Urine specific gravity and water hardness in relation to urolithiasis in persons with spinal cord injury

Study design: A matched case-control study. Objectives: To clarify the influence of urine specific gravity and drinking water quality on the formation of urinary stones in persons with spinal co rd injury (SCI). Setting: A rehabilitation center within a university hospital. Methods: Between 1992 and 1998, 63 stone cases (31 kidney, 27 bladder, and five both) and 289 age-duration-matched controls were recruited from a coho rt of SCI patients enrolled in an on-going longitudinal study. Data on urin e specific gravity and other characteristics of study participants were ret rieved from the database and medical charts. Community water supply informa tion was provided by the Alabama Department of Environmental Management. Mu ltivariable conditional logistic regression analysis was performed to evalu ate the association with stone formation. Results: SCI individuals who had urinary stones were more likely than contr ol subjects to use indwelling catheters and have decreased renal function. The occurrence of stones was not significantly related to gender, race, sev erity of injury, urinary tract infection, nor urine pH. After controlling f or the potential confounding from other factors, a continuously increasing stone occurrence with increasing specific gravity was observed (P=0.05); th is association was stronger for kidney (odds Ratio [OR]= 1.8 per 0.010 g/cm (3)) versus bladder stones (OR 1.2) and for recurrent (OR= 2.0) versus firs t stones (OR= 1.5). Increased water hardness was not significantly associat ed with a decreased stone occurrence. Conclusions: Study results suggest that maintaining urine specific gravity below a certain level might reduce the occurrence of urinary stones. This c ould be easily achieved by using a dipstick for self-feedback along with ap propriate fluid intake. For persons with SCI who are at an increased risk o f a devastating stone disease, this prophylactic approach could be very cos t-effective; however, this requires further confirmation.