False positive probability of TP in pregnant women

After pregnancy, pregnant women must undergo various prenatal examinations under the guidance of a doctor, among which TP screening is also a very important prenatal examination item, which is mainly to check whether the pregnant woman has syphilis problems. If a pregnant woman is tested positive for TP, she needs to go to the hospital for further screening to rule out syphilis in time. So what is the probability of a pregnant woman being tested false positive for TP?

If a pregnant woman tests positive for TP, the probability of a false positive is very low, because the test strips used for syphilis screening are relatively sensitive. Syphilis is a sexually transmitted disease that is more harmful to the population, so the testing is relatively strict and the chance of a false positive is not high. In addition, if the TP screening is positive, a further syphilis confirmation test should be performed. Only after further examination can it be confirmed, so the chance of a false positive in a syphilis test is relatively small.

Causes of false positive maternal syphilis:

① Specimen factors: Hemolysis and incomplete centrifugation of blood specimens can easily lead to misdiagnosis, while imperfect specimen preservation can lead to bacterial contamination and cause false positive reactions. Improper handling of blood specimens can lead to hemolysis. Studies have found that hemolysis can lead to false positive results in syphilis antibody testing by TP-ELISA, and the relationship is linear to a certain extent. Even if the centrifugal strength is increased, this phenomenon cannot be completely eliminated. When hemolysis occurs, various active enzymes and substances with enzyme-like activity in the intracellular fluid can non-specifically bind to the substrate. Hb has peroxidase-like activity, and its effect is similar to that of horseradish peroxidase. After hemolysis occurs, the reaction wells non-specifically adsorb Hb, which catalyzes the substrate to develop a certain degree of color, causing the OD value to be too high and producing a false positive. Incomplete centrifugation will result in unclean serum samples containing flocs, causing false positive reactions. In this study, false positives were caused by poor handling of serum samples, mainly because the specimens were hemolyzed during the collection process, which affected the test results and caused false positives;

②Disease factors: various malignant tumors, diabetes, measles, lupus erythematosus, rheumatoid arthritis, hepatitis C, tuberculosis, pregnancy, etc. cause the body to contain therapeutic antibodies, autoantibodies, rheumatoid factor, alpha-fetoprotein, etc. These special components have a certain adsorption effect during the reaction process, resulting in color development and causing false positives. Similar results are obtained for syphilis testing in the elderly, so a test titer of <1:9 is needed to exclude a syphilis diagnosis;

③Other factors: In addition to false positive tests caused by specimen factors, the abuse of antibiotics and unreasonable testing at the time may also cause false positive test results. In order to improve the diagnosis rate of early syphilis, standardized testing should be carried out.