Author Affiliations: Department of Surgery, University of Texas Medical Branch, Galveston (firstname.lastname@example.org).
To the Editor: Dr Domanski and colleagues1 reviewed 7 studies correlating creatine kinase (CK-MB) levels early after coronary artery bypass graft (CABG) surgery with survival. They concluded “mortality is an increasing function of CK-MB ratio” and “no threshold exists in the response relationship.” However, review of the individual studies suggests there is a threshold effect beyond which an increased risk of mortality exists. Figure 2 does not depict a linear relationship between CK-MB and 30-day mortality rate. Rather, the plot shows a quadratic or perhaps a threshold effect. The authors stated, “a linear term was used for the CK-MB ratio because this provided a better fit using a Cox model with covariates.” They did not state whether they tested for a nonlinear functional form of the CK-MB ratio, either by including polynomial functions or using a nonparametric method such as splines. The extreme CK-MB values may be driving the observed relationship between the continuous CK-MB variable and 30-day mortality rates.
Conti VR, Riall TS, Sheffield KM. Myocardial Enzyme Levels and Mortality After Coronary Artery Bypass Graft Surgery. JAMA. 2011;306(1):39-40. doi:10.1001/jama.2011.894