[Correction of urinary calcium levels for urine osmotic pressure].

The usefulness of the urinary calcium/creatinine (Ca/Cr) ratio, the oldest marker of bone resorption, is limited because of the influence of sodium intake and urine volume. The urinary Ca/Cr ratio was therefore compared with urinary calcium corrected for the urine osmotic pressure (Ca/OSM). Significant correlations are found between the Ca/Cr ratio and both creatinine (r = -0.386) and osmotic pressure (r = -0.473) in random urine samples from normal subjects, but similar correlations were not found for urine Ca/OSM. No significant correlation was found between Ca/OSM and Na/OSM, but the correlation coefficient between Ca/Cr ratio and Na/Cr ratio was 0.399. Thus, the Ca/Cr ratio is affected by urinary volume and sodium concentration, but the Ca/OSM ratio is not. The Ca/OSM ratio may therefore be a more accurate marker of bone resorption, because one of the main components of urinary osmotic pressure is sodium that inhibits tubular calcium reabsorption. The Ca/OSM ratios measured from 24-hour urine samples were found to be strongly correlated with the values measured from samples obtained during sleep (sleep urine, 0.823) and were slightly less strongly correlated with the values measured from second morning samples (0.641). Because bone resorption is especially active at night, the Ca/OSM ratio in sleep urine may be the most sensitive marker of bone resorption. The Ca/OSM ratio in sleep urine samples increased with age in normal women.