Studies have been done in which duplicate samples from known syphilitic patients in various stages of their disease and also from normal persons were sent to various laboratories. Besides this, many reports have appeared from laboratories all over the world comparing one test with another in various clinical stages of syphilis, in nonsyphilitic diseases, and in apparently normal persons. These results are summarized in Table 15-3.

Comparison of serologic tests for syphilis

Table 15-3 Comparison of serologic tests for syphilis (approximate percentage reported reactive)*

Table 15-3 demonstrates considerable variation in results. Several factors must be involved besides the inherent sensitivity and specificity of the individual tests themselves:

1. Antibiotic treatment may cause some previously reactive syphilitic patients to become nonreactive.

2. Some clinically normal persons may have unsuspected subclinical syphilis.

3. True BFP reactions, either acute or chronic, must be taken into account.

4. There is obvious variation in technique and ability between laboratories. Some laboratories introduce their own modifications into standard techniques.

5. The time of appearance differs for the various antibodies. In general, the FTA-ABS test result becomes positive in significant numbers of patients in the middle or end of the primary stage, followed by the MHA and RPCF, and then the VDRL. All of these procedures usually give positive results in the secondary stage, and also probably in the early latent stage.