Determination of red blood cells in urinary sediment: Do pH and specific gravity of urine matter?

Introduction: It has been postulated that erythrocyte cell lysis in urine is common in the case of low urine specific gravity and high urine pH. We aimed to verify the influence of urine dipstick pH and specific gravity on erythrocyturia in the urine sediment in the case of positive dipstick hemoglobinuria. Methods: We retrospectively collected data on dipstick specific gravity, pH, and urine sediment analysis done by nephrologists in the clinical and research urine laboratory at the Department of Nephrology, University Medical Centre Ljubljana. Results: During the 6-year observation period, we analyzed 843 second morning midstream urine samples with positive dipstick hemoglobinuria. Erythrocyturia in urinary sediment was detected significantly less often in urine samples with concomitant hemoglobinuria 1+ and pH < 6.0, (odds ratio (OR) 0.40; 95% confidence interval (CI) 0.21 - 0.76, p = 0.005). The difference was maintained in multivariate analysis including patient age, gender, and specific gravity (OR 0.32; 95% CI 0.15 - 0.65, p = 0.002). In samples with higher grade of hemoglobinuria (>= 2+), the impact of cell lysis in the case of low pH was negligible. Specific gravity did not have any influence on erythrocyte detection in urinary sediment. Conclusions: Urinary pH < 6.0 impaired the detection of erythrocytes in urinary sediment, while the dipstick measured urine specific gravity did not have any impact on it. Urine samples with low-grade hemoglobinuria and low pH without erythrocyte detection in urinary sediment should be evaluated again to avoid false-negative results.