Chlamydia trachomatis serovars L1, L2, L3 cause lymphogranuloma venereum (LGV). Which diagnostic test is most specific for confirming active LGV and distinguishing it from other serovars of C. trachomatis?

• A Frei test (skin test using LGV antigen)
• B Complement fixation test (CFT) titre >1:64
• C Nucleic acid amplification test (NAAT/PCR) with L-serovar-specific typing ✓
• D Microimmunofluorescence (MIF) detecting serovar-specific IgM

Explanation

NAAT (PCR) on swab specimens from the primary ulcer, bubo aspirate or rectal swab is the most sensitive and specific test for C. trachomatis. Serovar-specific genotyping (sequencing the ompA gene or using L2-specific primers) distinguishes LGV serovars (L1-L3) from non-LGV serovars (A-K). The Frei skin test (now discontinued) was the historical test but was insensitive and non-specific (positive for all C. trachomatis serovars). CFT titre >1:64 suggests LGV but is not serovar-specific. MIF detects serotypespecific antibody but cannot distinguish active from past infection as reliably as PCR.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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