Some people will be found positive for TP during routine pre-surgery exams and physical exams, leaving them wondering about the lack of any history of adverse exposures, rashes, and unusual symptoms. Why would this occur? Is this condition a syphilis infection?
This starts with the diagnostic criteria of syphilis. Serological tests are currently one of the main indicators for the diagnosis and treatment monitoring of syphilis, and include two main categories: non-syphilis spirochete antigen serological tests and syphilis serological tests. The most commonly used laboratory indicators in hospitals are RPR and TPPA, how to interpret them?
I. Classification of syphilis serological tests
There are two categories of non-syphilis spirochete antigen serologic tests (such as RPR or TRUST) and syphilis spirochete antigen serologic tests (such as TPPA or TPHA).
RPR has the following characteristics.
1. high sensitivity and low specificity of RPR.
2. changes in titer can be used as a reference for changes in disease and treatment effect
3.Stage I syphilis (disease duration less than 2 weeks) cannot be excluded when the test result is negative.
4, according to the titer drop, it can identify early or late latent syphilis, the former treatment titer drop fast, the latter titer drop slow or unchanged.
TPPA has the following characteristics.
1. high specificity and sensitivity.
2. It is a confirmatory test for the diagnosis of syphilis.
Second, about the false positive and false negative of syphilis serological test
1, false positive problem
(1) technical false positives: including reagents and operational reasons, such as high antigen sensitivity, serum specimen error or hemolysis or bacterial infection, serum is not fresh or test equipment contamination, immature test technology.
(2) Biological false positives: including acute and chronic biological false positive reactions. Non-syphilis spirochete antigen serologic test false positive rate is higher than syphilis spirochete antigen serologic test.
①Acute biological false positives are common in: non-syphilis spirochete antigen serologic test false positives are common in acute febrile diseases such as measles, chickenpox, rubella, upper respiratory tract infection, scarlet fever, streptococcal pneumoniae infection, active tuberculosis and after immunization. It mostly turns negative within 6 months and has a low titer (rarely more than 1:8), which can be given by TPPA to exclude.
② Chronic biological false positives: False positive serologic tests for non-syphilis spirochete antigens can persist for greater than six months, but most can turn negative within a few weeks to six months after the disease subsides. It is common in autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, rheumatic heart disease, dry syndrome, and chronic nephritis. False positives are 1%-2% in pregnant women and normal populations; false positives occur in 1% of senior citizens older than 70 years of age.
False positive syphilis spirochete antigen serologic test is common in systemic lupus erythematosus, rheumatoid arthritis, mixed connective tissue disease, scleroderma, liver cirrhosis, diabetes, lymphoma, pregnancy.
2. False-negative problem: Sometimes syphilis is highly suspected clinically but not supported serologically, which may be due to.
(1) RPR positive can only be detected 2-3 weeks after the appearance of sclerosing chancre, i.e. false negative may occur in the early stage of infection.
(2) Immediate treatment after infection with syphilis or late syphilis, with false negatives due to low serum reactin.
(3) The “pre-banding phenomenon” – some patients with stage II syphilis may have false negatives due to excessive anti-cardiolipin antibodies in the serum that suppress positive results.
Once the syphilis spirochete test is positive, it usually remains positive for life, regardless of treatment and disease activity, but 1-5% of patients with stage I syphilis turn negative 2-3 years after receiving treatment.
The diagnosis of syphilis must be combined with medical history, clinical manifestations, and serological tests. Once the three do not match, it must be considered together to exclude the possibility of false positives and false negatives.