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Diagnosis and Management of Rhabdomyolysis in the Absence of Creatine Phosphokinase: A Medical Record Review.

INTRODUCTION
Rhabdomyolysis is often encountered in austere environments where the diagnosis can be challenging due to the expense or unavailability of creatine phosphokinase (CPK) testing. CPK concentration ≥5,000 U/L has previously been found to be a sensitive marker for progression to renal failure. This study sought to propose a model utilizing an alternate biomarker to allow for the diagnosis and monitoring of clinically significant rhabdomyolysis in the absence of CPK.


MATERIALS AND METHODS
We performed a retrospective chart review of 77 patients admitted to a tertiary medical center with a primary diagnosis of rhabdomyolysis. A linear regression model with aspartate aminotransferase (AST) as the independent variable was developed and used to predict CPK ≥5,000 U/L on admission and CPK values on subsequent hospital days. The study was approved and monitored by the Institutional Review Board at Walter Reed National Military Medical Center.


RESULTS
Ln(AST) explained over 80% of the variance in ln(CPK) (adjusted R2 = 0.802). The diagnostic accuracy to predict CPK ≥5,000 U/L was high (AUC 0.959; 95% CI: 0.921-0.997, P < 0.001). A cut point of AST ≥110 U/L in our study population had a 97.1% sensitivity and an 85.7% specificity for the detection of a CPK value ≥5,000 U/L. The agreement between actual CPK and predicted CPK for subsequent days of hospitalization was fair with an intraclass correlation coefficient of 0.52 (95% CI: 0.38-0.63). The developed model based on day 1 data tended to overpredict CPK values on subsequent hospital days.


CONCLUSIONS
We propose a threshold concentration of AST that has an excellent sensitivity for detecting CPK concentration ≥5,000 U/L on day of admission in a patient population with a diagnosis of rhabdomyolysis. A formula with a fair ability to predict CPK levels based on AST concentrations on subsequent hospital days was also developed.