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Dark ground microscopy and treponemal serology for diagnosis of early syphilis

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There has been a recent increase in the incidence of infectious syphilis in the UK, especially in homosexual men. Dark ground microscopy (DGM) provides immediate diagnosis but requires a dark field microscope and trained staff and, because syphilis has been uncommon in the UK until recently, many genitourinary medicine physicians are unskilled in its use. Other direct detection methods such as direct fluorescent antibody staining (DFA-Tp) or polymerase chain reaction (PCR) are more difficult or slow and serological tests have limited sensitivity for primary syphilis. Workers in London have confirmed the usefulness of DGM.

Their retrospective case note review included 50 patients with primary syphilis (PS) and 36 with secondary syphilis (SS). The diagnosis of PS was based on the presence of ulcers of penis, anus, or outer lip and positive DGM and/or positive serology. SS was diagnosed from clinical signs (lesions of penis, scrotum, vulva, or extragenital skin) and positive serology or DGM. Lymph node aspirates for DGM were not performed. Treponemal enzyme immunoassay (EIA) was done for all patients and positive samples were tested further with rapid plasma reagin (RPR) and TPPA tests. Selected samples were sent for EIA-IgM testing.

DGM was performed in 31 of the 50 cases of PS and was positive in 30 (from lesions on the genitalia (23), anus (5), and lip (2). The test was positive at first visit in 24 patients. The reasons for not doing DGM in 19 patients were:- wrong clinical diagnosis of genital herpes (6), atypical ulcers and syphilis not suspected (7), no trained personnel available (4), DGM microscope not working (1), and inner lip lesion with possibility of contamination with non-syphilitic treponemes (1). Treponemal EIA-IgM testing was done for 27 cases of PS and was positive in 21, equivocal in four, and negative in two. DGM was performed in three EIA-IgM-equivocal cases and one negative and was positive in all four. Treponemal EIA (not IgM-specific) was initially negative in 17 cases of PS (34%) but DGM was positive in all 13 of these 17 on whom it was performed. EIA-IgM was positive in 12 of 14 EIA-negative cases tested.

All 36 cases of SS had positive treponemal serology on first testing. DGM in 19 cases was positive in 16. The DGM positive lesions were genital (7) and extragenital cutaneous (9). In 15 cases DGM was positive at the first visit. The reasons for not doing DGM in 17 cases were:- wrong diagnosis of rash (9), positive syphilis serology in asymptomatic HIV positive patients (2), no trained personnel (5), and reason not documented (1).

DGM is rapid and sensitive and allows immediate diagnosis, treatment, and partner notification. All genitourinary medicine clinics should have trained staff ready to do DGM.