Is urinary density an adequate predictor of urinary osmolality?

BACKGROUND: Urinary density (UD) has been routinely used for decades as a surrogate marker for urine osmolality (U(osm)). We asked if UD can accurately estimate U(osm) both in healthy subjects and in different clinical scenarios of kidney disease. METHODS: UD was assessed by refractometry. U(osm) was measured by freezing point depression in spot urines obtained from healthy volunteers (N = 97) and in 319 inpatients with acute kidney injury (N = 95), primary glomerulophaties (N = 118) or chronic kidney disease (N = 106). RESULTS: UD and U(osm) correlated in all groups (p < 0.05). However, a wide range of U(osm) values was associated with each UD value. When UD was ≤ 1.010, 28.4% of samples had U(osm) above 350 mOsm/kg. Conversely, in 61.6% of samples with UD above 1.020, U(osm) was below 600 mOsm/kg. As expected, U(osm) exhibited a strong relationship with serum creatinine (S(creat)), whereas a much weaker correlation was found between UD and S(creat). CONCLUSION: We found that UD is not a substitute for U(osm). Although UD was significantly correlated with U(osm), the wide dispersion makes it impossible to use UD as a dependable clinical estimate of U(osm). Evaluation of the renal concentrating ability should be based on direct determination of U(osm).