Two-tier Lyme testing begins with a crucial distinction: a positive Lyme screening test is not, by itself, a positive Lyme test. That distinction matters when syphilis is in the differential, because antibodies produced against Treponema pallidum can react with antigens used in some Lyme disease screening assays. Treating the screen as a diagnosis can lead a clinician to label a patient with syphilis as having Lyme disease.
Two-tier Lyme testing was designed in large part to prevent nonspecific reactions from becoming diagnoses. The first test is sensitive; the second asks whether the antibody response is specific enough to support Lyme disease. Published cases show that this second step has correctly rejected Lyme screening reactions later traced to syphilis. The safeguard is valuable—but incomplete unless clinicians investigate why the first test reacted. Syphilis serology itself can be negative in early infection, late disease, certain immunocompromised patients, and rare diagnostically difficult cases.
Why Syphilis Can Produce a Positive Lyme Screen
Lyme disease and syphilis are caused by different spirochetes—Borrelia burgdorferi (and related species) and Treponema pallidum. An immune response to one can sometimes recognize antigens used to test for the other.
The CDC lists syphilis among conditions capable of causing false-positive cross-reactions in Lyme serology. A 2023 review of Lyme serodiagnostic cross-reactivity described nonspecific recognition involving antigens such as FlaB, OspC, BmpA, and VlsE. Some of these proteins are highly immunogenic, but antibody binding is not exclusive to Lyme disease.[1] [2]
A reactive first-tier assay therefore only indicates that antibodies bound strongly enough to cross a laboratory threshold. It does not establish what produced those antibodies.
How Two-Tier Lyme Testing Filters Cross-Reactivity
CDC currently accepts two forms of two-tier Lyme serology:
- Standard two-tier testing: enzyme immunoassay (EIA) followed, if positive or equivocal, by IgM and IgG immunoblots.
- Modified two-tier testing: a second EIA using different antigen designs in place of immunoblots, when the assays are cleared for use together.
Under either approach, the overall result is positive only when the combined first- and second-tier findings meet the assay’s interpretation criteria.[3] [4]
The second tier is a specificity filter. Adding the immunoblot to a first-tier EIA raised specificity above 98% in a multicenter evaluation and above 99% in a later systematic review across clinical phases.[5] [6]
That filtering function is what protects a patient with syphilis from being diagnosed with Lyme disease solely on the basis of cross-reactive antibodies. If the first Lyme assay reacts but the second tier does not support Lyme disease, the correct laboratory interpretation is not “positive Lyme.” The discordance is a signal to reassess the clinical picture.
Six Cases in Which the Second Tier Pointed Away From Lyme
A 2011 report illustrates the system working as intended. A 44-year-old man developed visual-field defects and papilledema; neuroborreliosis was initially suspected. His Lyme screening assay was positive, but a C6-based confirmatory test was negative and Western blots on serum and cerebrospinal fluid did not confirm Lyme disease. VDRL and TPPA were positive, supporting the eventual diagnosis of neurosyphilis.[7]
The investigators then reviewed their laboratory database and identified five additional patients with positive Lyme screens but negative Western-blot confirmation whose reactions were attributed to T. pallidum antibodies. These six cases do not provide a population incidence, but they demonstrate the diagnostic mechanism. Without the second tier, the initial Lyme screen could have directed attention toward the wrong spirochetal infection.
The researchers’ recommendation remains useful: when Lyme screening is positive but confirmation is negative, syphilis serology should be considered. The second tier may prevent the wrong diagnosis while the rejected screen still supplies a clue to the right one.
The Cross-Reactivity Is Asymmetric, Not Strictly One-Way
A 2016 investigation found that syphilis-positive sera could react in Lyme screening assays and found no evidence, in the assays and specimens it studied, that prior B. burgdorferi infection produced false-positive syphilis screens.[8] That result should not be generalized to every syphilis assay. In an older study using the whole-organism FTA-ABS method, 9 of 40 Lyme disease sera reacted at a 1:5 dilution with T. pallidum antigen, although only one remained reactive at 1:10.[9]
For the diagnostic problem addressed here, the better-documented pattern remains syphilis antibodies triggering Lyme screening assays. The second tier in two-tier Lyme testing helps prevent that initial reaction from being interpreted as laboratory confirmation of Lyme disease. Reverse cross-reactivity is less consistently observed and depends on the syphilis assay used. The broader testing and stigma questions are examined in “Lyme Has Warriors. Syphilis Has Silence.”
