ผลตรวจหาเชื้อซิฟิลิสบวก แต่ไม่ใช่ซิฟิลิส: ผลบวกเท็จทางชีววิทยา

อัปเดตล่าสุด: 22 สิงหาคม 2026

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Few laboratory results cause as much distress as a reactive syphilis screening test in someone who was not expecting one — a routine check-up, a pre-employment screen, an antenatal panel. And in a meaningful proportion of those people, it is not syphilis. The phenomenon has a name: the biological false positive, or BFP. This article explains what it is, how it is settled, and — the part usually left out — why a false positive that refuses to go away is worth taking seriously for an entirely different reason.

Why the Test Can Be Wrong

VDRL and RPR are nontreponemal tests. They do not detect the syphilis organism at all — they detect antibodies against lipid material released when cells are damaged. Syphilis produces that pattern, but so do a number of other conditions, which is precisely why a reactive result is a reason to test further rather than a diagnosis. The companion treponemal test (TPHA, TPPA, FTA-ABS) looks for antibodies against the organism itself and is what settles the question. I set out the difference between the two in why there are two kinds of syphilis test.

Biological false positives are usually low-titre — typically below 1:8 — whereas active syphilis more often produces higher titres. That is a useful hint, not a rule, and it does not replace the confirmatory test.

Recognised causes

• Autoimmune disease — systemic lupus erythematosus, rheumatoid arthritis, and in particular antiphospholipid syndrome, discussed below
• Acute infections — viral illness, malaria, tuberculosis, and other febrile illnesses
• HIV infection
• กำลังตั้งครรภ์อยู่
• Recent vaccination
• Chronic liver disease
• Injecting drug use
• Older age and, less commonly, malignancy

A laboratory or clerical error is also always worth considering — a repeat sample is cheap. The distinction usually drawn is between acute false positives, which follow an infection or vaccination and settle within about six months, and chronic ones, which persist beyond that and are the group associated with autoimmune disease.

The Cause That Matters More Than the Others

A persistent biological false positive is one of the recognised laboratory features of antiphospholipid syndrome — an autoimmune condition that causes blood clots in the veins and arteries and recurrent pregnancy loss. This is the reason a chronic false positive should not simply be filed away with a shrug once syphilis has been excluded.

It matters most if you also have, or have had, any of the following — mention them to your doctor rather than waiting to be asked:

• A previous deep vein thrombosis or pulmonary embolism, or a stroke at a young age
• Recurrent miscarriage, stillbirth, or severe pre-eclampsia
• A known autoimmune diagnosis, or a low platelet count found on a blood test

In that situation, the false positive is not a nuisance result — it is a clue, and it belongs with a rheumatologist or physician rather than being dismissed. This is not urology, and I refer these patients on; but a positive test that nobody investigates is a missed opportunity, and it is the single most useful thing to know about this subject.

The Mirror Image: When the Test Is Falsely Negative

Less well known, and worth stating because it runs the other way: in very high-titre syphilis, the sheer quantity of antibody can overwhelm the reaction and produce a falsely negative VDRL or RPR. This is the prozone phenomenon, and it is resolved by the laboratory diluting the sample and retesting. It is uncommon, but it is a reason a negative screening test does not override a lesion or a rash that looks like syphilis — the clinical picture still counts.

What Happens Next

1. A treponemal test is done. Reactive nontreponemal test with a negative treponemal test, in someone without symptoms or exposure, is a biological false positive.
2. If the treponemal test is positive, it is not a false positive — that pattern means syphilis at some point in life: current, or past and treated. The history and the titre decide which, and การรักษา follows if needed.
3. A repeat after a few months distinguishes an acute false positive, which settles, from a chronic one, which does not and prompts the autoimmune question above.
4. Testing for the other infections is offered, since a screening panel that flagged one thing is a good moment to check the rest. HIV testing, and pre- or post-exposure prophylaxis, are handled by the Infectious Diseases department at Bangkok Hospital Headquarters rather than in the urology clinic.

In pregnancy this is not a wait-and-see situation. Untreated syphilis in pregnancy causes stillbirth and congenital syphilis, so a reactive result is confirmed promptly rather than at leisure, and treatment is given if there is doubt.

