Statistical Modeling and Aggregate-Weighted Scoring Systems in Prediction of Mortality and ICU Transfer: A Systematic Review

Risk of Bias Assessment

We scored the studies by adapting the Cochrane Collaboration tool for assessing risk of bias ³² (Appendix Table 5). Of the six studies, five received total scores between 1.0 and 2.0 (indicating relatively low bias risk), and one study had a score of 3.5 (indicating higher bias risk). Low bias studies^14,17-19,30 used large samples across multiple hospitals, discussed the choice of predictor variables and outcomes more precisely, and reported their measurement approaches and analytic methods in more detail, including imputation of missing data and model calibration.

In this systematic review, we assessed the predictive ability of EWSs using statistical modeling versus aggregate-weighted EWS models to detect clinical deterioration risk in hospitalized adults in general wards. From 2007 to 2018, at least five systematic reviews examined aggregate-weighted EWSs in adult inpatient settings.^33-37 No systematic review, however, has synthesized the evidence of EWSs using statistical modeling.

The recent evidence is limited to six studies, of which five had favorable risk of bias scores. All studies included in this review demonstrated superior model performance of the EWSs using statistical modeling compared with an aggregate-weighted EWS, and at least five of the six studies employed rigor in design, measurement, and analytic method. The AUC absolute difference between EWSs using statistical modeling and aggregate-weighted EWSs was 7% overall, moving model performance from fair to good (Table 2; Figure 2). Although this increase in discriminative power may appear modest, it translates into avoiding a 45% increase in WDR workload generated by an aggregate-weighted EWS, approximately two patient evaluations for each true positive case.

Results of our review suggest that EWSs using statistical modeling predict clinical deterioration risk with better precision. This is an important finding for the following reasons: (1) Better risk prediction can support the activation of rescue; (2) Given federal mandates to curb spending, the elimination of some resource-intensive false positive evaluations supports high-value care;³⁸ and (3) The Quadruple Aim³⁹ accounts for clinician wellbeing. EWSs using statistical modeling may offer benefits in terms of clinician satisfaction with the human–system interface because better discrimination reduces the daily evaluation workload/cognitive burden and because the reduction of false positive alerts may reduce alert fatigue.^40,41

Still, an important issue with risk detection is that it is unknown which percentage of patients are uniquely identified by an EWS and not already under evaluation by the clinical team. For example, a recent study by Bedoya et al.⁴² found that using NEWS did not improve clinical outcomes and nurses frequently disregarded the alert. Another study⁴³ found that the combined clinical judgment of physicians and nurses had an AUC of 0.90 in predicting mortality. These results suggest that at certain times, an EWS alert may not add new useful information for clinicians even when it correctly identifies deterioration risk. It remains difficult to define exactly how many patients an EWS would have to uniquely identify to have clinical utility.

Even EWSs that use statistical modeling cannot detect all true deterioration cases perfectly, and they may at times trigger an alert only when the clinical team is already aware of a patient’s clinical decline. Consequently, EWSs using statistical modeling can at best augment and support—but not replace—RRT rounding, physician workup, and vigilant frontline staff. However, clinicians, too, are not perfect, and the failure-to-rescue literature suggests that certain human factors are antecedents to patient crises (eg, stress and distraction,^44-46 judging by precedent/experience,^44,47 and innate limitations of human cognition⁴⁷). Because neither clinicians nor EWSs can predict deterioration perfectly, the best possible rescue response combines clinical vigilance, RRT rounding, and EWSs using statistical modeling as complementary solutions.

Our findings suggest that predictive models cannot be judged purely on AUC (in fact, it would be ill-advised) but also by their clinical utility (expressed in WDR and PPV): How many patients does a clinician need to evaluate?^9-11 Precision is not meaningful if it comes at the expense of unmanageable evaluation workloads, and our findings suggest that clinicians should evaluate models based on their clinical utility. Hospitals considering adoption of an EWS using statistical modeling should consider that externally developed EWSs appear to experience a performance drop when applied to a new patient population; a slightly higher WDR and slightly lower AUC can be expected. EWSs using statistical modeling appear to perform best when tailored to the targeted patient population (or are derived in-house). Model depreciation over time will likely require recalibration. In addition, adoption of a machine learning algorithm may mean that original model results are obscured by the black box output of the algorithm.^48-50

Findings from this systematic review are subject to several limitations. First, we applied strict inclusion criteria, which led us to exclude studies that offered findings in specialty units and specific patient subpopulations, among others. In the interest of systematic comparison, our findings are limited to general wards. We also restricted our search to recent studies that reported on models predicting clinical deterioration, which we defined as the composite of ICU transfer and/or death. Clinically, deteriorating patients in general wards either die or are transferred to ICU. This criterion resulted in exclusion of the Rothman Index,⁵¹ which predicts “death within 24 hours” but not ICU transfer. The AUC in this study was higher than those selected in this review (0.93 compared to 0.82 for MEWS; AUC delta: 0.09). The higher AUC may be a function of the outcome definition (30-day mortality would be more challenging to predict). Therefore, hospitals or health systems interested in purchasing an EWS using statistical modeling should carefully consider the outcome selection and definition.

Second, as is true for systematic reviews in general,⁵² the degree of clinical and methodological heterogeneity across the selected studies may limit our findings. Studies occurred in various settings (university hospital, teaching hospitals, and community hospitals), which may serve diverging patient populations. We observed that studies in university-based settings had a higher event rate ranging from 5.6% to 7.8%, which may result in higher PPV results in these settings. However, this increase would apply to both EWS types equally. To arrive at a “true” reflection of model performance, the simulations for PPV and WDR have used a more conservative event rate of 4%. We observed heterogenous mortality definitions, which did not always account for the reality that a patient’s death may be an appropriate outcome (ie, it was concordant with treatment wishes in the context of severe illness or an end-of-life trajectory). Studies also used different sampling procedures; some allowed multiple observations although most did not. The variation in sampling may change PPV and limit our systematic comparison. However, regardless of methodological differences, our review suggests that EWSs using statistical modeling perform better than aggregate-weighted EWSs in each of the selected studies.

Third, systematic reviews may be subject to the issue of publication bias because they can only compare published results and could possibly omit an unknown number of unpublished studies. However, the selected studies uniformly demonstrated similar model improvements, which are plausibly related to the larger number of covariates, statistical methods, and shrinkage of random error.

Finally, this review was limited to the comparison of observational studies, which aimed to answer how the two EWS classes compared. These studies did not address whether an alert had an impact on clinical care and patient outcomes. Results from at least one randomized nonblinded controlled trial suggest that alert-driven RRT activation may reduce the length of stay by 24 hours and use of oximetry, but has no impact on mortality, ICU transfer, and ICU length of stay.⁵³