Preventing a Lyme Diagnosis Is Only Half the Job
The cross-reactivity question and the false-negative question are separate. Two-tier Lyme testing can correctly reject Lyme disease, yet the subsequent investigation for syphilis can still fail when syphilis serology is nonreactive. The remainder of this article addresses that second diagnostic failure mode.
Two-tier Lyme testing answers a limited question: does the complete antibody pattern meet laboratory criteria for Lyme disease? It does not identify every reason for a reactive first-tier assay. Once Lyme disease is not confirmed, the diagnostic process should not automatically end.
A patient can fall into a gap between two testing systems. The Lyme algorithm correctly says the result does not support Lyme disease, while a single syphilis test may also be negative. Without attention to disease stage, immune status, symptoms, and the specific assays performed, a clinician may conclude that neither infection requires further consideration.
Syphilis testing is itself algorithmic. The traditional sequence begins with a nontreponemal test—RPR or VDRL—and confirms a reactive specimen with a treponemal test. The reverse sequence begins with an automated treponemal immunoassay, follows with a quantitative RPR or VDRL, and uses a second treponemal assay to resolve certain discordant results. CDC emphasizes that treponemal and nontreponemal tests should be used together.[10]
Why Syphilis Serology Can Be Negative
During very early primary syphilis, antibodies may not yet have reached detectable levels. Nontreponemal testing can also be falsely negative because of the prozone phenomenon, in which unusually high antibody concentrations interfere with the reaction in an undiluted specimen. CDC reports that prozone-related false-negative RPR results are rare—below 0.85% in two large referred populations—and occur more often in primary and secondary disease, although the phenomenon can appear at any stage. Clinicians should request dilution testing when signs or symptoms strongly suggest syphilis despite a nonreactive nontreponemal result.
The limitations of nontreponemal testing become more pronounced as infection becomes older. Sensitivity declines in late latent and tertiary syphilis, although the available estimates often come from small or incompletely characterized samples. Treponemal tests generally remain more sensitive in late disease, but they are not infallible.
Syphilis Test Performance by Stage and Condition
| Stage or Condition | Test Type | Estimated Sensitivity | Diagnostic Limitation and Next Steps |
|---|---|---|---|
| Early primary | Nontreponemal (RPR/VDRL) | Reduced during the serologic window period | Antibodies may not yet be detectable; repeat testing or direct detection may be appropriate when suspicion remains high. |
| Late latent | Nontreponemal (VDRL) | 63%–66% | Based on small international studies; VDRL alone cannot exclude infection. |
| Tertiary | Nontreponemal (VDRL/RPR) | 47%–64% | Sensitivity is limited in long-standing disease. |
| Tertiary | Treponemal (FTA-ABS) | Approximately 70.6% | Estimate from one study of only 17 cases with incompletely stated staging criteria. |
| Profound B-cell depletion | Treponemal and nontreponemal serology | No reliable population estimate | Case reports document persistent seronegativity; direct tissue detection may be required. |
Prozone is not a disease stage or a stage-specific sensitivity estimate. It is a rare interference phenomenon affecting nontreponemal testing. CDC reports prozone-related false-negative RPR results in fewer than 0.85% of specimens in two large referred populations; dilution testing should be requested when clinically indicated.[10]
Treponemal tests usually remain reactive for life and are generally much more sensitive in late disease, which is why modern algorithms combine test types. “Usually,” however, is not the same as “without exception.” The rarity of modern tertiary syphilis makes precise estimates difficult.
The 43-Year-Old Woman With a “Tree-Bark” Aorta
A 2023 forensic case shows how difficult the problem can become. A 43-year-old woman died suddenly from a ruptured thoracic aortic aneurysm. The inner surface of the aorta had the wrinkled “tree-bark” appearance classically associated with syphilitic aortitis, though not unique to it. Microscopy showed a predominantly perivascular plasma-cell infiltrate, and T. pallidum immunohistochemical staining identified multiple spirochete-like organisms.[11]
Postmortem femoral blood was nonreactive by both RPR and TPPA; PCR on formalin-fixed aortic tissue was negative. Previous antenatal screens had been negative and no exposure history was known. The authors still considered the gross and microscopic findings sufficiently suggestive to include the possibility of syphilis in the postmortem report, with potential implications for partners and children.
