How to Identify a False Positive Syphilis Test

False positives in syphilis testing can be confusing and concerning. Knowing how to identify them is crucial for accurate diagnosis and early intervention. Especially they’re more common in pregnant women, leading to many complications.
Syphilis tests sometimes yield false positives due to various factors like other medical conditions, certain infections, or recent vaccinations. Understanding these factors can help you interpret a false positive syphilis test more accurately and avoid unnecessary anxiety.
This guide will explain what causes false positives in syphilis testing, how to differentiate them from true positives, and the steps you can take if you receive unexpected test results. Let’s go in!

False Positive Syphilis Test

Syphilis is a bacterial sexually transmitted disease (STD). People infected with Syphilis bacteria may show symptoms intermittently over time. It can easily transmit from one person to another person, even from a pregnant lady to their baby. Diagnosing Syphilis is not a straightforward task. It usually requires two types of tests for a definitive diagnosis: treponemal and nontreponemal tests. But relying on one of them can return a false positive syphilis test.

A false positive syphilis test occurs when a patient receives a positive result for syphilis antibodies, though they have not been infected with the disease. Because the human body produces antibodies linked to Syphilis in other health conditions also. Relying on one type of test has the chance to return a false positive. A false positive syphilis test is more common with non-treponemal tests like False positive RPR or VDRL. But it can also happen with treponemal tests.

Some of the treponemal tests are:

• FTA-ABS (Fluorescent Treponemal Antibody Absorption)
• CLIA (Chemiluminescence Immunoassays)
• TP-EIA (T. Pallidum Enzyme Immunoassay)
• MHA-TP (Microhemagglutination Assay For Antibodies To Treponema Pallidum)
• TPHA (T. Pallidum Hemagglutination Assay)
• TP-PA (T. Pallidum Particle Agglutination Assay)

Most healthcare professionals prefer treponemal tests first, as they are more definitive.

What is Treponema pallidum?

Treponema pallidum is the bacterium responsible for syphilis infection. Most syphilis tests are designed to detect antibodies produced by the immune system in response to this organism rather than detecting the bacterium directly. Because some medical conditions can trigger similar antibody responses, false positive syphilis test results may occasionally occur.

Infections that Can Test Positive for Syphilis

• Genital Herpes
• Rocky Mountain Spotted Fever
• Behçet Syndrome
• Viral Exanthem
• Mononucleosis
• Atopic Dermatitis
• Erythema Multiforme
• Lymphoma
• Contact Dermatitis
• Pityriasis Rosea

In addition, Malaria, HIV, Hepatitis C, and other infections can also cause a false positive syphilis test.

What Can Cause a False Positive Syphilis Test?

Several medical conditions and immune responses can cause false positive syphilis test results. Common causes include:

Possible Cause	How It May Affect Testing
Pregnancy	Temporary antibody changes
Autoimmune diseases (e.g., lupus)	Cross-reacting antibodies
HIV infection	Altered immune response
Hepatitis C	Antibody interference
Malaria	Non-specific antibody production
Recent vaccinations	Temporary immune activation
Intravenous drug use	Biological false-positive reactions
Certain cancers	Immune system abnormalities

These conditions are more likely to affect non-treponemal tests such as RPR and VDRL.

Points to Note about a False Positive Syphilis Test

• Hormonal changes and immune system alterations during pregnancy can sometimes cause a false positive syphilis test.
• Certain cancers, including lymphoma, can interfere with syphilis testing and result in false positives.
• Intravenous drug use can produce antibodies that may cause a false positive reaction in syphilis tests.
• Some vaccinations, particularly those that stimulate a robust immune response, can be a reason for false positive syphilis tests.
• Laboratory errors, such as cross-contamination or improper handling of samples, can cause false positive results.
• Infections caused by other treponemal bacteria (like yaws or pinta) can cross-react and produce false positives in the Syphilis tests.
• Chronic alcoholism and liver cirrhosis can sometimes interfere with syphilis tests, leading to false positives.

