Given a creatine kinase level of 371 U/L (reference 28–366 U/L), anti‑double‑stranded DNA antibody <1, anti‑Smith antibody <0.2, and pending erythrocyte sedimentation rate, C‑reactive protein, homocysteine, and repeat antinuclear antibody, what is the likely diagnosis and recommended management?

Evaluation of Mildly Elevated CK with Negative Autoantibodies

Your mildly elevated CK of 371 U/L (just above the upper limit of normal) with negative anti-dsDNA and anti-Smith antibodies most likely represents a benign or non-autoimmune cause rather than inflammatory myositis or systemic lupus erythematosus, and your immediate next steps should focus on completing the pending inflammatory markers (ESR, CRP) and assessing for clinical symptoms of muscle weakness versus pain alone.

Clinical Context and Differential Diagnosis

Your laboratory picture shows:

• Minimally elevated CK (371 vs. upper limit 366 U/L) – only 1.4% above normal
• Negative lupus markers (anti-dsDNA <1, anti-Smith <0.2) – effectively ruling out SLE
• Pending inflammatory markers (ESR, CRP) – critical for determining inflammatory vs. non-inflammatory causes

Key Diagnostic Distinction

The most important clinical question is whether you have:

• True muscle weakness (difficulty standing, lifting arms, climbing stairs) → suggests myositis
• Muscle pain/myalgia alone without weakness → suggests polymyalgia rheumatica (PMR), drug effect, or non-inflammatory causes

This distinction is critical because muscle weakness with elevated CK indicates myositis requiring urgent evaluation, while pain without weakness has a broader differential 1, 2.

Likely Diagnoses Based on Your Presentation

If You Have Muscle Pain WITHOUT Weakness:

1. Polymyalgia Rheumatica (PMR) – most likely if:

   • Age >60 years
   • Proximal shoulder/hip girdle pain and stiffness
   • Markedly elevated ESR/CRP (typically ESR >40 mm/hr)
   • Normal or minimally elevated CK (as in your case)
   • Morning stiffness lasting >45 minutes 3

2. Drug-induced myopathy – consider if taking:

   • Statins (most common cause of isolated CK elevation)
   • Other medications affecting muscle

3. Benign CK elevation – can occur with:

   • Recent exercise or physical activity
   • Intramuscular injections
   • Idiopathic hyperCKemia 4

If You Have Muscle Weakness WITH Pain:

Inflammatory myositis becomes more concerning, though your minimally elevated CK makes severe myositis less likely. Typical myositis presents with:

• CK elevation ≥3× upper limit of normal (≥1098 U/L in your reference range)
• Proximal symmetric weakness
• Difficulty with functional tasks 1, 2

Recommended Management Algorithm

Immediate Assessment (While Awaiting Lab Results):

1. Document clinical symptoms precisely:

   • Can you stand from a seated position without using arms?
   • Can you lift arms above head without difficulty?
   • Is your primary complaint pain or actual weakness?
   • Any dysphagia, dyspnea, or cardiac symptoms? 1

2. Review medication list:

   • Stop statins temporarily if taking them 1
   • Identify any other myotoxic drugs

3. Check for systemic symptoms:

   • Fever, weight loss, rash (dermatomyositis)
   • Headache, visual changes, jaw claudication (giant cell arteritis with PMR) 3

Once ESR/CRP Results Return:

Scenario A: High ESR/CRP (ESR >40, CRP elevated) + Pain Without Weakness

→ Likely PMR

• Consider trial of prednisone 15-20 mg daily 3
• Expect dramatic improvement within 1-3 days if PMR
• Refer to rheumatology for confirmation and long-term management
• Obtain additional baseline labs: RF, anti-CCP, TSH, vitamin D 3

Scenario B: High ESR/CRP + Muscle Weakness Present

→ Possible myositis despite modest CK elevation

• Urgent rheumatology referral 1
• Additional testing needed:
  • Aldolase, LDH, AST, ALT
  • Troponin and ECG (to exclude myocarditis – life-threatening)
  • Myositis-specific antibodies panel
  • Consider EMG and muscle MRI 1, 5
• May require muscle biopsy if diagnosis uncertain 1

Scenario C: Normal or Low ESR/CRP

→ Non-inflammatory cause most likely

• Reassess for drug-induced myopathy (especially statins)
• Consider metabolic causes: hypothyroidism (check TSH), vitamin D deficiency
• If asymptomatic, may represent benign hyperCKemia 4
• Repeat CK in 2-4 weeks off potentially offending medications

Critical Red Flags Requiring Urgent Evaluation

Seek immediate medical attention if you develop:

• Dysphagia, dysarthria, or dyspnea (bulbar/respiratory muscle involvement)
• Chest pain or palpitations (myocarditis – can be fatal)
• Severe progressive weakness limiting mobility
• Dark urine (rhabdomyolysis) 1, 5

These symptoms with elevated CK warrant:

• Immediate troponin, ECG, echocardiogram
• Hospitalization consideration
• High-dose corticosteroids (prednisone 1 mg/kg or IV methylprednisolone) 1

Important Caveats

Your negative anti-dsDNA and anti-Smith effectively exclude lupus, but note that:

• Most patients with immune checkpoint inhibitor-related myositis are seronegative for traditional myositis antibodies 1
• Approximately 55% of patients with CK >1000 U/L referred to rheumatology have inflammatory myopathy, but your CK is barely elevated, making this less likely 4
• ESR/CRP discordance (one high, one low) can occur with infection, renal disease, or low albumin and may complicate interpretation 6

Bottom Line

With CK only marginally elevated and negative lupus markers, await your ESR/CRP results while carefully documenting whether you have true weakness versus pain alone. If high inflammatory markers with pain but no weakness, empiric treatment for PMR is reasonable with rapid rheumatology follow-up. If any weakness is present, urgent rheumatology evaluation with expanded myositis workup is mandatory regardless of the modest CK elevation 1, 3.