This case does not prove that every negative RPR and TPPA combination conceals infection. It documents strong pathologic suspicion that could not be reconciled with negative serology and PCR. The authors recommended early consultation with microbiology specialists and consideration of newer techniques, including 16S rRNA sequencing, in similarly difficult cases.
Modern Cases Confirm That Complete Seronegativity Can Occur
Seronegative syphilis is uncommon but not theoretical. In 2026, researchers described a 44-year-old man with lymphoma receiving the B-cell-depleting agent obinutuzumab. He developed a characteristic palmoplantar eruption and systemic symptoms; repeated treponemal IgG, RPR, and TPPA tests remained nonreactive, with prozone ruled out. Skin biopsy showed a plasma-cell-rich pattern, and T. pallidum immunohistochemistry demonstrated numerous organisms. The eruption resolved after penicillin while serology stayed negative.[12]
Similar cases have been reported with rituximab and ofatumumab. These immunocompromised examples do not justify treating every negative syphilis test as suspect. They establish a narrower point: antibody-based tests require a measurable antibody response. When B-cell function is profoundly suppressed—or when disease stage and specimen limitations reduce the signal—clinical findings and direct detection become more important.
What a Discordant Lyme Result Should Mean
When a first-tier Lyme screen is reactive but the second-tier immunoblot or EIA is negative, the result should not be treated as laboratory confirmation of Lyme disease. Depending on symptom duration and exposure history, early Lyme disease may still require clinical follow-up or appropriately timed repeat testing. At the same time, clinicians should consider alternative causes of the first-tier reaction, including syphilis, relapsing-fever borreliae, Epstein–Barr virus, and certain autoimmune conditions identified in CDC guidance.
Diagnostic Action Checklist for Discordant Lyme Results
- Audit the specific syphilis algorithm used: Verify whether traditional screening, with a nontreponemal test first, or reverse screening, with a treponemal immunoassay first, was performed.
- Request serum dilution: Rule out prozone interference in nonreactive RPR or VDRL tests when clinical suspicion remains high.
- Order orthogonal treponemal testing: Use a different treponemal test format and antigen target, such as TPPA, when appropriate to the algorithm.
- Assess immunosuppressive history: Review active or past anti-CD20 monoclonal antibody therapies that can profoundly blunt antibody responses, including rituximab and obinutuzumab.
- Consult specialists about direct detection: When compelling clinical or pathologic findings conflict with serology, consult infectious-disease and microbiology specialists about tissue biopsy, immunohistochemistry, PCR, or 16S rRNA sequencing.
A reactive Lyme screen does not prove syphilis any more than it proves Lyme disease. It is a clue that must be resolved through the complete Lyme algorithm, the appropriate syphilis algorithm, and the clinical evidence.
The Safeguard Works Only If the Investigation Continues
Two-tier Lyme testing can help prevent syphilis from being misdiagnosed as Lyme disease because it refuses to let a broadly reactive screening result stand alone. The six-case report shows the design working: the first test pointed toward Lyme, the second tier rejected that conclusion, and syphilis testing revealed the actual diagnosis.
The remaining danger is diagnostic complacency. A negative second-tier Lyme result does not explain the initial reaction, and a negative syphilis result is not equally definitive at every stage or in every patient. The “tree-bark” aorta case shows profound pathologic suspicion despite negative serology and PCR. The B-cell-depleted cases separately demonstrate that serology can remain negative even when tissue-based testing identifies the organism.
Testing algorithms are tools for disciplined interpretation. The Lyme second tier can stop the wrong diagnosis; patient safety depends on taking the next step and asking what caused the first test to react.
Medical note: This article is educational and does not diagnose any individual. People with possible exposure, compatible symptoms, pregnancy, neurologic or ocular symptoms, immune suppression, or unexplained discordant test results should seek evaluation from qualified clinicians using current testing and treatment guidance.
