Archived Issues

2006, Volume 17, Number 2

Interpreting the New Syphilis Screening Tests

By David F. Keren, M.D.

At Warde Medical Laboratory, we screen all sera with the Syphilis Antibody Test (SAT) (a treponeme-specific assay). Because the Syphilis Antibody Test can give false positive results, all positive tests are automatically confirmed by an FTA-ABS. However, the SAT is treponeme-specific and like the FTA-ABS confirmation test, it remains positive for the patient’s lifetime. Therefore, while useful in the initial identification of the infection, SAT cannot be used to follow therapy. Because of this, all sera with a positive SAT also receive the standard RPR to determine if the infection is recent and to follow the response to therapy. If the RPR is positive, this is most consistent with a recent infection. However, if the Syphilis Antibody Test and the FTA-ABS are both positive, but the RPR is negative, this is most consistent with a past infection that has been treated.

Below are the interpretations we provide for our current testing.

1. Syphilis antibody test           Negative
If there is clinical suspicion that the patient may have early acute primary syphilis, repeat testing in 12-14 days is recommended. Serology tests may be falsely negative in patients with immunodeficiency.

2 Syphilis antibody test            Positive
   RPR                                           Positive
   FTA-ABS                                  Positive

The syphilis antibody test is confirmed as positive with the FTA-ABS. Both of these tests remain positive for years and do not decline with therapy. The RPR is positive. The most likely explanation for this combination is active syphilis. The RPR titer is useful to follow treatment.

3. Syphilis antibody test          Positive
    RPR                                         Negative
    FTA-ABS                                Positive

The syphilis antibody test is confirmed as positive with the FTA-ABS. Both of these tests remain positive for years and do not decline with therapy. The RPR is negative. The RPR titer declines and often disappears after appropriate antibiotic therapy. The most likely explanation for this combination is that the patient had syphilis which was treated previously. Note, in untreated late stage syphilis, the RPR titer declines and may be absent. Clinical correlation with history of treatment is recommended.

4. Syphilis antibody test         Positive
    RPR                                        Negative
    FTA-ABS                               Negative

The syphilis antibody screening test did not confirm with the FTA-ABS. The most likely explanation for this is a false positive syphilis antibody test. If there is clinical suspicion that the patient may have early acute primary syphilis, repeat testing in 12-14 days is recommended.

5. Syphilis antibody test         Positive
    RPR                                        Positive
    FTA-ABS                               Negative

The syphilis antibody screening test did not confirm with the FTA-ABS. The RPR (a nontreponemal test) is positive. The most likely explanation for this is a false positive syphilis antibody test and false positive RPR test. If there is clinical suspicion that the patient may have early acute primary syphilis, repeat testing in 12-14 days is recommended.

As always, we encourage you to call Warde Medical Laboratory directly with any questions you have about Syphilis testing that we have performed on your patient’s serum.

Warde
Medical
Laboratory