A patient presents with acute polyarthritis involving the sacroiliac joint, knee, and shoulder, an elevated antistreptolysin O (ASO) titer (201 U/mL from a reliable laboratory and 400 U/mL from a local laboratory) and a normal serum uric‑acid level—what is the most likely diagnosis and how should it be managed?

Acute Rheumatic Fever Following Group A Streptococcal Infection

This patient most likely has acute rheumatic fever (ARF) and requires immediate antibiotic therapy to eradicate streptococcal infection, followed by long-term secondary prophylaxis to prevent recurrent episodes and permanent cardiac damage. 1

Diagnostic Assessment

Applying the Jones Criteria

The diagnosis of ARF requires either ≥2 major manifestations or 1 major + ≥2 minor manifestations plus laboratory evidence of recent streptococcal infection. 1

This patient presents with:

• Major criterion: Migratory polyarthritis involving sacroiliac joint, knee, and shoulder 1
• Laboratory confirmation: Elevated ASO titre (201 IU/mL from reliable lab, 400 IU/mL from local lab) confirms prior streptococcal infection 1, 2
• Minor criteria likely present: Elevated inflammatory markers (ESR/CRP should be documented) and possibly fever 1

Critical Cardiac Evaluation Required

All patients with suspected ARF must undergo immediate cardiac assessment because cardiac involvement determines prognosis and duration of prophylaxis. 1

• 12-lead ECG is mandatory to assess for prolonged PR interval (first-degree AV block) 1
• Transthoracic echocardiogram is required to identify valve lesions, pericardial effusion, or ventricular dysfunction 1
• Cardiac involvement may be subclinical and still counts as a major criterion 1

Distinguishing ARF from Post-Streptococcal Reactive Arthritis

The sacroiliac joint involvement is particularly significant—research demonstrates a strong correlation between elevated ASO titres and SI joint disease (p<0.0001). 3 While post-streptococcal reactive arthritis (PSRA) typically presents with non-migratory arthritis, older age of onset (mean 42 years), and lacks carditis 4, this patient's presentation warrants full ARF evaluation given the polyarticular involvement and need to exclude cardiac disease.

Immediate Management

Antimicrobial Therapy (10-Day Course Mandatory)

First-line treatment to eradicate streptococcal infection: 1, 2

• Penicillin V 500 mg orally twice or three times daily for 10 days, OR
• Amoxicillin 50 mg/kg orally once daily for 10 days

For penicillin allergy: cephalosporins (if no anaphylaxis history), clindamycin, or azithromycin based on local resistance patterns 1, 2

A 10-day course is non-negotiable—shorter courses increase ARF recurrence risk. 2

Long-Term Secondary Prophylaxis (Duration Based on Cardiac Status)

Cardiac Status	Prophylaxis Duration	Citation
No carditis	5 years OR until age 21 (whichever is longer)	[1,2]
Carditis without residual valve disease	10 years OR until age 21 (whichever is longer)	[1,2]
Residual valve disease	Lifelong prophylaxis	[1,2]

Important Differential Considerations

Septic Arthritis Must Be Excluded

Septic arthritis requires urgent joint aspiration and cannot resolve spontaneously without antibiotics. 5 However, if this patient is afebrile, hemodynamically stable, and has migratory (not fixed) joint involvement, septic arthritis is effectively ruled out. 5

Kocher criteria help differentiate septic arthritis: fever >101.3°F, ESR ≥40 mm/h, WBC ≥12,000/µL, inability to bear weight 1. CRP >2.0 mg/dL strongly predicts septic arthritis. 1

Gout is Excluded

Normal uric acid effectively excludes gout in this clinical context. 6, 5 While the European League Against Rheumatism notes that uric acid may transiently fall during acute attacks 5, the combination of elevated ASO titre, polyarticular involvement including SI joint, and normal uric acid makes gout extremely unlikely.

Axial Spondyloarthritis Considerations

While SI joint involvement raises the question of axial spondyloarthritis, the acute polyarticular presentation with elevated ASO titre points to post-streptococcal disease. 6 The elevated ASO titre is not a feature of primary spondyloarthritis 6, and the temporal relationship to streptococcal infection is diagnostic.

Follow-Up Protocol

Re-evaluate clinically 3-5 days after initiating antibiotic therapy. 1, 2

Continue weekly follow-up until the patient is completely asymptomatic. 1, 2

Monitor for late complications: 1, 2

• Post-streptococcal glomerulonephritis (hematuria, proteinuria, edema, hypertension)
• Delayed carditis development
• Recurrent arthritis if prophylaxis is not maintained

Critical Pitfalls to Avoid

Do not delay cardiac evaluation—subclinical carditis changes the entire prophylaxis strategy from 5 years to potentially lifelong. 1

Do not prescribe NSAIDs for symptomatic relief without first confirming cardiac status, as undiagnosed carditis requires different management. 6

Do not assume a single elevated ASO titre represents active infection—ASO peaks at 3-6 weeks post-infection and remains elevated for months. 1, 7 The diagnosis requires clinical manifestations plus elevated titres, not titres alone. 2

Do not stop antibiotic prophylaxis prematurely—even patients without carditis require 5 years or until age 21 to prevent recurrent ARF and subsequent cardiac damage. 1, 2