We thank Dr Koopman for his interesting observations.1 Consistent with our findings in the chronic kidney disease population,2 he found that urine osmolality was associated with mortality only when the analysis was not adjusted for confounders. The association faded when adjusted for age, sex, and ethnicity. Maximal urine concentrating ability decreases with age, possibly explaining these results.3 A longer follow-up might also be necessary to predict mortality. These data show first that urine osmolality may be a specific biomarker of the kidney because optimal urine concentrating ability requires tubular, peritubular capillary, and interstitial integrity.