Syphilis Test Types Compared — RPR, VDRL, TPPA Explained
Syphilis testing involves an alphabet soup of acronyms: RPR, VDRL, TPPA, FTA-ABS, EIA. They're not interchangeable. Each measures something slightly different, and modern syphilis diagnosis usually requires two tests to confirm — a "two-step algorithm" that has changed in the last decade.
This guide tells you what each test does, when each is used, and what the numbers mean for your diagnosis.
The short answer
- Two categories of syphilis tests: non-treponemal (RPR, VDRL) and treponemal (TPPA, FTA-ABS, EIA, CIA).
- Non-treponemal tests measure your immune response to fats released by syphilis-damaged cells. Good for tracking treatment, can have false positives.
- Treponemal tests detect antibodies specifically against the syphilis bacterium. More specific, stay positive for life.
- Modern diagnosis requires BOTH categories (the "reverse sequence" or "traditional" algorithm).
- A single test is rarely conclusive by itself.
- Treatment monitoring uses the non-treponemal titer (RPR or VDRL).
How syphilis testing works at all
Syphilis bacterium (Treponema pallidum) is too thin to grow easily in lab cultures. So we don't test for the bacterium directly in most cases — we test for antibodies the immune system makes against it.
Two kinds of antibodies, two kinds of tests:
Non-treponemal antibodies
The body produces antibodies against fats (lipids) that get released when syphilis damages cells. These antibodies are useful but not specific — other conditions (pregnancy, autoimmune disease, recent vaccination) can produce them too.
Tests: RPR (Rapid Plasma Reagin) and VDRL (Venereal Disease Research Laboratory).
Treponemal antibodies
The body also produces antibodies specifically against syphilis bacterium proteins. These are highly specific — they only appear with actual syphilis exposure. They also tend to stay positive for life, even after treatment.
Tests: TPPA, FTA-ABS, EIA, CIA, TP-PA.
Each test, explained
RPR (Rapid Plasma Reagin)
- Category: Non-treponemal
- What it measures: Antibodies to lipids
- Format: Card test that detects "flocculation" (clumping)
- Result format: A titer (1:1, 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, etc.)
- Use: Initial screening AND treatment monitoring
- Sensitivity: ~85% primary, ~99% secondary syphilis
- Specificity: ~98% (can have false positives)
- Falls with successful treatment (titer should drop 4-fold by 6-12 months)
VDRL (Venereal Disease Research Laboratory)
- Category: Non-treponemal
- Use: Same as RPR — historical name, both still in use
- Best use case: Cerebrospinal fluid (CSF) testing for neurosyphilis (RPR can't be used on CSF)
For blood testing, RPR and VDRL are essentially equivalent.
TPPA (Treponema Pallidum Particle Agglutination)
- Category: Treponemal
- What it measures: Antibodies specific to syphilis bacterium
- Format: Particles coated with treponemal antigens
- Result: Reactive or non-reactive (and sometimes a titer)
- Use: Confirming a positive non-treponemal test, OR as initial test in reverse-sequence algorithm
- Sensitivity: ~98% primary, ~100% later stages
- Specificity: ~98–99%
- Stays positive for life after exposure, even with treatment
FTA-ABS (Fluorescent Treponemal Antibody Absorption)
- Category: Treponemal
- What it measures: Same as TPPA (specific antibodies)
- Format: Fluorescent dye + microscope
- Status: Largely replaced by TPPA in modern labs but still in use
- Sensitivity and specificity: Similar to TPPA
EIA / CIA (Enzyme Immunoassay / Chemiluminescence Immunoassay)
- Category: Treponemal (and sometimes non-treponemal versions)
- What it measures: Treponemal antibodies — uses automated lab equipment
- Use: Increasingly used as initial screening in the reverse sequence algorithm because they're automated and easy to run
- Result: Positive/negative + sometimes index value
- Pros: High throughput, easy
- Cons: Slightly higher false positive rate than TPPA → requires confirmation
The two diagnostic algorithms
This is where it gets interesting. There are two ways to combine these tests:
Traditional algorithm (older, still used in some labs)
- Screen with a non-treponemal test (RPR or VDRL)
- If reactive → confirm with a treponemal test (TPPA, FTA-ABS)
- Both positive = syphilis
- Non-treponemal positive but treponemal negative = false positive
Issue: misses some primary/early-stage and late syphilis cases where non-treponemal can be negative.
Reverse sequence algorithm (modern, preferred)
- Screen with a treponemal EIA/CIA
- If reactive → reflex to a non-treponemal test (RPR)
- If RPR is reactive too → syphilis confirmed; titer used for staging and treatment monitoring
- If RPR is non-reactive → confirm with another treponemal test (TPPA)
- Both treponemal positive, RPR negative = past infection (treated or latent)
- Discordant treponemal results = ambiguous, repeat testing
The reverse sequence is the CDC-recommended algorithm in the US and most countries with high-volume automated labs.
