Suspected Macrocreatine Kinase Type 1 in an Asymptomatic Young Cat

A 1‐year‐old castrated male Japanese domestic cat was referred for persistently elevated serum creatine kinase (CK) level (> 2000 U/L), which was detected incidentally. The cat showed no clinical signs, with no abnormalities on physical examination, radiography, and ultrasonography. Serum CK isoenzyme analysis by agarose gel electrophoresis revealed an atypical high distribution of CK‐MB (90%). However, CK‐MB–specific quantification by chemiluminescent immunoassay showed significantly low concentration (< 0.1 ng/mL). Based on these observations, we suspected macro‐CK type 1. Serum CK is used as a biomarker of muscle injury in both humans and other mammals. Macro‐CK type 1, a complex of CK and immunoglobulin, has been recognized as the cause of persistently elevated CK levels with or without disease in human medicine. On the other hand, reports of macro‐CK are scarce in veterinary medicine. Based on this case, when persistently elevated CK was detected in clinically healthy cats, this disease should be included as a differential diagnosis. Further studies are required to establish the diagnosis of macro‐CK type 1 in cats.