Performance of the bioLytical Multiplex HIV 1/2 and Syphilis rapid test on serum in a laboratory evaluation for syphilis.

Syphilis, a sexually transmitted infection, is resurging with large outbreaks worldwide. In pregnant females, vertical transmission can result in stillbirths and serious sequelae for the fetus. Syphilis serology is primarily used to make the diagnosis, but the testing platforms and confirmatory tests are often centralized; therefore, the time to a result is usually more than 2 days. Most affected populations in the current outbreaks can be transient and easily lost to follow up. Therefore, rapid testing can assist with timely detection, treatment, and linkage to care. We evaluated the performance of the INSTI Multiplex HIV-1/2 Syphilis rapid test, using a large panel of negative and confirmed positive sera with varying rapid plasma reagin (RPR) titers. Sera with RPR titers of 1:8 and greater consistently gave a positive rapid test result (93.8%-100%; 95% CI: 82.5%-100%), whereas samples with RPR titers of 1:2 and lower were often negative (7.6%-56.5%). Rapid test positive samples generally had a syphilis CMIA S/CO ratio of 19.78 and higher, although there was no specific ratio that separated all positive from negative rapid results. In infectious syphilis, RPR titers are often 1:8 and greater and should be reliably detected with this rapid test (accuracy 97.2%), thereby supporting immediate treatment and linkage to care with a high level of confidence. Additionally, a result is available within minutes and therefore could be included on acute care STAT laboratory test menus while the individual is an in-patient or awaiting assessment in the Emergency Department. IMPORTANCE: We examined the performance of the syphilis antibody component of a rapid multiplex device and found that sera with RPR titers of 1:8 and greater, often found in infectious syphilis, were highly likely to test positive. This device would be suitable for providing STAT syphilis serology testing for suspect cases waiting in the Emergency Department or as inpatients. Positive results can support immediate treatment and linkage to care, especially for pregnant females and transient persons who are often lost to follow up.