Biological False Positive RPR Test

A biological false positive RPR test occurs when the Rapid Plasma Reagin (RPR) test returns a reactive result despite the absence of syphilis infection. This happens because the RPR test detects antibodies that can also be produced during other medical conditions, including autoimmune disorders, viral infections, pregnancy, aging, and certain chronic diseases.

For this reason, a positive RPR result is typically followed by a treponemal-specific confirmatory test before diagnosing syphilis.

VDRL False Positive Causes

The Venereal Disease Research Laboratory (VDRL) test is a non-treponemal screening test commonly used for syphilis detection. Although highly useful, VDRL false positive results can occur due to:

• Pregnancy
• Autoimmune disorders such as lupus
• Tuberculosis
• HIV infection
• Hepatitis
• Malaria
• Recent vaccinations
• Advanced age

Because VDRL is a screening test, positive results should always be confirmed with a treponemal-specific assay.

Positive RPR but Negative Treponemal Test: What Does It Mean?

Sometimes a patient may receive a positive RPR result while a treponemal-specific antibody test remains negative. In many cases, this pattern suggests a false positive screening result rather than an active syphilis infection.

• Healthcare providers typically review:
• Clinical symptoms
• Medical history
• Risk factors
• Repeat testing results

before making a final diagnosis.

False Positive Syphilis Test During Pregnancy

Pregnancy can occasionally cause false positive syphilis screening results because hormonal and immune system changes affect antibody production. Since untreated syphilis can seriously affect pregnancy outcomes, healthcare providers usually perform confirmatory treponemal testing whenever an initial screening test is positive.

Most pregnant women with false positive screening results do not have an active syphilis infection after confirmatory testing is completed.

How Common are False Positive Syphilis Tests?

False positive syphilis tests are not as common as it seems. However, they are more common in pregnant women because of high antibody activity. Any acute illness can trigger antibodies in women, which can interfere with Syphilis tests.

Other than that, a false positive syphilis test occurs 11% of time with nontreponemal tests, whereas 14–40% of treponemal tests cause the same by using the reverse algorithm. So, both types of tests are used for a definitive diagnosis of Syphilis.

How to Read Syphilis Test Results?

Reading test results of Syphilis involves understanding both treponemal and nontreponemal tests. As mentioned, in most cases, treponemal tests are conducted first as they are specific to Syphilis and are less likely to produce false positives. A positive treponemal test almost confirms the presence of the infection. However, nontreponemal tests are also conducted for more definitive diagnosis.

In both cases, the test results are reported in terms of the concentration of specific antibodies measured in titers. Higher titer levels indicate an active infection, whereas low levels suggest a past infection or successful treatment.

When diagnosing Syphilis, healthcare providers use the results of both tests along with the patient's medical history. Considering various other factors, your doctor will interpret the rest of the results in the right context.

What if you Receive a False Positive for Syphilis

Remain Calm: A false positive syphilis test is more common than not, and further testing is necessary to confirm the diagnosis. Don’t be anxious about receiving false results.

Review Initial Test Type: Learn whether the initial test was non-treponemal (e.g., RPR or VDRL) or treponemal (e.g., FTA-ABS, TP-PA). As mentioned, nontreponemal tests are more prone to false positives.

Confirmatory Testing: Request a confirmatory test to validate the initial results. It could be a more specific treponemal test to validate the initial results.

Seek a Second Opinion: If you're unsure or uncomfortable with the results, get a second opinion from another healthcare professional to relieve yourself of anxiety and doubt.

Avoid Self-Diagnosis: Always rely on professional medical advice rather than self-diagnosing based on online information.

A positive screening result alone should never be used to diagnose syphilis. Healthcare providers rely on confirmatory testing, clinical findings, and patient history before making a final diagnosis. Individuals should always discuss test results with a qualified healthcare professional rather than relying solely on self-interpretation.

Conclusion

Understanding syphilis test results and the possibility of false positives is essential for early intervention of the infection. Confirm with further tests, consult your healthcare provider, and avoid self-diagnosis. Accurate interpretation and professional guidance ensure proper treatment and peace of mind. Always seek a second opinion if you have any doubts.