Cardy and Potter reviewed the predictive value of serum creatine kinase (CK), electromyography (EMG) and MRI for diagnosing muscle disease. The authors are from the County Hospital Hereford and University Hospital Coventry in England.
Patient selection: suspected muscle disease
Initial tests:
(1) serum CK in IU/L (CK > 1000 IU/L sensitivity 48%, specificity 94% for abnormal muscle biopsy)
(2) MRI (sensitivity 92%, specificity 89% for abnormal muscle biopsy)
MRI changes in inflammatory myositis consists of high signal intensity on fat-suppressed T2-weighted and short T1 inversion recovery (STIR).
Follow-up tests:
(1) muscle biopsies at a site identified on MRI
For use in selected patients:
(1) myositis-specific antibodies
(2) EMG (sensitivity 74%, specificity 67% for abnormal muscle biopsy)
The MRI can be used to find the distribution of muscle involvement and to determine an optimum location for muscle biopsy. The yield for a muscle biopsy is generally low if the serum CK is low AND if no abnormal areas are seen on MRI.