How should I manage a patient presenting with a 39 °C fever, myalgias, elevated creatine kinase (CK) of 3500 U/L, C‑reactive protein (CRP) of 80 mg/L, neutrophil predominance, and only a sore throat?

Immediate Management: Acute Infectious Pharyngitis with Systemic Inflammatory Response

This patient most likely has acute bacterial pharyngitis (likely Group A Streptococcus) with a systemic inflammatory response, and should receive empiric antibiotics immediately while ruling out life-threatening complications including myocarditis and rhabdomyolysis.

Critical Initial Assessment

Immediate Workup Required

Cardiac evaluation is mandatory given the markedly elevated CK (3500 U/L) and inflammatory markers:

• Troponin level - to exclude myocardial involvement 1
• ECG and echocardiogram - myocarditis can present with elevated CK and must be ruled out urgently 1
• Urinalysis - to check for myoglobinuria/rhabdomyolysis 1
• Liver enzymes (AST, ALT), LDH, aldolase - these can be elevated in muscle inflammation and help differentiate hepatic vs. muscle injury 1, 2
• Throat culture or rapid strep test - to confirm bacterial pharyngitis
• Blood cultures if toxic-appearing - given high fever and inflammatory markers

Key Clinical Context

The CK of 3500 U/L is significantly elevated (>10x upper limit of normal), but this level of CK elevation occurs in 28% of febrile patients and does not necessarily indicate specific pathology 3. However, the combination of high fever (39°C), myalgias, CRP 80 mg/L, and neutrophil predominance suggests acute bacterial infection rather than viral myositis.

Diagnostic Reasoning

Why This is Likely Bacterial Pharyngitis:

• Fever ≥38°C with sore throat - meets criteria for significant infection 4
• CRP 80 mg/L - markedly elevated, consistent with bacterial infection (CRP >30 mg/L increases likelihood of bacterial etiology) 5
• Neutrophil predominance - strongly suggests bacterial rather than viral infection
• Myalgias with elevated CK - common in febrile illnesses, not specific for myositis 3

Why Viral Myositis is Less Likely:

While viral myositis can present with myalgias and elevated CK 6, it typically:

• Occurs during the recovery phase of viral infections, not acutely
• More common in children than adults
• Usually presents with proximal muscle weakness, not just myalgias
• The neutrophil predominance argues against viral etiology

Management Algorithm

Step 1: Rule Out Life-Threatening Complications (FIRST PRIORITY)

If troponin elevated or any cardiac symptoms:

• Urgent cardiology consultation
• Continuous cardiac monitoring
• Hold any potential myocarditis triggers
• Consider hospitalization 1

If myoglobinuria present or CK continues rising:

• Aggressive IV hydration to prevent acute kidney injury
• Monitor renal function closely
• Consider hospitalization 1

Step 2: Initiate Empiric Antibiotic Therapy

Start antibiotics immediately for presumed bacterial pharyngitis:

• Penicillin or amoxicillin (first-line for strep pharyngitis)
• Azithromycin if penicillin-allergic
• Do not delay antibiotics waiting for culture results given systemic inflammatory response 5

Step 3: Symptomatic Management

• Acetaminophen or NSAIDs for fever and myalgias (if no contraindications and renal function normal) 1
• Adequate hydration
• Rest

Step 4: Close Monitoring

Recheck within 24-48 hours:

• CK levels - should begin declining with treatment
• Renal function - ensure no progression to rhabdomyolysis
• Clinical response to antibiotics
• Repeat troponin if initially elevated

Critical Pitfalls to Avoid

1. Do not assume this is simple viral myositis - the neutrophil predominance and acute presentation with pharyngitis point to bacterial infection

2. Do not miss myocarditis - elevated CK can reflect cardiac muscle injury, which is life-threatening. Troponin and ECG are non-negotiable 1

3. Do not attribute elevated transaminases to liver disease - AST/ALT elevation in this context likely reflects skeletal muscle injury, not hepatitis 2

4. Do not withhold antibiotics - the combination of high fever, elevated CRP >30 mg/L, neutrophilia, and pharyngitis warrants empiric bacterial coverage 5

5. Do not order extensive autoimmune workup initially - while the guidelines for immune checkpoint inhibitor-related myositis 7, 1, 7 recommend extensive rheumatologic panels, those apply to a completely different clinical context (cancer patients on immunotherapy). This patient needs infection treatment first.

When to Consider Alternative Diagnoses

If no improvement after 48-72 hours of antibiotics:

• Consider viral myositis (though less likely given presentation)
• Evaluate for drug-induced myopathy (check medication list, especially statins) 8, 2
• Consider early rheumatology referral if CK remains elevated and muscle weakness develops 1
• Consider EMG and muscle MRI if diagnosis remains uncertain 1

Expected Clinical Course

With appropriate antibiotic therapy:

• Fever should resolve within 24-48 hours
• CK should begin declining by 48-72 hours 9
• Myalgias should improve within 3-5 days
• Complete recovery expected within 1-2 weeks

If CK continues rising or exceeds 10,000 U/L, urgent hospitalization is indicated for aggressive hydration and monitoring for rhabdomyolysis complications 1.