Reading your results — a practical guide
"RPR non-reactive, treponemal negative"
No syphilis at the level of detection. (Caveat: in very early primary syphilis — before antibodies develop — both can be negative. If you have a chancre, get tested again in 4 weeks.)
"RPR non-reactive, treponemal positive"
Possibilities:
- Successfully treated past syphilis — treponemal tests stay positive for life
- Very early primary with treponemal antibodies but no non-treponemal yet
- Late latent syphilis — non-treponemal can wane to negative
Discuss with provider. May need additional testing.
"RPR reactive, treponemal positive"
Syphilis present. The RPR titer indicates infection level:
- 1:1 to 1:4 = lower titer, possibly late/latent or treated
- 1:8 to 1:32 = mid-range, often early infection
- 1:64+ = high titer, often active early infection
Stage determination requires clinical info (symptoms, exposure history, prior testing).
"RPR reactive, treponemal negative"
Likely false positive. Causes:
- Pregnancy
- Autoimmune disease (lupus, RA)
- Other infections (mononucleosis, malaria, HIV)
- Recent vaccination
- IV drug use
May need repeat testing or alternative treponemal test.
Titers and what they mean
A "titer" is the highest dilution at which the test is still reactive. 1:64 means the serum was diluted 64-fold and still reacted. Higher = more antibody = generally more active infection.
Four-fold change is meaningful. From 1:32 → 1:8 is a four-fold decrease, suggesting treatment is working. From 1:32 → 1:16 (two-fold) is within test variability — not significant.
Treatment monitoring
After treatment, the RPR titer should drop:
- Primary or secondary syphilis: four-fold drop by 6 months
- Early latent: four-fold drop by 12 months
- Late latent: four-fold drop by 24 months (often slower; some never seroconvert)
If the titer doesn't drop as expected, it can mean:
- Re-infection (most common in MSM with multiple partners)
- Treatment failure (rare with penicillin)
- "Serofast" state — titer plateaus at a low level, neither dropping nor rising
If the titer rises four-fold after dropping, it usually means re-infection.
The treponemal test stays positive for life — once you've had syphilis, you'll always test positive on TPPA/FTA-ABS. This is why monitoring uses RPR.
Other syphilis-related tests
Dark-field microscopy
Direct visualization of T. pallidum in fluid from a chancre. Diagnostic for primary syphilis if positive. Rarely used now — requires immediate sample handling.
PCR
Detects syphilis DNA. Used in research and reference labs. Very specific. Not standard outpatient testing.
CSF VDRL
Tests cerebrospinal fluid for neurosyphilis. Done by lumbar puncture when neurosyphilis is suspected (visual changes, hearing changes, neurologic symptoms in a syphilis patient).
CSF FTA-ABS
Treponemal test on spinal fluid — used as backup when CSF VDRL is negative but neurosyphilis is still suspected.
Common questions
"Why did my partner test positive and I test negative?"
Window period or asymptomatic exposure with insufficient bacterial dose. Re-test in 4–6 weeks.
"Can the test be wrong?"
False positives happen (especially with non-treponemal tests). False negatives happen (especially in very early or very late stages). The two-step algorithm catches most errors.
"Why do I still test positive after treatment?"
Treponemal tests stay positive for life. Your RPR titer should be dropping. As long as RPR is going down (or has stabilized at a low level), you're fine.
"Can syphilis come back?"
The infection itself doesn't reactivate (unlike herpes or HIV). But re-infection from a new exposure is possible and common. Treponemal test stays positive; RPR rises if re-infected.
"Do home syphilis tests work?"
Some at-home test kits exist (mail-in blood collection). They use the same lab assays. Generally reliable when properly collected. Treatment requires a doctor either way.
"How long after exposure can I test?"
- 3–6 weeks for primary syphilis (chancre stage)
- 6 weeks for reliable serology
- 3 months for high-confidence ruling-out
When to retest
- 3, 6, 12 months after treatment
- 3 months after a new exposure
- Every 3–6 months if you're a high-risk patient (MSM, multiple partners, history of syphilis)
- Pregnancy: first prenatal visit, third trimester, and at delivery
Bottom line
Syphilis testing:
- RPR/VDRL = non-treponemal, used for screening + treatment monitoring (titer drops with cure)
- TPPA/FTA-ABS/EIA = treponemal, used for confirmation (positive for life)
- Reverse sequence algorithm is the modern standard
- Both kinds positive = syphilis (active or past)
- RPR titer is what you watch after treatment
- Treponemal positivity is permanent and not a sign of failed treatment
If you have a positive syphilis test: confirm with the other test type, get the titer, work with your provider to determine stage and treat. Modern syphilis is fully curable with penicillin.
For more, see Syphilis symptoms by stage, Syphilis epidemic 2026, STI testing window periods, and our Syphilis pillar guide.
