Can we trust the math: A quality assurance project evaluating validity of postvoid residual estimation by subtraction

Drewry A1, Sergeant A1, Giles D L1, Brown H1, Heisler C A1

Research Type

Clinical

Abstract Category

Anatomy / Biomechanics

Abstract 698
Non Discussion Abstracts
Scientific Non Discussion Abstract Session 36
Prospective Study Voiding Dysfunction Surgery
1. University of Wisconsin-Madison Department of Obstetrics and Gynecology
Links

Abstract

Hypothesis / aims of study
A normal postvoid residual (PVR), defined as the volume of urine left in the bladder at the completion of micturition, ranges from 30 – 100 mL, with drainage via a short, female plastic catheter providing the most effective measurement [1]. In urogynecology, PVR is often estimated using subtraction in retrograde fill voiding trials (VTs) if void occurs immediately after (within ten minutes of) bladder filling and voided volume does not exceed volume instilled by more than 50 mL [2]. Our objective was to investigate the validity of PVR assessed via subtraction in postoperative urogynecology patients.
Study design, materials and methods
For this quality assurance project, we collected prospective data from postoperative patients with indwelling catheters who presented to our urogynecology clinic for retrograde fill VT. All included patients had undergone a retrograde fill VT on post-operative day zero or one and had an elevated PVR, for which they opted to have indwelling Foley catheter placed rather than intermittent self-catheterization. VTs were performed by a urogynecology clinic nurse using our standard practice: 1) The drainage bag is placed inferior to the patient’s bladder and allowed to drain to gravity until no additional urine drains; 2) The drainage bag is removed and an open 60 mL catheter-tip syringe is connected to the Foley; 3) Sterile normal saline is instilled via gravity to patient comfort or 300 mL; 4) The Foley catheter is removed; 5) The patient voids in a bladder hat on a standard toilet; 5) Urine is transferred from a bladder hat to a graduated cylinder to confirm voided volume. In our standard practice, PVR is calculated via subtraction (volume instilled – volume voided = PVR) and catheterization is not routinely performed. For this quality improvement project, we assessed PVR via subtraction and then using ultrasound followed by bladder drainage using a short, female plastic catheter. We considered a PVR of <100 to be normal. We categorized PVR interpretations according to subtraction versus catheterization to ascertain the validity of PVR assessed by subtraction when compared to the most effective existing method.
Results
We evaluated seven post-operative urogynecology patients during this month long quality assurance project. PVR calculated by subtraction was within 100 mL of PVR obtained by catheterization in 5/7 (71%), but the other two patients had discrepancies of 250 mL and 140 mL. The interpretation of the PVR as normal versus elevated changed for three patients (43%) when assessing with catheter versus subtraction. Using subtraction alone to assess PVR would have resulted in diagnosing two patients with normal PVR whose true PVRs were elevated (175 mL and 190 mL).
Interpretation of results
Even with a limited sample size, this quality assurance project demonstrates the limitations of using the subtraction method to assess PVR. The two patients with elevated PVRs by catheterization but not by subtraction would have been misclassified in the algorithm used in the Trial Of Mid-Urethral Slings (TOMUS).
Concluding message
Subtraction does not provide a valid assessment of PVR for post-operative urogynecology patients in our clinic. We suggest that a more rigorous research approach be applied to this question.
Figure 1
References
  1. Haylen, B.T., de Ridder, D., Freeman, R.M. et al. An International Urogynecological Association (IUGA)/International Continence Society (ICS) joint report on the terminology for female pelvic floor dysfunction. Int Urogynecol J (2010) 21:5. https://doi.org/10.1007/s00192-009-0976-9
  2. Ferrante KL, Kim H-Y, Brubaker L, et al. Repeat Post-Op Voiding Trials: An Inconvenient Correlate with Success. Neurourology and urodynamics. (2014) 33(8):1225-1228. doi:10.1002/nau.22489.
Disclosures
Funding None Clinical Trial No Subjects Human Ethics not Req'd This study incurred minimal risk and no harm to patients as it was an anonymous abstraction clinical data from the medical record. Helsinki Yes Informed Consent No
27/03/2024 16:57:44