Is Creatine Kinase (CK) different from Creatine Phosphokinase (CPK) in a patient suspected of having myositis?

CK and CPK Are the Same Enzyme

Creatine Kinase (CK) and Creatine Phosphokinase (CPK) are identical—they are simply two different names for the same enzyme. The terminology "CK" is now preferred in modern medical literature, though "CPK" remains in common clinical use. 1

Why This Matters in Myositis Evaluation

The key clinical issue is not the nomenclature, but rather understanding that CK/CPK levels are critical for distinguishing true myositis from myalgia-like syndromes:

When CK/CPK is Elevated (Suggests Myositis)

• Myositis typically shows markedly elevated CK levels (median ~2650 IU/L, ranging from 335 to 20,270 IU/L in checkpoint inhibitor-associated cases). 1
• Elevation indicates actual muscle damage with rhabdomyolysis, requiring urgent evaluation for life-threatening complications including myocarditis. 1
• Even with elevated CK, muscle biopsy remains the gold standard when diagnosis is uncertain. 2

When CK/CPK is Normal (Suggests Alternative Diagnosis)

• Polymyalgia rheumatica-like syndrome presents with severe myalgia and fatigue but CK levels remain within normal limits, distinguishing it from true myositis. 1
• Patients have pain without true weakness, and MRI/EMG should not show myopathy. 1
• However, normal CK does not completely rule out myositis—amyopathic or hypomyopathic dermatomyositis can present with normal or only mildly elevated CK despite active muscle inflammation. 2, 3, 4

Critical Clinical Pitfalls

Do not rely solely on CK/CPK levels for diagnosis:

• In connective tissue diseases (SLE, rheumatoid arthritis, Sjögren's syndrome), CK values are frequently low (geometric mean 13-35 IU/L), and normal values can occur despite active myositis. 3
• Check additional muscle enzymes (AST, ALT, LDH, aldolase) as one may be elevated when CK is normal, increasing diagnostic sensitivity. 2
• Consider MRI and EMG, which can detect muscle inflammation even with normal CK levels. 2

Diagnostic Algorithm for Suspected Myositis

Measure CK/CPK immediately along with:

• Inflammatory markers (ESR, CRP) to assess disease activity 1
• Cardiac troponin I (more specific than troponin T in skeletal muscle disease) and ECG to rule out myocarditis 1
• Additional muscle enzymes (AST, ALT, LDH, aldolase) if CK is normal but clinical suspicion remains high 2

If CK is markedly elevated (>3x normal) with weakness:

• Initiate high-dose corticosteroids immediately (prednisone 1 mg/kg/day or IV methylprednisolone). 1, 5
• Search for life-threatening manifestations: bulbar symptoms (dysphagia, dysarthria), dyspnea, chest pain, palpitations. 1
• Perform cardiac evaluation systematically—myocarditis occurs in >60% of checkpoint inhibitor-related myositis cases and drives the ~20% mortality rate. 1

If CK is normal but weakness persists:

• Do not exclude myositis—proceed with MRI of proximal muscles and EMG. 2
• Consider muscle biopsy, particularly if other enzymes are elevated or imaging shows abnormalities. 2