Syphilis is a sexually transmitted disease (STD) caused by the bacterium Treponema pallidum. Serologic tests remain the gold standard to diagnose and confirm syphilis infection. They are widely adopted using non-treponemal or treponemal tests.

Diagnosing syphilis early and taking prompt treatment is vital because of the serious complications it can cause in the body and even during pregnancy.

Syphilis is highly contagious and even with preventive measures such as the use of condoms, there is still a risk of catching it.

When are lab tests to diagnose syphilis indicated?

Laboratory tests to diagnose syphilis are indicated in all the following cases:

  • Persons exhibiting syphilis symptoms to confirm the presence of syphilis
  • All persons belonging to the high-risk group such as
    • People with multiple partners
    • Men having sex with men
    • Intravenous drug users
    • Sex workers
  • A person who had sex with  a syphilis patient
  • Those who recently visited high syphilis risk zones like Canada and especially Alberta
  • Pregnant women are routinely screened
  • Blood donors to confirm that their blood is not infected

Through testing not only has the presence of syphilis been diagnosed but unsuspected asymptomatic cases in the general population have been detected.

There are two different types of serologic tests to diagnose syphilis depending on the type of antigen the antibodies are directed against.

  1. Treponemal tests detect antibodies to T pallidum proteins.
  2. Nontreponemal tests detect antibodies against lipoidal antigens, damaged host cells, and possibly from treponemes.

Both tests are used to confirm the infection and assess whether the disease is active. Because of the introduction of automated and rapid treponemal testing, doctors now widely order a treponemal test as the first diagnostic tool.

Nontreponemal blood tests to screen for syphilis

Nontreponemal tests (NTT) measure how much the blood sample reacts to an antigen, which may suggest T. pallidum infection. The test screens for antibodies that may not be specific to syphilis but can still trigger the condition.

Nontreponemal tests are the first line of diagnostic testing for syphilis and are used to screen for syphilis even in the absence of symptoms.

The two most common nontreponemal tests for syphilis include:

  1. Venereal disease research laboratory (VDRL) test
  2. Rapid plasma Reagin (RPR)

1. VDRL blood test for syphilis

The blood VDRL test for syphilis detects the antibodies, which the body produces to fight the syphilis infection. The presence of these antibodies in the blood tells you that syphilis infection is present.

The antibodies in the blood can be present in the blood for years even after the infection is cured. This test can therefore tell you about the past infection as well as the present.

What is the window period for syphilis?

There is a window period during which the VDRL test will come falsely negative even though the person is infected. The window period is the period, which corresponds to the time taken for the antibodies to show up in blood after the individual has got initially infected. In the case of syphilis, the window period is three months.

As it is necessary for the antibodies to show up in blood for the test to show a positive result, testing during this period will show a negative result even though the individual is infected.

In such a case of a negative result, the test should be repeated after three months to confirm the absence of syphilis. If the infection is present, it will show up after three months. The VDRL test uses the microscope to detect the antibodies and proclaim the result.

Ideal testing time to do VDRL test after suspected infection

The chances of antibodies for syphilis infection showing up in the blood period-wise are as follows:

  • Possible Detection – 1-2 weeks after the initial infection
  • Most Likely Detection – 6 weeks       “                    “
  • Highest Accuracy – 12 weeks              “                   “

The VDRL test has limitations in early cases and late stages of syphilis as regards accuracy.

2. Rapid Plasma Reagin (RPR) test

The rapid plasma reagin (RPR) test for syphilis also detects the antibodies in the blood like the VDRL test. It is used

  • To determine the prevailing status of the disease
  • To evaluate response to treatment and
  • To check for re-infection

The RPR is a more advanced test than the VDRL test. It is the most preferred nontreponemal test for syphilis because of its ready-to-use technique and effectiveness. However, this test cannot be used to test cerebrospinal fluid in neurosyphilis where the VDRL test can be used.

VDRL and RPR test limitations

The VDRL and the RPR tests are found to be non-effective to diagnose syphilis in the later stages. Again, both these tests can give false reactive results in certain medical conditions such as acute viral infections (measles, hepatitis), pregnancy, drug addiction, some autoimmune diseases, and recently taken vaccinations.  A positive result from both these tests requires confirmation by doing the treponemal test (FTA-ABS test described below).

Treponemal blood tests to diagnose syphilis infection

Treponemal tests are also used as diagnostic tests in patients with nonreactive nontreponemal test results but with signs and symptoms of late syphilis.

There are many treponemal tests currently available, and some of the most commonly used ones are described below.
  1. Darkfield microscopic study of the serous fluid
  2. Fluorescent treponemal antibody absorption (FTA-ABS) test
  3. Treponema pallidum particle agglutination assay (TPPA)
  4. Syphilis Enzyme Immunoassay (EIA) test.

1. Darkfield microscopic study of the serous fluid

In this test, fluid or tissue from the chancre (sore or ulcer) is taken by scraping the chancre and examining the scraped material under a special microscope. This helps to directly visualize the presence of syphilis bacteria if present.

This method of diagnosing syphilis is only possible in the primary stage and in the early second stage when the chancre of syphilis is present.

2 . Fluorescent treponemal antibody absorption (FTA-ABS) test

The FTA-ABS test is a treponemal test, which examines the blood for antibodies to the syphilis bacteria three to four weeks after the suspected exposure. The cerebrospinal fluid can also be tested by the FTA-ABS test to diagnose neurosyphilis.

This test is more specific than the nontreponemal tests and remains positive for a longer time. It is used as a confirmatory test after positive results are obtained on the VDRL and/or RPR test.

3. Treponema pallidum particle agglutination assay (TPPA)

This test is also a treponemal test, which detects the antibodies in the blood to the syphilis bacteria. It is also used as a confirmatory test to confirm the diagnosis of syphilis after nontreponemal tests like VDRL and/or RPR show positive results. However, cerebrospinal fluid cannot be tested by this method to rule out neurosyphilis.

4. Syphilis Enzyme immunoassay (EIA) test.

This is a new screening test for syphilis, which examines the blood for IgM and IgG antibodies specific for Treponema pallidum, the bacillus that causes syphilis. It is a very sensitive automated test.

A negative EIA will indicate the patient has not been infected with syphilis till three months ago.

A positive result should be confirmed by the VDRL or the RPR test. Test reports are available within 24 to 48 hours after receipt of the blood sample by the lab.

Lab testing of cerebrospinal fluid to diagnose neurosyphilis

If syphilitic involvement of the nervous system is suspected, it may become necessary to test and confirm the presence of neurosyphilis. The cerebrospinal fluid (CSF) is taken from the spinal cord by a procedure called the lumbar puncture or the spinal tap. The spinal fluid is then tested by the VDRL test.

CSF VDRL test for syphilis is recommended in all cases of untreated syphilis with a duration of greater than one year. A negative case does not mean you do not have syphilis. Only in 25% of the cases of early syphilis, the syphilis organism can be identified. In case of a negative result, further tests are required and then the patient is put on a course of antibiotics.

Screening for syphilis in pregnancy

All pregnant women are screened for syphilis by doing the Venereal Disease Research Laboratory (VDRL) test or the Rapid Plasma Reagin (RPR) test.

Screening should be done at the first antenatal visit and at the beginning of the third trimester. High-risk women should also be tested before delivery.

All women should get tested for STDs when they’re pregnant, even if they think you’ve never had any STD symptoms. These tests will rule out the presence of chlamydia, hepatitis B, herpesgonorrhea, syphilis, and HIV.