that understanding the relationship between osmolality and tonicity adds important nuance to the interpretation of measured serum osmolality. The second case demonstrates how the inherent limitations of the physical examination for volume status contribute to imprecision and inter-individual variation in volume status assessment and underscores the importance of interpreting laboratory values with nuance rather than relying rigidly on strict cutoff values as branch points in clinical reasoning. The third case illustrates that more complex presentations may not fit neatly within any single algorithm and require more nuanced clinical judgment. Together, these cases demonstrate how a diagnostic approach to hyponatremia rooted in physiology provides learners with greater insight into the underlying pathophysiology and may improve diagnostic accuracy.