18.3 Treponema, Borrelia, and TB Infection Serodiagnosis

18.3 Treponema, Borrelia, and TB Infection Serodiagnosis

Quick Answer: RPR/VDRL titer tracks syphilis activity and treatment response; treponemal EIA, FTA-ABS, and TP-PA usually stay positive. Traditional screening is RPR then treponemal confirmation; reverse screening is treponemal EIA/CIA, then RPR, then TP-PA if the RPR is negative. Lyme is two-tier (EIA then immunoblot, or two EIAs); erythema migrans is a clinical diagnosis and early serology is often negative. IGRA (QFT, T-Spot) and PPD detect TB infection and do not split latent from active disease. Smear, NAAT, and culture diagnose active TB. West Nile and other arbovirus encephalitis serology is SM-only.

Outline III.E.5–6 (guideline 2025-09-25) names stages and serodiagnosis of Treponema pallidum and Borrelia burgdorferi, then serodiagnosis of tuberculosis infection with IGRA and PPD. Method principles for RPR versus treponemal tests were 18.1. This section is the clinical algorithm. III.E.4 (West Nile and other arboviruses) remains SM-only; do not treat it as M-tested content.

Syphilis stages and the two antibody families

T. pallidum subsp. pallidum is sexually transmitted (and vertically). Stages organize both the clinic and the laboratory.

Nontreponemal tests (RPR, VDRL) detect reagin antibody to cardiolipin–lecithin–cholesterol. Report reactive plus a titer (highest dilution still reactive). Use the titer to:

• Estimate activity (high in secondary disease).
• Follow treatment response (a four-fold decline, for example 1:32 to 1:8, is serologic cure language).
• Detect reinfection as a four-fold rise after a new exposure.

Nontreponemal tests are not specific. Biological false positives occur in pregnancy, SLE and other autoimmune disease, malaria, endocarditis, and injection-drug use. That is why a reactive RPR is confirmed with a treponemal test. VDRL on CSF is the named neurosyphilis nontreponemal test: a reactive CSF VDRL is highly specific in the right fluid, but a negative CSF VDRL does not exclude neurosyphilis because sensitivity is limited. Do not follow treatment with FTA-ABS titers; treponemal tests are not built as activity markers.

Treponemal tests (EIA, CIA, FTA-ABS, TP-PA) use T. pallidum antigens. They are more specific than RPR and usually remain positive for life after treated or untreated infection. They do not distinguish treated past syphilis from untreated latent disease by themselves. FTA-ABS is an absorbed immunofluorescence assay; TP-PA is a gelatin-particle agglutination. Automated EIA/CIA is what most laboratories now use as the reverse-sequence screen.

Traditional versus reverse algorithms

Traditional algorithm: screen with RPR (or VDRL). If reactive, confirm with a treponemal test (TP-PA or FTA-ABS historically). A reactive pair is syphilis (current or treated, interpreted with titer and history). A reactive RPR with a negative treponemal test is a biological false-positive RPR.

Reverse algorithm: screen with a treponemal EIA or CIA. If reactive, reflex RPR with titer. If the RPR is also reactive, report syphilis and use the titer for activity and follow-up. If the EIA is reactive and the RPR is nonreactive, perform a second treponemal test (TP-PA). TP-PA reactive plus RPR nonreactive usually means old treated syphilis, very early infection, or late disease with a faded nontreponemal titer—clinical history decides. TP-PA nonreactive plus EIA reactive plus RPR nonreactive is usually a false-positive screening EIA.

Both algorithms require two different methods. Do not report a lone CIA as “active syphilis,” and do not skip the titer: public health and the clinician need the RPR dilution, not just a qualitative reactive.

Prozone belongs on secondary-syphilis items: if the clinical picture is classic and the undiluted RPR is nonreactive, dilute and repeat before calling the patient seronegative.

Lyme disease: two-tier timing

Borrelia burgdorferi (and related genospecies) is transmitted by Ixodes ticks. Early localized disease is erythema migrans (EM), a slowly expanding annular rash days to weeks after the bite. Disseminated disease includes multiple EM lesions, facial-nerve palsy, meningitis, and later arthritis.

Standard two-tier testing: a first-tier EIA or IFA. If negative, stop (unless early disease is still suspected and a later specimen is planned). If equivocal or positive, reflex immunoblot (Western blot).

• First 30 days of illness: IgM and IgG blots may be used. IgM blot is positive when 2 of 3 bands (23/OspC, 39, 41 kDa) are present.
• After 30 days: use IgG blot only (5 of 10 bands). Isolated IgM positivity late is a common false-positive and should not be called Lyme.

Modified two-tier testing (MTTT) uses two different EIAs (often a whole-cell or enzyme immunoassay followed by a VlsE/C6 or similar EIA) instead of immunoblot. CDC accepts validated MTTT. The exam idea is the same: screen, then a second serologic method—not a single EIA as proof.

Timing with EM is the highest-yield trap. In the first days of a classic EM rash, two-tier serology is often still negative. EM is a clinical diagnosis in an endemic setting; do not delay treatment for a blot, and do not report a negative EIA as “not Lyme” when the stem describes a bull’s-eye rash four days after a recognized tick bite. Sensitivity rises in disseminated and late disease. Serology is not a test of cure; antibody persists. Do not screen asymptomatic patients with a lone IgM blot.

TB infection: IGRA versus PPD, and what neither test is

Tuberculin skin test (PPD/TST) injects purified protein derivative intradermally and is read as induration at 48–72 hours. Prior BCG vaccine and some nontuberculous mycobacteria cause false-positive TST. The patient must return for a second visit. Boosting can confuse serial testers.

IGRA measures IFN-γ released by T cells after stimulation with M. tuberculosis–specific antigens (ESAT-6, CFP-10), which are missing from BCG and from most NTM (exceptions include M. kansasii, M. marinum, and M. szulgai). Two assay formats are named:

• QuantiFERON-TB Gold Plus (QFT): ELISA of IFN-γ in plasma from nil, TB1, TB2, and mitogen tubes. TB minus nil above cutoff is positive. Indeterminate means high nil (background IFN-γ) or low mitogen (anergy, immunosuppression, tube error)—repeat or switch methods; do not call it negative.
• T-Spot.TB: ELISPOT that counts IFN-γ–secreting cells. Also includes nil and positive-control wells.

IGRA advantages: one visit, no BCG cross-reaction from vaccine, less reader subjectivity. IGRA does not distinguish latent TB infection from active TB. A positive QFT in a patient with fever and a cavity does not prove that the cavity is TB, and a negative IGRA does not exclude active disease, especially in advanced immunosuppression.

Active TB is a microbiology diagnosis: acid-fast smear, NAAT (for example Xpert MTB/RIF), and culture, as in Chapter 14. IGRA and PPD belong on pre-employment, contact, and latent-infection questions. They do not replace the AFB bench when the stem is night sweats, hemoptysis, and a cavitary infiltrate.

Do not add West Nile IgM, dengue, or other arbovirus encephalitis panels to this chapter as if they were M content. Those serologies are SM-only on III.E.4. If an M stem describes encephalitis, the tested viral CSF pathogens remain the Chapter 15 list (enterovirus, HSV), not an arbovirus EIA.