The relationship between urine osmolality and specific gravity

Background. In general, there is a good correlation between the specific gravity and osmolality of a urine sample. In certain clinical conditions, such as uncontrolled diabetes mellitus, nephrotic syndrome, after the administration of intravenous radiocontrast material or saline diuresis, dependence upon specific gravity for determining the concentrating ability will result in over- or underestimation. Methods: We studied the relationship between specific gravity and osmolality in vitro with simulated urines of varying composition. Urine samples from patients with different clinical conditions were also analyzed. Results: The in vitro curves for sodium chloride, urea, creatinine, glucose, contrast dye, and albumin were plotted (specific gravity versus osmolality). We found a linear correlation between the specific gravity and osmolality of the 6 substances that were studied and for their combinations. The urine samples obtained from patients with different clinical conditions documented that reliance on specific gravity could over- or underestimate the urine osmolality. Conclusions: We concluded that in those clinical conditions, urine osmolality should always be determined and it should not be estimated based on specific gravity.