Performance and user evaluation of a novel capacitance-based automatic urinometer compared with a manual standard urinometer after elective cardiac surgery

Anton Eklund, Martin Slettengren, Jan van der Linden

In the intensive care setting, most physiologic parameters are monitored automatically. However, urine output (UO) is still monitored hourly by manually handled urinometers. This study evaluated an automatic urinometer (AU) and compared it with a manual urinometer (MU). This prospective study was carried out in the intensive care unit of a cardio-thoracic surgical clinic. In postoperative patients (n = 34) with indwelling urinary catheters and an expected stay of 24 hours or more, hourly UO samples were measured with an AU (Sippi®, Observe Medical, Gothenburg, Sweden, n = 220) or a MU (UnoMeter™ 500, Unomedical a/s, Birkeroed, Denmark, n = 188), and thereafter validated by cylinder measurements. Malposition of instrument at reading excluded measurement. Data were analyzed with the Bland-Altman method. The performance of the MU was used as minimum criteria of acceptance when the AU was evaluated. The loss of precision with the MU due to temporal deviation from fixed hourly measurements were recorded (n = 108). A questionnaire, filled out by the ward staff (n = 28), evaluated the ease of use of the AU compared with the MU. Bland-Altman analysis showed a smaller mean bias for the AU, +1.9 ml, compared with the MU, +5.3 ml (p < 0.0001). There was no statistical difference in measurement precision between the two urinometers, defined by their limits of agreement (±15.2 ml vs. ±16.6 ml, p = 0.11). The mean temporal variation with the MU was ±7.4 minutes (±12.4%), limits of agreement ±23.9 minutes (±39.8%), compared with no temporal variation with the AU (p < 0.0001). The ward staff considered the AU easy to learn and rated it higher than the MU (p < 0.0001). The AU was non-inferior to the MU and significantly better in terms of bias, temporal deviation and staff opinion, although the clinical relevance of these findings may be open to discussion.