Background/introduction Diagnosis of primary syphilis is confirmed by serological tests suggestive of the stage of the disease. In most cases specific (IgM/IgG) and non-specific (RPR) antibodies develop in response to primary treponemal infection. Lack of development of such antibodies is known to occur amongst HIV positive patients but is unusual among HIV negative patients with no significant comorbidities.

Aim(s)/objectives To present a cluster of four cases of primary syphilis from our clinic with unexpected serological results.

Methods We describe four unusual cases of HIV-negative MSM, all of whom presented with penile lesions (three chancres and an atypical lesion). These cases were identified by clinicians between July 2014 and January 2015. Clinical and laboratory records were retrospectively interrogated. Clinical photographs will be used to illustrate these cases.

Results In all cases, patients with no history of previous syphilis returned positive results on one or more specific treponemal serological tests with persistently negative RPR. Three cases had recent negative syphilis screening at our clinic. Darkfield microscopy also failed to demonstrate T. pallidum in those with chancres. In all cases, treatment of presumed syphilis led to the resolution of the lesions.

Discussion/conclusion These cases demonstrate the ongoing difficulties with treponemal diagnostic test interpretation. There are reports in the literature that men over 35 may be more likely to return a false negative RPR result, but overall prevalence of false negative RPR in primary syphilis is uncertain. Over-reliance on serology may result in under diagnosis of syphilis even in HIV-negative MSM.