Serological tests for syphilis: there are two categories
I. Non-syphilis spirochete antigen serologic test.
It is the use of cardiolipin, lecithin and cholesterol as antigens to check the reactants in the serum for the initial screening test and efficacy observation. Can be divided into the following four tests, the principle is basically the same.
1.Venereal disease research laboratory slide test (VDRL): Because the reagents need to be ready to use, so domestic laboratories rarely use it. However, VDRL is the only serological method that can be used for the diagnosis of neurosyphilis.
2.Rapid plasma reactin ring card test (RPR): It is performed on a special paper sheet by adding a certain amount of special charcoal powder, which can make the antigen and antibody appear to agglutinate, and the result can be observed with the naked eye.
3.Toluidine red test (TRUST): Toluidine red is added to the reagent instead of charcoal powder particles to make the results more easily observable.
4.No heating serum reactin test.
Defects of the non-syphilis spirochete antigen serologic test.
1, specificity: because reagents can appear in a variety of diseases, such as acute viral infections, autoimmune diseases, connective tissue disease, intravenous drug users and pregnant women, so such tests sometimes have false-positive reactions.
2, sensitivity: non-syphilis spirochete antigen serologic test sometimes appear weakly positive or negative results, and clinically like the second stage of syphilis, this serum should be diluted to do quantitative testing, such as positive results, the antibody excess caused by the front band phenomenon. 1-2% of patients with second-stage syphilis can appear this phenomenon and syphilis serum false-negative reaction. In addition, because the appearance of reactive elements after infection with syphilis is later than the specific syphilis spirochete antibodies, late syphilis reactive elements and may turn negative, so this type of test is not suitable for stage 1 and 3 syphilis, and is not sensitive to occult syphilis and neurosyphilis. Therefore, this kind of test to do the initial screening of syphilis, there is a certain number of missed detection.
Advantages of non-syphilis spirochete antigen serologic test.
1, non-syphilis spirochete antigen serologic test shows a gradual decrease in titer with the course of treatment, so it can be used as an indicator of efficacy observation.
2, such methods are inexpensive.
Second, syphilis spirochete antigen serologic test.
Using syphilis spirochetes as antigens to detect anti-spirochete IgM and/or IgG antibodies in the serum, its sensitivity and specificity are high.
Five methods are commonly used as follows.
1, fluorescent syphilis spirochete antibody uptake test.
2, syphilis spirochete hemagglutination test (TPHA).
3, syphilis spirochete gelatin agglutination test.
4.Syphilis spirochete enzyme-linked immunosorbent assay (TP-ELISA).
5.Syphilis spirochete rapid diagnostic test (TP-RT).
Fluorescent syphilis spirochete antibody uptake test
The FTA-ABS is the most sensitive of all spirochete tests and has a high specificity, and is considered the “gold standard” for syphilis detection. The keys to the reliability of the test are standardized results, high quality fluorescent-labeled antibodies, and appropriate dilutions. Since laboratory procedures are cumbersome and subjective interpretation often produces erroneous results, the addition of labeled fluorochromes can reduce errors in standardization and increase readability and reproducibility.
Syphilis spirochete hemagglutination assay (TPHA)
Principle: Ultrasonically lysed syphilis spirochetes are used as antigens to sensitize aldehydized and tanned sheep or avian red blood cells, which bind to syphilis spirochete antibodies in human serum or plasma, producing an agglutination reaction observable to the naked eye.
Advantages: The method is easier to operate and more stable than FTA-ABS, its sensitivity is similar to that of FTA-ABS except for early syphilis, and TPHA is easier to operate than FTA-ABS for bulk testing of large samples, which is a commonly used confirmatory test for syphilis spirochetes in domestic laboratories.
Disadvantages: Since red blood cells are biologically active, they may produce non-specific agglutination, and the storage time is short, and the difference between batches is large, so there are some problems in actual use. At present, the commercial TPHA kits used in China have not passed the national batch test.
Syphilis spirochete gelatin agglutination test (TPPA)
Principle: The principle of TPPA is basically the same as that of TPHA. It uses synthetic inert gelatin particles instead of red blood cells as carriers in TPHA test, and is an upgrade of TPHA.
Advantages.
1.Stable reagent and small inter-batch variation.
2. Clearer and more precise interpretation of results.
3.Sensitivity and specificity are further improved compared with TPHA.
4. Passed the national batch test.
Disadvantage: The results are subjective and difficult to be automated.
Syphilis spirochete enzyme-linked immunoassay (TP-ELISA).
Principle: The syphilis spirochete antigen is encapsulated in a polypropylene plate, and then the serum to be tested and the enzyme-labeled antigen are added to form a double antigen sandwich to detect IgG and IgM antibodies at the same time.
Advantages.
1.Higher sensitivity and specificity.
2, lower price, suitable for screening a large number of samples.
3, can realize the automation and objectivity of the operation.
Disadvantages: The antigens and processes used in the ELISA kits are different, resulting in large differences in the sensitivity and specificity of the assay, which should be carefully evaluated by clinical laboratories before selecting ELISA reagents.