Methods for Judging False Positives in Syphilis Testing

Methods for Judging False Positives in Syphilis Testing

Syphilis is a sexually transmitted disease caused by the Treponema pallidum spirochete. Currently, both the prevalence and incidence of syphilis are on the rise globally. As the number of cases increases, the clinical manifestations and disease stages of syphilis have also become more diverse. In clinical laboratory practice, it is common to encounter cases where syphilis test results are positive, but further verification confirms these to be false positives. Syphilis and its serological testing, are relatively complex; therefore, we should exercise caution when explaining test results to patients.

Due to the diversity of its clinical manifestations, the diagnosis of syphilis currently relies mainly on serological testing, as equipment such as dark-field microscopes or fluorescence microscopes is not widely available. For initial screening tests, non-specific syphilis tests like colloidal gold assay and TRUST (Toluidine Red Unheated Serum Test) are simple to perform and yield rapid results, but they have a relatively high probability of producing false positives. Confirmatory tests such as TPHA (Treponema Pallidum Hemagglutination Assay) and TPPA (Treponema Pallidum Particle Agglutination Assay) are widely used due to their simplicity of operation, high specificity, and good sensitivity.

Biological False Positive for Treponema Pallidum: A positive reaction for non-specific /specific antibodies against Treponema pallidum, caused by biological factors (other than Treponema pallidum itself), disease-related factors, or physiological factors.

False Positive Results in Treponema Pallidum Serological Tests: From a clinical perspective, certain pathological or physiological conditions of the tested individual (such as hepatitis, pregnancy, aging, etc.) may interfere with the test and lead to false positive results.

From a laboratory perspective, after ruling out random errors and technical false positives caused by factors such as improper collection or preservation of clinical samples (e.g., hemolysis or contamination), differences in detection performance among testing systems and methodologies, and laboratory operations, a positive reaction in syphilis serological tests still persists.

After Treponema pallidum infects a host, the body produces non-specific antibodies against Treponema pallidum and specific antibodies against Treponema pallidum. Based on the type of antibody detected, biological false positives for Treponema pallidum can be classified into the following three categories: biological false positives for non-specific antibodies against Treponema pallidum, biological false positives for specific antibodies against Treponema pallidum, and dual biological false positives for both non-specific and specific antibodies against Treponema pallidum.

2.1 Biological False Positive for Non-Specific Antibodies Against Treponema Pallidum:

The incidence of biological false positives for non-specific antibodies against Treponema pallidum ranges from approximately 0.2% to 0.8%, accounting for the highest proportion among the three categories of biological false positives for Treponema pallidum (and is thus referred to simply as "biological false positive" in some contexts). Non-specific antibody tests for Treponema pallidum detect lipoidal antibodies in serum—specifically, antibodies (also called "reagin") produced by the body in response to stimulation from phospholipids released when Treponema pallidum damages host cells, or from lipoids on the pathogen’s surface. The antigens coated in the detection reagents include cardiolipin, lecithin, and cholesterol. Data shows that biological false positives for non-specific antibodies against Treponema pallidum can occur in individuals with over 60 types of diseases, including autoimmune diseases (e.g., systemic lupus erythematosus), acute viral infections (e.g., human immunodeficiency virus infection), and tumors (e.g., bone tumors). Additionally, this phenomenon may also be observed in the elderly, drug users, and pregnant women.

2.2 Biological False Positives for Specific Antibodies Against Treponema Pallidum: