Methods for studying adverse events on surgical wait lists

Abstract

Studies on access to health care are often interested in determining the relationship between the length of delay and the risk of adverse events while waiting. When access to elective surgery is rationed by wait lists, patients requiring and suitable for the operation are subject to competing events of surgery, pre-operative death, unplanned emergency admission or cancellation of surgery for other reasons. Methodologically, measuring the probability of adverse events in patients on wait lists for treatment is similar to quantifying the risk of an event during follow-up in a population exposed to competing risks. Although the Kaplan–Meier method is commonly used to summarize the probability of a primary event, such as death before surgery or unplanned emergency admission, over wait-list time, the estimates produced by this method are only interpretable in a hypothetical situation when all competing risks are eliminated without altering the risk of primary event. In this paper, we consider two other measures for summarizing wait-list data—cumulative incidence and conditional probability function of a competing event. To describe application of these methods in studying outcomes of delaying on surgical wait lists, we used data from a population-based prospective registry which collects the information on access to elective bypass surgery. The summary probabilities were quantified separately in two groups: high-severity at presentation were patients with either persistent unstable angina or stable angina and extensive coronary artery disease, and low-severity at presentation were stable symptomatic patients with limited disease. Our results suggest that the protracted delay for surgical revascularization carries a significant risk of death even in cardiac surgery patients judged to be low-severity at presentation.