Biological False‑Positive Rapid Plasma Reagin (RPR) Results

Prevalence in the General Population

In unselected adults, the baseline rate of a biological false‑positive RPR is 0.6 %–1.3 %. 1

Among individuals living with HIV infection, the false‑positive RPR rate rises to ≈10.7 %, roughly ten‑fold higher than in the general population. 1
Hepatitis B virus infection is associated with a false‑positive RPR rate of ≈8.3 %. 2
Hepatitis C virus infection shows a false‑positive RPR rate of ≈4.5 %. 1

Pregnant persons have a false‑positive RPR rate of ≈0.6 %. 2
Patients older than 60 years exhibit a modestly higher false‑positive rate (0.34 %) compared with younger adults (0.25 %). 1
Autoimmune diseases—particularly systemic lupus erythematosus and other connective‑tissue disorders—are linked with an increased likelihood of a false‑positive RPR. 2
Intravenous drug use is associated with elevated false‑positive RPR frequencies. 1

When the RPR titer is ≤ 1:8 (including the lowest tested dilution of 1:1), 27 %–31 % of reactive results represent biological false‑positives. 1

The prozone phenomenon—a false‑negative RPR caused by extremely high antibody concentrations—occurs in secondary syphilis with high titers and does not apply to low‑titer (1:1) results. 3

Do not treat for syphilis when the RPR is reactive but the treponemal test is negative, because both tests must be positive for a syphilis diagnosis. 2
Investigate underlying causes of a false‑positive RPR by screening for the associated conditions listed above (HIV, hepatitis B/C, autoimmune disease, pregnancy). 1

All statements are supported by the cited clinical evidence from Clinical Infectious Diseases* (2020).

The Centers for Disease Control and Prevention (CDC) guidelines indicate that non-reactive results on all three tests (initial treponemal antibody screen, RPR, and confirmatory TPPA) signify "no laboratory evidence of syphilis" 4, 5
The American College of Physicians and other medical guidelines recommend retesting in 2-4 weeks if recent exposure is suspected, as treponemal antibodies typically appear 1-4 weeks after infection, and nontreponemal antibodies (RPR) appear slightly later but are reliably positive by 4-6 weeks in primary syphilis 5

If the exposure occurred within the past 2-4 weeks, negative results do not definitively exclude early primary syphilis, as the patient may be in the "window period" before antibodies develop, according to the CDC and other guidelines 5
The sensitivity of both treponemal and nontreponemal tests is reduced in very early infection (first 1-3 weeks after exposure), making repeat testing essential if exposure was recent, as recommended by medical guidelines 5

If exposure was within the past 4 weeks, the CDC recommends repeating complete syphilis serology (treponemal antibody screen with reflex to RPR and TPPA) in 2-4 weeks from the initial test, and examining carefully for primary chancre or ulcer at potential exposure sites 5, 4
Consider empiric treatment with benzathine penicillin G 2.4 million units IM as a single dose if the patient is at high risk for loss to follow-up or if clinical suspicion is very high based on exposure circumstances, as suggested by medical guidelines 5
If exposure was more than 6-8 weeks ago, negative results effectively rule out syphilis infection, and no further testing or treatment is needed unless new exposure occurs, according to medical guidelines 5