Q. What other tests are required for diagnosis of syphilis?
Following are different types of tests available that can diagnose syphilis:
1. Antibody tests (serology): Antibody tests are most commonly used. These tests detect antibodies in the blood and cerebrospinal fluid (CSF). Two types of tests available for syphilis testing are nontreponemal antibody test and treponemal antibody test. Any of these two types can be used for syphilis screening, but it must be followed by a second test that uses a different method to confirm a positive result and to diagnose active syphilis.
Nontreponemal antibody tests—These are called "nontreponemal", as they detect antibodies that are not specifically directed against the syphilis bacteria Treponema pallidum. Besides syphilis, these antibodies can also be produced in several other conditions (false positive results). The tests are highly sensitive but are non-specific in nature. False-positive results can be seen in IV drug use, pregnancy, Lyme disease, certain types of pneumonia, malaria, tuberculosis, or certain autoimmune disorders including lupus. A positive screening result must be confirmed with a more specific (treponemal) test. Nontreponemal tests include RPR (Rapid Plasma Reagin) test and VDRL (Venereal Disease Research Laboratory) test.
· Treponemal antibody tests: These blood tests detect antibodies that specifically target Treponema pallidum. They are highly specific for syphilis. However, once a person is infected and these antibodies develop, they remain in the blood for life. On the other hand, nontreponemal antibodies typically disappear in a well treated person after about 3 years. Therefore, a positive treponemal screening results must be followed by a nontreponemal test (such as RPR) to differentiate between an active infection (or reinfection) and one that occurred in the past and was successfully treated.
Treponemal antibody tests include FTA-ABS, TP-PA, and MHA-TP. FTA-ABS (Fluorescent treponemal antibody absorption) test is used to diagnose neurosyphilis. Sometimes instead of FTA-ABS test, TP-PA (T. pallidum particle agglutination assay) test is performed because it is more specific and there are fewer false positives. MHA-TP (Microhemagglutination assay) is another confirmatory method, however, this test is used much less commonly now. Immunoassays (IA): in more recent years, several automated tests have been developed, making them convenient for screening purposes.
2. Direct detection of bacteria—These tests are less commonly used now. Dark field microscopy may be used in the early stages of syphilis when a suspected syphilis sore (chancre) is present. It involves obtaining a scraping of the sore, placing it on a slide, and examining it with a special instrument called a dark-field microscope.
3. Molecular testing (polymerase chain reaction, PCR): This test detects genetic material from the bacteria in the sample obtained from the sore, in the blood, or in CSF.
Q. What do you mean by false-negative TPHA test results?
False negative means negative results of TPHA test in the presence of syphilis infection. The body does not always produce antibodies specifically in response to the syphilis bacteria, so the test is not always accurate. False-negatives may occur in people with early- and late-stage syphilis. Hence, to accurately detect syphilis other tests are required as well.
Q. What do you mean by false positive TPHA results?
False positive means positive results in the absence of syphilis. TPHA test is not specific for syphilis, as it does not differentiate between other treponemal antibodies from syphilis, although it is highly sensitive. A false-positive TPHA can be encountered in infectious mononucleosis, leprosy, and connective tissue disorders.
Q. Is there any risk associated with this test?
There is no risk associated with the test. However, since this test involves a needle prick to withdraw the blood sample, in very rare cases, a patient may experience increased bleeding, hematoma formation (blood collection under the skin), bruising or infection at the site of needle prick.
Q. Is there any special preparation required for the test?
No, there is no special preparation required for the test.