Evaluation of an automatic urinometer including use of silicone oil to decrease biofilm formation due to proteinuria, hemoglobinuria and bacterial growth

Abstract: Background: A new capacitance-based automatic urinometer (AU) facilitates continuous urine output (UO) measurement, which may help to predict and diagnose acute kidney injury (AKI). To prevent mismeasurement due to bacterial, albumin or free hemoglobin biofilm, a water-soluble capsule with silicone oil has been integrated in the device. Aims: To assess: the performance of a new capacitance-based AU in adult patients in a cardiothoracic intensive care unit (ICU) and compare it with a manual urinometer (MU) in regard of bias, precision, temporal deviation and to evaluate the staff’s opinion of the AU (Study I); a modified capacitance-based AU in comparison with an MU regarding measuring bias among patients ≤10 kg in a pediatric intensive care unit and to evaluate the staff’s opinion of the AU (Study II); whether a silicone oil-coated polypropylene plastic surface, as used in an AU, may reduce early microbial biofilm formation and to identify the silicone oil target; to compare polypropylene with polystyrene and low with medium viscosity silicone oil regarding the propensity to impede biofilm formation (Study III); if silicone oil added to the measuring chamber of the AU may prevent the rise in capacitance due to albumin or free hemoglobin biofilm, allowing the device to function for longer periods of time (Study IV). Methods: Study I-II were prospective observational cohort studies, whereas Study III-IV were experimental prospective in vitro studies. Study I: 34 postoperative patients had their hourly UO registered with either an AU (n=220) or an MU (n=188), which were validated by cylinder measurements and analyzed using the Bland-Altman method. The temporal deviation of the MU measurements was recorded (n=108) and at the end, the nursing staff (n=28) evaluated the AU. Study II: The hourly diuresis was measured using either an AU (n=127) or an MU (n=83) in 12 children (weight ≤10 kg) and validation was carried out using a measuring cylinder. Thereafter, the nursing staff (n=18) evaluated the AU. Study III: Clear flat-bottomed wells of either polypropylene or polystyrene were pretreated with silicone oil of low or medium viscosity, after which a panel of microbes, including common uropathogenic bacteria and Candida albicans, were added. The plates were left for 3 days and the amount of biofilm formation was assessed using the crystal violet assay. Study IV: A solution of Ringer’s acetate mixed with either albumin or free hemoglobin was run through an AU with either a water-soluble capsule with silicone oil (n=20) or not (n=20) and the derived 400-500 capacitance measurements, respectively, were retrieved from the AU device and analyzed. Results: Study I: The AU had a smaller mean bias (+1.9 mL) than the MU (+5.3 mL) (p<0.0001). Defined by their limits of agreements (±15.2 mL AU vs. ±16.6 mL MU, p=0.11), the measurement precision of the two urinometers were similar. The AU had inherently no temporal deviation, whereas the mean temporal deviation of the MU was ±7.4 minutes (±12.4%) (p<0.0001). The nursing staff rated the AU significantly higher than the MU in terms of user-friendliness, measuring reliability, efficacy and safety. Study II: The AU and the MU had a mean bias of −1.1 mL (CI, -0.6 to -1.5) and -0.6 mL (CI, ±0.0 to -1.2) respectively (p=0.21). The participating staff considered the AU significantly easier to learn, use and handle compared with the MU. Study III: Polypropylene plastic exhibited less biofilm growth than polystyrene. Silicone oil, irrespective of viscosity, significantly decreased biofilm formation by common uropathogenic bacteria, including ESBLproducing and multi-drug resistant strains, as well as C. albicans. E. coli curli fimbriae were established as the main focus of silicone oil. Study IV: The mean increase in capacitance with albumin 3 g/L group was 257±96 without and 105±32 with silicone oil, respectively, during 24 hours. After ten hours of registration, differences between the two albumin groups reached statistical significance. For the free hemoglobin groups (0.01 g/L), the mean increase in capacitance was 190±174 with silicone oil and 324±78 without. A significant difference between the free hemoglobin groups was seen after 20 hours and onwards. Conclusions: For adult postoperative patients, the AU was non-inferior to the MU with regard to measuring precision and significantly better than the MU in terms of bias and temporal deviation (Study I); for children weighing ≤10 kg, the urinometers were comparable in performance (Study II); staff consistently appraised the AU significantly higher than the MU in terms of user-friendliness, reliability, safety and efficacy (Study I and II). Both low and medium viscosity silicone oil coating of a polypropylene surface decreased biofilm formation from common uropathogenic bacteria including Candida albicans and the biofilm-promoting factor curli fimbriae was identified as a plausible target (Study III); coating of the capacitance measurement membrane of the AU by albumin or free hemoglobin significantly disturbed the capacitance measurement capability of the AU, and this could be prevented by incorporating silicone oil in the device (Study IV).