Objective: To investigate the association between urine-specific gravity and oncological outcomes in patients with non-muscle-invasive bladder cancer. Methods: We identified 433 primary non-muscle-invasive bladder cancer patients who underwent transurethral resection between 2002 and 2016. The association between urine-specific gravity and tumor recurrence was statistically evaluated. Results: A total of 211 (48.7%) patients received adjuvant bacillus Calmette–Guérin therapy. During the median follow-up period of 60 months, 155 (35.8%) patients experienced at least one tumor recurrence. Of them all, 95 (21.9%) and 338 (78.1%) patients had high (>1.020) and low (≤1.020) urine-specific gravity, respectively. The Kaplan–Meier curve suggested that recurrence-free survival was significantly lower in patients with a high urine-specific gravity; however, the multivariate analysis failed to show that urine-specific gravity is significantly associated with tumor recurrence. In 222 (51.3%) patients who had not received bacillus Calmette–Guérin therapy, the Kaplan–Meier curve also suggested that recurrence-free survival was significantly lower in patients with a high urine-specific gravity. Multivariate analysis showed that age >70 years (hazard ratio 1.69, P = 0.02), grade 3 tumor (hazard ratio 1.81, P = 0.03) and high urine-specific gravity (hazard ratio 1.87, P < 0.01) were independent risk factors for tumor recurrence. Conclusion: High urine-specific gravity is an independent risk factor for tumor recurrence in non-muscle-invasive bladder cancer patients who have not received bacillus Calmette–Guérin therapy. Our results suggest that hydration status might have some clinical impacts on bladder tumor recurrence.
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