One last thing, because it is the question everyone actually wants answered: a biological false positive does not mean you have had syphilis, and it is not something you can pass to anyone. It is a quirk of what the test measures.

คำถามที่พบบ่อย

What is a biological false positive syphilis test?

A reactive VDRL or RPR in a person who does not have syphilis. These are nontreponemal tests: they detect antibodies against lipid material released from damaged cells rather than the syphilis organism itself, so several other conditions can make them reactive. False positives are usually low-titre, typically below 1:8. A treponemal test such as TPHA, TPPA or FTA-ABS settles the question.

What causes a false positive VDRL or RPR?

Autoimmune disease including lupus, rheumatoid arthritis and antiphospholipid syndrome; acute infections such as viral illness, malaria and tuberculosis; HIV infection; pregnancy; recent vaccination; chronic liver disease; injecting drug use; and older age. Laboratory or clerical error is also worth excluding with a repeat sample.

My VDRL is positive but TPHA is negative. Do I have syphilis?

In someone with no symptoms and no exposure, that combination indicates a biological false positive rather than syphilis. It does not mean you have had syphilis in the past, and it is not infectious to anyone. If the treponemal test is positive, that is a different situation entirely: it means syphilis at some point in life, either current or past and treated, and the history and titre decide which.

Should a persistent false positive be investigated?

Yes. A false positive that persists beyond about six months is one of the recognised laboratory features of antiphospholipid syndrome, an autoimmune condition causing venous and arterial clots and recurrent pregnancy loss. It is particularly worth pursuing in anyone with a previous clot, a stroke at a young age, recurrent miscarriage or stillbirth, a known autoimmune diagnosis, or a low platelet count. That assessment belongs with a rheumatologist or physician rather than a urologist.

Can pregnancy cause a false positive syphilis test?

Yes, pregnancy is a recognised cause. It is also the situation in which the answer is needed quickly rather than after a wait, because untreated syphilis in pregnancy causes stillbirth and congenital syphilis. A reactive antenatal result is confirmed promptly with a treponemal test, and treatment is given where doubt remains.

Can a syphilis screening test be falsely negative?

Yes, though it is uncommon. In very high-titre syphilis the quantity of antibody can overwhelm the reaction and give a falsely negative VDRL or RPR — the prozone phenomenon — which the laboratory resolves by diluting the sample and retesting. A negative screening test therefore does not override a sore or rash that looks like syphilis; the clinical picture still counts. A test taken very soon after exposure can also be negative before antibodies develop.

นัดหมายแพทย์

Dr. Soarawee Weerasopone provides confidential testing and treatment for sexually transmitted infections at โรงพยาบาลกรุงเทพ สำนักงานใหญ่ และที่โรง สมิติเวช ศรีราชา พยาบาลชลบุรี – ฝ ศัลยศาสตร์ยูโรวิทยา ่ายบริการ 088-022-1445. HIV testing and PrEP or PEP are handled by the Infectious Diseases department rather than the urology clinic. Questions about cost are answered by the hospital rather than by Dr. Soarawee — for Bangkok Hospital, by email to bhquro@bdms.co.th.

โรงพยาบาลกรุงเทพฯ มีบริการแพทย์ทางไกลสำหรับผู้ป่วยที่ไม่สามารถมาพบแพทย์ด้วยตนเอง รวมถึงผู้ป่วยต่างชาติ กรุณาแจ้งล่วงหน้าทางอีเมลไปยังแผนกศัลยกรรมทางเดินปัสสาวะที่ bhquro@bdms.co.th. It is a useful way to review results you already have; blood testing itself requires an in-person visit. Samitivej Sriracha is in-person only.

ข้อจำกัดความรับผิดชอบ This article is written and reviewed by Dr. Soarawee Weerasopone, a board-certified urologist at Bangkok Hospital Headquarters. It is educational only, is not medical advice, and does not create a doctor–patient relationship. A reactive syphilis screening result should always be interpreted by a doctor with your history in front of them rather than from a web page, and it should never be ignored. Investigation of antiphospholipid syndrome is not urological and is referred to a rheumatologist or physician. No diagnosis, prescription or individual medical advice is given through personal messaging channels or social media, and Dr. Soarawee operates no public social media account — any account offering private consultation in his name is fraudulent.