A 10-year-old boy diagnosed with acute rheumatic fever has an ASO titre of 400 Todd units at admission and 800 Todd units when repeated three weeks later. What is the correct interpretation of this finding?

• A The rise confirms active streptococcal pharyngeal infection requiring antibiotics
• B The titre is nonspecific and carries no diagnostic weight in rheumatic fever
• C The rising titre indicates recent preceding streptococcal infection, fulfilling laboratory evidence for the diagnosis of rheumatic fever ✓
• D A rising titre indicates progression of carditis and predicts severity

Explanation

ASO antibodies appear one to three weeks after streptococcal pharyngitis and peak around three to six weeks, so a rising titre between paired sera documents recent streptococcal infection, satisfying the Jones criterion for laboratory evidence of antecedent streptococcal disease. By the time rheumatic fever manifests, throat cultures are usually negative and antibiotic therapy does not alter the course of rheumatic fever.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

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