The risk of round numbers and sharp thresholds in clinical practice

Summary

Mortality risks of pneumonia patients exhibit surprising statistical patternsTo investigate and categorize the real-world impact of clinical decision-making thresholds, we analyzed mortality risks using real-world data to find surprising statistical patterns in patient outcomes. We conducted this exploratory data analysis by training generalized additive models (GAMs)3 to estimate mortality risk as the additive effects of individual risk factors. The GAMs demonstrated high predictive accuracy, comparable to fully-connected neural networks and XGBoost baselines (Supplementary Table 2), and provided high-resolution, nonlinear, component functions that can be sequentially analyzed. Our analysis used the 1989 MedisGroups Comparative Hospital Database (MCHD) pneumonia dataset4. We see two repeated statistical patterns that routinely indicate underlying treatment effects: (1) discontinuities in risk, and (2) counter-causal non-monotonicities in risk.Discontinuities in risk often reveal threshold-based treatment effects in clinical data. Blood urea nitrogen (BUN) exhibits a discontinuous association with mortality risk (Fig. 3a), with a non-monotone structure. Intrinsic (i.e., untreated) risk is likely to increase smoothly with increasing BUN, so this discontinuous population risk suggests that different treatments are being applied at different BUN levels. Lower levels of BUN are associated with survival; however, there is a rapid rise in mortality risk from 30–40 mg/dL followed by a plateau in risk from 40–80 mg/dL. The pattern of risk at BUN of 40 mg/dL is stronger for male patients than female patients, with the risk for female patients rising continuously from 30–50 mg/dL, suggesting a stronger adherence to threshold-based decisions for male patients than for female patients. Similarly, systolic blood pressure shows a discontinuous association with mortality risk (Fig. 3b). There is a sharp discontinuous rise in risk as systolic blood pressure decreases below 80 mmHg,...