Variations in daily urine solute output in patients with NDI, CDI, SIADH, and NSIAD: Clinical implications.

Introduction: We aimed to study the effects of daily variation in solute intake on urine volume in patients with syndrome of inappropriate secretion of antidiuretic hormone secretion (SIADH), syndrome of nephrogenic antidiuresis (NSIAD), central diabetes insipidus (CDI), or nephrogenic diabetes insipidus (NDI).
Materials and Methods: In 6 patients with CDI, 7 patients with NDI, 7 patients with SIADH, and 2 patients with NSIAD we had the opportunity to have 24-hour urine collections during normal diet before any treatment. We had two 24-hour urine collections with a difference of at least 20% in solute output (measured by the formula urine osmolality × urine volume). In 1 patient with NDI taking a high protein diet we analyzed the effect of a normal protein intake on diuresis. With respect to patients with NDI and CDI we included only patients with a urine osmolality lower than 110 mOsm/kgH ₂ O, and for SIADH/NSIAD we included only patients with a urine osmolality between 500 and 700 mOsm/kgH ₂ O.
Results: When the data of the patients with CDI/NDI were pooled, a high correlation between urine volume and solute output was observed (R = 0.83; p < 0.001). In 1 patient with X-linked NDI, we decreased urine volume simply by decreasing protein intake. If we pooled the data concerning SIADH/NSIAD, a correlation was observed between urine volume and solute output (R = 0.94; p < 0.001). As expected, increasing solute intake is beneficial in SIADH/NSIAD, while decreasing it decreases diuresis in NDI.
Conclusion: Daily variations in solute output affect urine volume in NDI/CDI/SIADH/NSIAD, this could affect serum sodium (SNa) despite no variation in fluid intake. In SIADH, if solute intake is lower than usual for a few days it may significantly influence the SNa level despite no variation in fluid intake.