Pre-operative Waterlow score and outcomes after kidney transplantation

Background Waterlow scoring was introduced in the 1980s as a nursing tool to risk stratify for development of decubitus ulcers (pressure sores) and is commonly used in UK hospitals. Recent interest has focussed on its value as a pre-op surrogate marker for adverse surgical outcomes, but utility after kidney transplantation has never been explored. Methods In this single-centre observational study, data was extracted from hospital informatics systems for all kidney allograft recipients transplanted between 1(st) January 2007 and 30(th) June 2020. Waterlow scores were categorised as per national standards; 0-9 (low risk), 10-14 (at risk), 15-19 (high risk) and >= 20 (very high risk). Multiple imputation was used to replace missing data with substituted values. Primary outcomes of interest were post-operative length of stay, emergency re-admission within 90-days and mortality analysed by linear, logistic or Cox regression models respectively. Results Data was available for 2,041 kidney transplant patients, with baseline demographics significantly different across Waterlow categories. As a continuous variable, the median Waterlow score across the study cohort was 10 (interquartile range 8-13). As a categorical variable, Waterlow scores pre-operatively were classified as low risk (n = 557), at risk (n = 543), high risk (n = 120), very high risk (n = 27) and a large proportion of missing data (n = 794). Median length of stay in days varied significantly with pre-op Waterlow category scores, progressively getting longer with increasing severity of Waterlow category. However, no difference was observed in risk for emergency readmission within 90-days of surgery with severity of Waterlow category. Patients with 'very high risk' Waterlow scores had increased risk for mortality at 41.9% versus high risk (23.7%), at risk (17.4%) and low risk (13.4%). In adjusted analyses, 'very high risk' Waterlow group (as a categorical variable) or Waterlow score (as a continuous variable) had an independent association with increase length of stay after transplant surgery only. No association was observed between any Waterlow risk group/score with emergency 90-day readmission rates or post-transplant mortality after adjustment. Conclusions Pre-operative Waterlow scoring is a poor surrogate marker to identify kidney transplant patients at risk of emergency readmission or death and should not be utilised outside its intended use.