Final answer:
To confirm a syphilis infection after a positive initial test, the FTA-ABS or TP-PA tests, which are treponemal serologic tests, are typically used due to their specificity to T. pallidum antigens. These tests are superior to the VDRL test, which is susceptible to false positives and better suited for screening and determining if the infection is active.
Step-by-step explanation:
Confirming Syphilis Infection
When confirming a syphilis infection after another method tests positive, the preferred tests are those that measure antibodies directed against T. pallidum antigens. The recommended confirmatory tests are the treponemal serologic tests such as the Fluorescent treponemal antibody absorption (FTA-ABS) test and the Treponema pallidum particle agglutination (TP-PA) test. Although the Indirect Fluorescent Antibody (IFA) test provides important complement and is less likely to generate false-positives, confirmatory testing specifically requires treponemal tests for their specificity to spirochete antigens.
The VDRL test, a nontreponemal test, is known to generate false positives and is more suitable for screening purposes and determining if an infection is currently active rather than confirming the infection. A patient with a positive IFA and negative VDRL result might indicate early or treated infection, or a biological false-negative VDRL, necessitating further confirmation with a test like FTA-ABS or TP-PA.
For neurosyphilis, a variety of factors must be assessed including the VDRL-CSF test, which is highly specific but not sensitive enough on its own for a conclusive diagnosis.