Chlamydia trachomatis serovars L1, L2, and L3 cause lymphogranuloma venereum (LGV). The diagnostic confirmatory test distinguishing LGV from other C. trachomatis genital infections is:

• A NAAT (PCR) for C. trachomatis from genital swab — all serovars are detected identically
• B Frei test (intradermal LGV antigen skin test) — gold standard
• C Complement fixation test titre ≥1:64 is sufficient for LGV diagnosis
• D NAAT detection of C. trachomatis with subsequent molecular typing/genotyping to confirm L serovars ✓

Explanation

Standard C. trachomatis NAAT detects all serovars (A–L3) identically by 16S rRNA or ompA targets. To confirm LGV, molecular typing (real-time PCR using L-serovar-specific targets or ompA sequencing) is required. This is critical because LGV is treated with 21 days of doxycycline vs 7 days for urogenital chlamydia. The Frei test is obsolete and poorly sensitive. Complement fixation titres ≥1:64 support LGV diagnosis but are not specific; they cross-react with other Chlamydia species and are now rarely used.

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

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Written and medically reviewed by the StethoPrep medical team.