The presence of the RPR acronym on a blood test report can raise questions. This test is a common screening tool for syphilis infection. Understanding its meaning, interpretation, and the steps that follow a result is essential for good health management. This article aims to provide clear and factual information about the RPR test.

What is the RPR (syphilis) test?

The bacterium Treponema pallidum causes syphilis. Healthcare professionals use the RPR (Rapid Plasma Reagin) blood test to screen for this infection. This test does not detect the bacterium itself but looks for antibodies that the immune system produces to fight it.

These antibodies, called reagins, are directed against substances released by cells damaged by the bacterium. The RPR test is therefore a non-treponemal test. It functions as an alert system indicating a possible infection. Its rapidity and low cost make it an excellent initial screening tool.

Healthcare professionals widely use it because syphilis can develop silently for many years. Early detection is therefore crucial to implement effective treatment and avoid long-term complications. Due to its non-specific nature, a positive RPR test result always requires confirmation by another test.

Why is it important to understand this test?

The RPR test is a key marker of sexual and overall health. If left untreated, syphilis can have serious repercussions on many organs, especially the heart and nervous system. Screening is therefore a major public health issue. It is estimated that there are over 7 million new cases of syphilis worldwide each year.

An undetected infection can evolve through several phases over decades. Late complications can be severe. Early detection through the RPR test allows for simple and effective antibiotic treatment that prevents these risks.

RPR also plays a fundamental role in treatment monitoring. After a course of antibiotics, the antibody level detected by RPR should decrease. This reduction confirms that the treatment has worked. A lack of decrease may indicate reinfection or treatment failure, requiring new management.

How to read and understand your RPR analysis

The laboratory presents the result of an RPR test in two ways on your report.

Result presentation

• Qualitative: The result is simply indicated as “Positive” or “Negative”.
• Quantitative: The result is expressed by a titer, which corresponds to a dilution ratio (e.g., “1:8”, “1:32”). A high titer means a greater concentration of antibodies.

The general interpretation is as follows:

• A negative result means that no antibodies were detected.
• A positive result suggests a possible infection, current or past, and must be confirmed.

The physician uses the quantitative titer to assess the activity of the infection and to monitor how the patient responds to treatment. For example, a titer of 1:64 indicates a more active infection than a titer of 1:8.

Checklist for a first analysis

1. Locate the “RPR” or “VDRL” line (a similar test).
2. Identify the status: positive or negative.
3. If the result is positive, note the titer (e.g., 1:16).
4. Check if a confirmation test (like TPHA or FTA-ABS) was performed in parallel.
5. Compare the titer with any previous results.

RPR Test and Syphilis Stages

The RPR test screens for syphilis. Without treatment, this infection evolves through several phases.

Primary Syphilis

This first phase appears approximately 3 weeks after infection. It manifests as a chancre (a painless ulceration) at the infection site. At this stage, the RPR test can be positive in 70 to 80% of cases, often with a low titer. However, a test performed too early may be negative.

Secondary Syphilis

It occurs a few weeks or months later. Skin rashes, particularly on the palms and soles of the feet, are typical. The RPR test is positive in almost 100% of cases, with often very high titers (e.g., > 1:32), reflecting a strong immune reaction.

Latent Syphilis

This is a symptom-free phase that can last for years. The person remains contagious during the first year (early latency). RPR remains positive, with titers tending to slowly decrease over time.

Tertiary Syphilis

This late phase (10 to 30 years after infection) is rare today thanks to treatments. It is characterized by severe neurological or cardiovascular damage. RPR can be positive but often with a low titer.

Special Situations with a Positive RPR

Biological False Positives

The RPR test can be falsely positive. It detects antibodies that are not specific to syphilis and can appear in other contexts:

• Pregnancy
• Autoimmune diseases (lupus)
• Certain viral infections (mononucleosis, hepatitis)
• Advanced age

In these cases, the RPR titer is generally low (< 1:8) and the treponemal confirmation test (TPHA, etc.) will be negative.

Jarisch-Herxheimer Reaction

This inflammatory reaction (fever, headache) can occur within hours of starting antibiotic treatment. It is due to the release of toxins by destroyed bacteria. It is a transient reaction that does not mean an allergy to the antibiotic.

Practical Advice and Follow-up

What to do after an RPR test result?

• Negative RPR: If you have no risk factors, a routine check-up is sufficient. If you had a risky exposure, a new test is recommended after 6 weeks.
• Positive RPR: A medical consultation is essential. A confirmation test will be performed. If syphilis is confirmed, antibiotic treatment will be prescribed.

Follow-up after treatment

Monitoring the RPR titer is crucial to confirm healing.

• A check-up is performed at 3, 6, and 12 months after treatment.
• The titer should decrease by at least fourfold in 6 to 12 months (for example, from 1:64 to 1:16 or less).
• A lack of decrease requires a new evaluation.

Prevention

• Consistently use a condom.
• Undergo regular screening for sexually transmitted infections (STIs) in case of multiple partners.
• Inform partners in case of a positive diagnosis so they can get tested and treated.

Frequently Asked Questions about RPR and Syphilis

Can the RPR test remain positive after effective treatment?

Yes. This is called a “serological scar.” For some individuals, RPR remains positive at a very low titer (e.g., ≤ 1:8) for life. What is important is the significant drop in titer after treatment, not its complete negativization.

What is the difference between RPR and TPHA?

RPR is a non-treponemal test, useful for screening and treatment monitoring. TPHA is a treponemal test, highly specific for the bacterium. It confirms the diagnosis but generally remains positive for life, even after healing. Therefore, it cannot be used to monitor treatment effectiveness. The two are often used together.

How to interpret a positive RPR in a pregnant woman?

When facing a positive RPR in a pregnant woman, the doctor must immediately perform a confirmation test. If this test confirms syphilis, the doctor will initiate treatment without delay. Treatment prevents the mother from transmitting the bacterium to the fetus, thereby preventing congenital syphilis and its serious consequences.

Can an RPR be falsely negative?

Yes, it’s rare. A falsely negative result occurs when the body has not yet produced enough antibodies at the beginning of the infection. Another case, the “prozone effect,” occurs when an excessively high antibody concentration interferes with the reaction. The laboratory corrects this phenomenon by diluting the patient’s sample.

Should a positive RPR with a negative TPHA be treated?

No. This situation corresponds to a “biological false positive.” The RPR is positive for a reason other than syphilis, and the TPHA, which is more specific, confirms the absence of Treponema pallidum infection. No syphilis treatment is necessary.

Additional resources

• To deepen your knowledge, click here.
• To extend your knowledge and decipher other markers, more articles are available here.

Confused by your blood test results?

Get instant clarity. AI DiagMe interprets your blood test results online in minutes. Our secure platform translates complex medical data into an easy-to-understand report. Take control of your health today. Visit aidiagme.com to get your personalized insights now.