Relationship of body mass index, serum creatine kinase, and acute kidney injury after severe trauma

Abstract

Objectives: Body mass index (BMI) is associated with acute kidney injury (AKI) after trauma, but underlying mechanisms are unclear. Body mass index correlates with both excess adiposity and increased muscle mass. Since the latter could predispose to severe rhabdomyolysis after trauma, we hypothesized that the BMI-AKI association may be partially explained by a direct relationship of BMI with serum creatine kinase (CK).

Methods: Prospective cohort study of 463 critically ill patients admitted to a level I trauma center from 2005 to 2015 with Injury Severity Score of >15 and serum CK measured in the first 7 days. We defined AKI by AKI Network creatinine criteria. We used simple linear regression to determine the association of BMI with peak CK and multivariable logistic regression to adjust the BMI-AKI association for peak CK and confounders.

Results: Median age was 43 years, 350 (76%) were male, 366 (79%) had blunt mechanism, and median Injury Severity Score was 24. Body mass index was associated with peak CK (R = 0.05, p < 0.001). Acute kidney injury developed in 148 patients (32%), and median time to peak CK was 29 hours (interquartile range, 15-56 hours) after presentation. Body mass index was significantly associated with AKI in multivariable models adjusted for age, race, sex, diabetes, injury mechanism and severity, and red blood cell transfusions (odds ratio [OR], 1.31 per 5 kg/m; 95% confidence interval [CI], 1.09-1.58; p = 0.004). Adding peak CK to the model partially attenuated the association of BMI with AKI (OR, 1.26 per 5 kg/m; 95% CI, 1.04-1.52; p = 0.018), and peak CK was also associated with AKI (OR, 1.19 per natural log; 95% CI, 1.00-1.41; p = 0.049). Peak CK remained associated with AKI when restricted to patients with values of <5,000 U/L (OR, 1.31 per natural log; 95% CI, 1.01-1.69; p = 0.043).

Conclusion: Serum CK correlated with BMI and partially attenuated the association of BMI with AKI after major trauma, suggesting that excess muscle injury may contribute to the BMI-AKI association.

Level of evidence: Epidemiologic study, level III.

Figure 1:. Scatterplot of body mass index (BMI) vs. natural log-transformed creatine kinase (CK)

Superimposed on the scatterplot is the estimated association of BMI with serum peak CK (solid line), 95% confidence limits (dashed lines), and p-value derived from a linear regression model. Note: Exclusion of the outlier patient with BMI 58.8 did not significantly change the slope of the regression line or statistical significance of the association.

Figure 2:. Predicted risk of AKI by peak CK elevation

Adjusted probability of acute kidney injury (AKI) across the range of peak serum creatine kinase (CK) values. Estimated probabilities (line) with 95% confidence intervals (gray shading) determined using post-estimation marginal analysis after multivariable logistic regression modeling. Estimated probabilities adjusted for age, race, sex, diabetes status, trauma mechanism, abbreviated injury scale (AIS) abdomen score, and packed red blood cell transfusion within 24 hours. Natural log-transformed CK levels were used in the multivariable model and the CK axis is on the natural log scale, though for clarity the corresponding non-transformed CK values are labeled across the x-axis.