Reply to the Letter to the Editor: How Accurate Are the Surgical Risk Preoperative Assessment System (SURPAS) Universal Calculators in Total Joint Arthroplasty?

To the Editor, We appreciate the thoughtful feedback from Henderson et al. [4] regarding our study assessing the accuracy of the Surgical Risk Preoperative Assessment System (SURPAS) universal calculators in total joint arthroplasty (TJA) [5, 6]. We agree that comparisons of universal and procedure-specific surgical risk calculators are an important area of research—research that will ultimately inform which models are the most accurate for which procedures. We recognize that model accuracy needs to be balanced with other design criteria, such as simplicity and ease of use. It makes no sense to develop tools that are too burdensome for use. However, without knowing how much accuracy is sacrificed for simplicity, the balance is impossible to strike. We also agree that heterogeneity plays an important role in the calculation of the concordance statistic (C-index). Therefore, in a givenmodel, the C-index for the full sample is expected to be higher than the C-index for a subset of patients who are more homogeneous in model input variables. Nonetheless, we believe that quantifying how much the C-index declines for these patient subgroups remains important. At the extreme, if the group of individuals undergoing a procedure has no observed variation among the model inputs, we could expect the C-index to be 0.50 (no better than chance), meaning the model should not be used for that procedure. Henderson et al. [4] are correct that we should not assume procedurespecific models are more accurate than universal models. That needs to be carefully evaluated. SURPAS C-indexes for TJA were highly variable in our study. The TJA mortality model with a C-index of 0.82 demonstrates strong discrimination, despite having a lower C-index than the original SURPAS sample (0.94). Nonetheless, to our knowledge, the C-index of 0.82 is higher than that in all existing TJAspecific mortality models, and therefore represents the current benchmark to beat for developers of future TJAspecific models. We found considerably lower SURPAS C-indexes for other complications after TJA, such as venous thromboembolism (0.56), infection (0.62), and cardiac or transfusion events (0.62). Although not exactly comparable, more-accurate TJA-specific models for cardiac complications (minus transfusions) already exist and represent the current accuracy benchmarks [2, 3]. Defining the best subgroups that balance relevance to clinical practice, ease of use, and predictive accuracy remains an active area of investigation. As we discussed in our paper [6] and as Henderson et al. [4] highlight, calculating risk models for every Current Procedural Terminology-coded procedure in the American College of Surgeons National Surgical Quality Improvement Program (NSQIP) database will not be feasible. However, the NSQIP has developed procedure(RE: Trickey AW, Ding Q, Harris AHS. How accurate are the surgical risk preoperative assessment system (SURPAS) universal calculators in total joint arthroplasty? Clin Orthop Relat Res. 2020;478:241-251). Both authors certify that neither they, nor any members of their immediate families, have any commercial associations that might pose conflict of interest in connection with the submitted article. The opinions expressed are those of the writers, and do not reflect the opinion or policy of CORR or The Association of Bone and Joint Surgeons. A. W. Trickey (✉), 1070 Arastradero Rd., MC5552, Stanford, CA 94305 USA, Email: atrickey@stanford.edu