Analysis of the Causes of Syphilis False Positive Results

Analysis of the Causes of Syphilis False Positive Results

Syphilis is a sexually transmitted disease caused by the Treponema pallidum spirochete. It is a disease unique to humans, with both symptomatic and asymptomatic syphilis patients serving as the sources of infection. The skin lesions, their secretions, and the blood of individuals infected with syphilis contain treponemal antibodies. Serological testing for Treponema pallidum, conducted using blood samples collected in clot activator&gel tubes, is the primary basis for syphilis diagnosis. Whenever a false positive result occurs, the clinical laboratory will investigate potential causes, including operational errors, issues with clot activator&gel tubes, and reagent-related problems. In fact, there are numerous contributing factors: clinical pathological factors and instrument malfunctions can also lead to false positives. False positive syphilis test results severely impact the physical and mental health of the tested individuals, as well as their family life and work. Timely identification of syphilis false positives and accurate interpretation of Treponema pallidum serological test results pose significant challenges and difficulties for clinicians and laboratory technicians in the diagnosis and treatment of syphilis.

Definition of Syphilis False Positive: A biological false positive for Treponema pallidum refers to a positive reaction for non-specific and/or specific antibodies against Treponema pallidum that is caused by biological factors other than Treponema pallidum itself, disease-related factors, or physiological factors.

False Positives Caused by Various Pathological Factors:

Specific Antibody False Positive:

• Having been infected with or exposed to Treponema pallidum before, but with relatively mild symptoms that went unnoticed, and the infection resolved spontaneously without treatment.
• Common internal medical diseases in the elderly, such as coronary heart disease, cerebrovascular disease, diabetes mellitus, and leukemia.
• Symbiotic spirochetes in the human body may induce the production of cross-reactive antibodies against specific antigens.
• Pregnancy.
• Rheumatoid arthritis, systemic lupus erythematosus, colon cancer, lymphosarcoma, hepatitis C, liver cirrhosis, AIDS, leprosy, genital herpes, etc.

Non-Specific Antibody False Positive:

• Acute biological false positive reactions, such as those caused by viral diseases including rubella, measles, chickenpox, infectious mononucleosis, viral hepatitis, etc.
• Chronic biological false positive reactions, such as those caused by diseases including leprosy, systemic lupus erythematosus, rheumatoid arthritis, Sjögren's syndrome, Hashimoto's thyroiditis, polyarteritis nodosa, and rheumatic heart disease, etc.
• Other diseases that can lead to the production of lipoidal antibodies.

From a clinical perspective, certain pathological or physiological responses of the tested individual themselves (such as hepatitis, pregnancy, aging, etc.) may affect the test results, leading to false positives in Treponema pallidum serological tests. 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 between testing systems and methodologies, and laboratory operations, positive reactions in syphilis serological tests may still occur.

Impact of Instrumentation on False Positive and False Negative Results in Syphilis Testing: When enrolling cases, relevant influencing factors (including infectious diseases, autoimmune diseases, pregnancy and lactation period, etc.) were excluded. A total of 19 factors potentially associated with false positive and false negative results in syphilis testing were selected for inclusion. After analyzing and excluding irrelevant indicators under certain single factors, it was found that laboratory instrumentation, laboratory technical operations, reagents, reference standards, clot activator&gel tubes, testing methods, and medical history were correlated with erroneous syphilis test results under single-factor analysis. Multivariate analysis was then conducted, and the results showed that laboratory instrumentation, laboratory technical operations, reagents, reference standards, clot activator&gel tubes, and testing methods are independent influencing factors for false positive and false negative results in syphilis testing. Laboratory instruments and equipment may be contaminated by other samples, which can cause cross-reactions of syphilis antibodies or the binding of contaminated antigen components to detection reagents, thereby resulting in false positive results. Non-specific antibodies such as rheumatoid factors may be present in laboratory instruments and equipment; these antibodies can bind to syphilis antibodies, leading to false positive results. Different syphilis detection methods have different sensitivities, and some methods may fail to detect antibodies with low titers, thus resulting in false negative results. Contamination during laboratory technical operations, errors in sample processing, and interference from reaction conditions (such as temperature, time, pH, etc.) may all affect the specificity of immune reactions, leading to false positive results. Meanwhile, issues with sample quality, improper selection of detection methods, and inaccurate operational techniques may all cause distortion in result interpretation, resulting in false positive results. Regarding the impact of reagents and reference standards on false positive or false negative results in syphilis testing, the main mechanisms include cross-reactions between reagents, non-specific binding, contamination, antigen deficiency, antibody binding inhibition, and technical limitations.

In the daily syphilis testing conducted by hospitals, the probability of false positives is analyzed to be 2-4% based on the testing volume. Since clot activator&gel tubes are in direct contact with blood samples, they are also a factor frequently suspected by the clinical laboratory. If clot activator&gel tubes do cause false positives in syphilis testing, given the uniformity of the production line for such tubes, all these blood collection tubes would lead to false positive results.

To sum up, the conditions related to the laboratory have a significant impact on the occurrence of false-negative and false-positive results in syphilis testing. Emphasis should be placed on the standardization of testing procedures, implementation of quality control processes, regular review and update of testing methods, as well as verification and validation of test results. Comprehensive prevention and control measures should be adopted to reduce the incidence of such erroneous results.

References

Gu Weiming, Yang Tianci. Expert Consensus on the Management of Biological False-Positive Results in Serological Tests for Treponema pallidum [J]. Chinese Journal of Laboratory Medicine, May 2023, Vol. 46, No. 5

(https://med.wanfangdata.com.cn/Periodical/Issue?id=zhyxjy&year=2025&issue=6)

Zhang Zhiqiang, Chen Cheng, et al. Analysis of Causes of False-Negative and False-Positive Results in Syphilis Testing [J]. China Science and Technology Journal Database (Medicine), 2023, No. 12, pp. 41-44 (4 pages in total)

(https://med.wanfangdata.com.cn/Periodical/Issue?id=zhyxjy&year=2025&issue=6)