The virulent spirochete tests would probably replace all others if they were easier to do, and they would be easier, for example, if virulent spirochetes could be cultured. But they still have a few drawbacks, and it is possible that reagin type tests would continue to be needed in any event. The reagin tests usually become positive earlier in a few weeks after the chancre appears whereas TPI type tests may not be positive for several months; so that tests like the VDRL are very valuable for early diagnosis. Furthermore, the TPI test may remain positive longer after clinically successful treatment of late syphilis; and hence a positive reagin type test may be a better guide to the need for continuing such treatment. Yet this, too, unfortunately, is not quite so simple. A positive reagin test in late latency, in the absence of any symptoms or recent history of progressive syphilis, does not usually warrant treatment. People are not infectious under these conditions, and since their syphilis is quiescent and may be healed, one lets them alone. But there is some reason to believe that a persistently positive TPI test in late syphilis without symptoms may mean residual infection not entirely eliminated by treatment. I will tell you more about that in the next chapter.

The TPI test is the most elaborate and technically demanding of all blood tests for syphilis. Two circumstances are made use of in it. The first is a way of keeping the spirochetes alive and moving for eighteen to twenty four hours or so no easy trick, but it can be done. The second is the fact that when such spirochetes are mixed under exacting conditions with certain materials, including blood serum which has the syphilitic antibodies in it, the spirochetes stop moving and are killed. The mixture is kept for the eighteen to twenty four hours during which spirochetes without added antibodies would still be found moving. The test is read under the darkfield microscope. It is positive in syphilis and in all the nonvenereal treponematoses, but negative in all other BFP's; and it is used especially to resolve such cases.

A good deal of work has been done to try to simplify a test based on the TPI principle that is, a test using the same spirochetal antigen and its antibody but without the need to keep the spirochetes alive. The best of these, which has come to be widely adopted, is called the FTA ABS test, meaning "fluorescent treponemal antibody absorption." It uses the same rabbit virulent spirochetes but kills them so that they keep for some time, making it possible for the laboratory using them to do without a rabbit colony and without the need to keep inoculating and processing testicles. And it includes a fluorescent dye which, when mixed under the right conditions with the spirochetes and syphilitic blood serum, makes the dye stick to the spirochetes so that, when seen under a modified darkfield microscope, they look greenish and fluorescent against a dark background. At first this test was found to be too sensitive; it was positive with normal human serum which contains antibodies to "normal" spirochetes. But if the test serum is first treated with the Reiter treponeme, these "normal" antibodies are removed. This is the "absorption" part of the test. The specific antibodies still remain to fluoresce with rabbit virulent spirochetes.