คำอธิบายการตรวจเลือดหาโรคซิฟิลิส: ทำไมจึงมีสองประเภท

อัปเดตล่าสุด: 30 สิงหาคม 2569

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Syphilis is one of the sexually transmitted infections I see regularly. How it presents depends on the stage — a painless genital ulcer early on, a rash on the palms and soles later. A blood test is the only way to confirm the diagnosis, and that is where the confusion starts, because there are two different families of blood test and each has its own strengths and limits.

This matters more than a technical detail. Patients who receive a positive result without a full explanation often leave badly frightened, and end up going from clinic to clinic looking for a second opinion that says something different. Understanding what each test actually measures usually removes the fear entirely.

1. Nontreponemal tests — the screening tests

These are used first, when syphilis is suspected. Their strength is that they reflect active infection, which also makes them the tests used to check whether treatment is working.

1. VDRL (Venereal Disease Research Laboratory)
2. RPR (Rapid Plasma Reagin)

Their weakness is that they can turn positive when there is no syphilis at all — a biological false positive, which several other conditions can cause. That is precisely why a positive screening test is never the end of the story. They can also read falsely negative very early in infection, before antibodies have developed, so a recent exposure with a negative result may need repeating.

2. Treponemal tests — the confirmation tests

If a VDRL or RPR comes back positive, the diagnosis has to be confirmed with a treponemal test.

1. TP-PA (Treponema Pallidum Particle Agglutination)
2. FTA-ABS (Fluorescent Treponemal Antibody Absorbed)

These are highly accurate for syphilis. Their limitation is the one that causes the most distress: a treponemal test stays positive for the rest of your life, even after the infection has been completely cured. It tells you that you have encountered syphilis at some point — not that you have it now. This is why treatment can never be judged by a treponemal test alone.

One practical note, because it confuses people who compare results between hospitals: some laboratories now run the two in the opposite order, using an automated treponemal test as the initial screen and then adding a VDRL or RPR afterwards. If your treponemal test came back first, that is a laboratory workflow rather than a mistake. The logic of interpreting the pair together does not change.

Reading the two together

The practical rule is simple once you have it: the confirmation test answers whether it was ever syphilis, and the screening test answers whether it is active now and whether the treatment is working. Treatment response is followed by watching the VDRL or RPR titre fall over the months after treatment. A permanently positive treponemal test alongside a falling titre is exactly what a successfully treated patient looks like.

Two practical points that follow from all this. A confirmed diagnosis means current sexual partners need testing and treatment as well, whatever your own result now shows. And because syphilis travels in company, testing for other sexually transmitted infections at the same time is standard rather than an extra.

Three things that must not be left out of the conversation

HIV testing. Syphilis and HIV travel together more closely than any other pair in this field: the ulcer of early syphilis breaks the skin barrier and raises the risk of acquiring HIV, and the two are frequently diagnosed in the same person. An HIV test belongs with a syphilis diagnosis as a matter of routine, not suspicion. HIV testing is not carried out in the urology clinic here — at Bangkok Hospital Headquarters it is handled by the Infectious Diseases department, and I refer patients there. Testing and treatment for other sexually transmitted infections, and HPV vaccination, are available with me.

Pregnancy. If you are pregnant, may be pregnant, or your partner is, this changes from a matter about you to a matter about the baby. Syphilis crosses the placenta and can cause serious harm to an unborn child, and it is one of the few such infections that is almost entirely preventable — treatment during pregnancy protects the baby. Antenatal screening exists precisely for this reason. Tell your obstetric team, and do it early, because the timing of treatment during pregnancy affects how well the baby is protected.

Symptoms in the eyes, ears or nervous system. Syphilis can affect these at any stage, not only late in the disease, and when it does the treatment is different and more intensive. Seek assessment the same day — in Thailand you can call 1669 — for any change in vision, new eye pain or redness, new hearing loss, ringing in the ears or vertigo, severe headache with neck stiffness, or new weakness, numbness or confusion. Eyesight in particular can be lost, and the window to protect it is short. Say that you have syphilis, or a positive syphilis test, when you arrive — it is the single piece of information that changes what happens next.

Syphilis is a very treatable infection — treatment is straightforward when it is done correctly. What turns it into a frightening experience is usually not the disease but the test results arriving without an explanation. If you have a positive result and nobody has walked you through what it means, ask. Once you can read your own results, the panic tends to disappear.

หากท่านมีข้อกังวลเกี่ยวกับผลการตรวจเลือดซิฟิลิส หรือต้องการการวินิจฉัยและการรักษาโรคติดต่อทางเพศสัมพันธ์ นพ.สรวีร์ วีระโสภณ ให้บริการปรึกษาเฉพาะทางที่รักษาความลับของท่าน ที่โรงพยาบาลกรุงเทพสำนักงานใหญ่ นัดหมายปรึกษาแพทย์. นัดหมายที่โรงพยาบาลสมิติเวช ศรีราชา ได้โดยโทรติดต่อแผนกทางเดินปัสสาวะที่ 088-022-1445. Questions about the cost of testing or treatment should go to the hospital directly — for Bangkok Hospital, by email to bhquro@bdms.co.th.

คำถามที่พบบ่อยเกี่ยวกับการตรวจเลือดซิฟิลิส

ข้อจำกัดความรับผิดชอบ This content is written and reviewed by Dr. Soarawee Weerasopone, a board-certified urologist at Bangkok Hospital Headquarters. It is intended for educational purposes only and does not constitute medical advice. Blood test results must be interpreted by a doctor who knows your history — do not draw conclusions about your own results from this article alone. No medical advice, diagnosis or prescription is provided through personal messaging channels or social media. Always consult a qualified healthcare professional before starting any medical treatment. In an emergency, attend the nearest emergency department — in Thailand the emergency number is 1669.