The impact of temperature and urinary constituents on urine viscosity and its relevance to bladder hyperthermia treatment

Abstract Purpose: The aim of this study was to determine the kinematic viscosity of human urine and factors associated with its variability. This value is necessary for accurate modelling of fluid mechanics and heat transfer during hyperthermia treatments of bladder cancer. Materials and methods: Urine samples from 64 patients undergoing routine clinical testing were subject to dipstick urinalysis and measurement of viscosity with a Cannon-Fenske viscometer. Viscosity measurements were taken at relevant temperatures for hyperthermia studies: 20 °C (room temperature), 37 °C (body temperature), and 42 °C (clinical hyperthermia temperature). Factors that might affect viscosity were assessed, including glucosuria, haematuria, urinary tract infection status, ketonuria and proteinuria status. The correlation of urine specific gravity and viscosity was measured with Spearman’s rho. Results: Urine kinematic viscosity at 20 °C was 1.0700 cSt (standard deviation (SD) = 0.1076), at 37 °C 0.8293 cSt (SD = 0.0851), and at 42 °C 0.6928 cSt (SD = 0.0247). Proteinuria appeared to increase urine viscosity, whereas age, gender, urinary tract infection, glucosuria, ketonuria, and haematuria did not affect it. Urine specific gravity was only modestly correlated with urine viscosity at 20 °C (rho = 0.259), 37 °C (rho = 0.266), and 42 °C (rho = 0.255). Conclusions: The kinematic viscosity of human urine is temperature dependent and higher than water. Urine specific gravity was not a good predictor of viscosity. Of factors that might affect urine viscosity, only proteinuria appeared to be clinically relevant. Estimates of urine viscosity provided in this manuscript may be useful for temperature modelling of bladder hyperthermia treatments with regard to correct prediction of the thermal conduction effects.