Diagnosis

A diagnosis of McArdle disease is suggested by patient history. Clinical findings may be absent during physical examination. Tests like ischemic forearm test, laboratory analysis, and electromyography are performed if the doctor suspects McArdle disease in a patient.

Tests for McArdle disease include:

Blood tests:

Creatine kinase level: Elevated in more than ninety percent of patients with the disease
Fasting blood glucose
Urine analysis: Routine urine studies are performed. Myoglobinuria (described previously) is found in half of the patients after exercise.
Liver function tests
Biochemical assay is required for the definitive diagnosis of McArdle disease.

Ischemic forearm test:

Ischemic forearm test can be used to diagnose a number of muscle disorders. The test assesses the chemical reactions and products of muscle activity. In this test, forearm muscles are made to exercise after compromising the blood supply to the area. Blood levels (obtained from the area) of lactate and ammonia are measured before and after muscle activity (i.e. exercise). In McArdle disease, lactate levels fail to rise while ammonia levels show an exaggerated rise.

Electromyography

An electromyograph records the electrical waves associated with the activity of skeletal muscles. Unlike other glycogen storage diseases (GSD), findings upon electromyography may be normal in McArdle disease.

Muscle biopsy

Sample of muscle tissue is obtained and examined. Assay of muscle enzyme activity (to confirm the diagnosis) also requires muscle biopsy.

Gene testing for mutations may also be performed.