Elevation of creatine kinase in amyotrophic lateral sclerosis. Potential confusion with polymyositis.

Serum creatine kinase (CK) levels were recorded in 100 patients with amyotrophic lateral sclerosis (ALS). CK concentrations were elevated in 43% of the patients, with a mean level of 240 U/liter and a range of 59-1,327 U/liter (male normal less than 95 U/liter, female normal less than 59 U/liter). All patients with elevated CK had muscle weakness. Seven patients were initially misdiagnosed as having polymyositis and given high dose corticosteroids without clinical benefit, prior to the diagnosis of ALS. Such diagnostic confusion can be avoided by an awareness that CK levels cannot be used to reliably differentiate between ALS and primary muscle disease such as polymyositis.