Interpreting Syphilis Serology
Interpreting syphilis test results can be difficult, particularly if previous test results and treatment histories are unknown. A key role of the population/public health unit (PHU), some sexual health clinics and staff from syphilis registers in some regions is to assist with the accurate interpretation of syphilis serology.
Questions about test results can also be directed to the Specialist Microbiologist (a pathologist) responsible for authorising the laboratory test results or a sexual health physician, or infectious diseases physician, if available.
The following information is required to interpret syphilis test results for the purpose of diagnosing and staging syphilis:
- sexual history, symptoms and clinical signs AND
- treponemal test result AND
- Rapid Plasma Reagin (RPR) test result AND
- previous syphilis serology results (if available) AND
- history of treatment (if available)
Serology in the different stages of syphilis infection
The Syphilis Decision Making Tool provides a systematic way to review syphilis results to determine if treatment indicated:
Practical Examples: Syphilis Serology Interpretation
(De-identified cases have been provided below for training purposes)
|
Syphilis Serology | ||||
|---|---|---|---|---|
|
Specimen: Serum | ||||
|
Test name |
Result |
Flag |
Ref-Range |
Units |
|
T. pallidum Total Ab (EIA) |
Detected | |||
|
RPR |
4 | |||
|
Patient syphilis testing history | |||
|---|---|---|---|
|
Date |
RPR |
TPPA |
EIA |
|
02/02/2023 |
Not detected | ||
|
15/01/2024 |
Negative |
Not detected | |
|
13/04/2024 |
RPR 1:16 |
TPPA Pos (+3) |
T. pallidum Total Ab detected |
|
13/06/2024 |
RPR 1:128 | ||
|
13/08/2024 |
RPR 1:16 | ||
|
Evidence of successful treatment of active syphilis is provided by a four-fold or greater decline in RPR titre within 12 months of commencing treatment, Contact a Clinical Microbiologist for advice if required. | |||
Explanation/interpretation:
- Patient presented early in syphilis infection returning an RPR of 1:16 on 13/04/2024 – treatment was not initiated at this stage as the patient had left prior to receiving results. Patient was recalled on 14/04/2024, but failed to return at this point
- Patient returned to clinic, where the patient management system prompted subsequent testing on 13/06/2024. As indicated in the results the titre had increased to RPR 1:128 – treatment was initiated in this consultation.
- Patient was asked to return for subsequent testing to make sure treatment was effective. As of the 13/08/2024 the RPR was back to 1:16, indicating a decline in titre, greater than four-fold. Hence the treatment was effective.
- The final consult on the 13/11/2024, shows a RPR of 1:4, indicating the titre had returned to baseline or close to baseline.
|
Serum Treponemal Serology | ||||
|---|---|---|---|---|
|
Request number |
Date |
RPR |
TPPA |
EIA |
|
12345678 |
13/04/2020 |
Not detected | ||
|
12548451 |
13/06/2022 |
Not detected | ||
|
15488784 |
13/02/2024 |
Reactive (1:256) |
Pos |
Detected |
|
14984541 |
19/02/2024 |
Reactive (1:256) |
Detected | |
|
25451521 |
22/05/2024 |
1:16 | ||
|
Evidence of successful treatment of active syphilis is provided by a four-fold or greater decline in RPR titre within 12 months of commencing treatment, Contact a Clinical Microbiologist for advice if required. | ||||
|
Other tests ran separate from the above serology results. 13/02/2024 – Nucleic Acid Amplification Test (NAAT) – swabbing of chancre: reactive 11/02/2024 – PoCT on presentation in clinic: non-reactive | ||||
Explanation/interpretation:
Sexual health nurse was made aware of a sore on patient, identified as a possible syphilis chancre. Swabbed and sent sample away for NAAT testing. Additionally, ran a PoCT at the time of presentation, which returned a non-reactive result. PoCT was done in attempt to reduce time to diagnosis. A Syphilis PoCT where possible should always be conducted in parallel with serology testing.
In this case the PoCT was a false negative, emphasising the importance of swab and/or serology when there is a high clinical suspicion of syphilis. A limitation of Syphilis PoCT is that it is unlikely to pick up an early-stage infection (first three weeks of infection).For most people it will show up within 2-4 weeks but can take up to 3 months. It is important to know however that a syphilis PoCT has marginally inferior sensitivity and specificity compared with serology meaning it is still a good option for testing if available.
|
Name: |
xxxxx |
DOB: |
04/03/85 |
|
Sex: |
Male | ||
|
Community: |
xxxxx |
HRN: |
123456789 |
|
Date Tested |
Treponemal Test |
Result |
Treponemal Test 2 |
Result 2 |
RPR |
Treatment |
|---|---|---|---|---|---|---|
|
21/11/2022 |
EIA |
Negative | ||||
|
5/07/2024 |
EIA |
Positive |
TPPA |
Positive |
1:64 | |
|
27/07/2024 |
EIA |
Positive |
TPPA |
Positive |
1:256 |
Benzathine benzylpenicillin 2.4 mu IM |
In the absence of test result within last 2 years (prior to July 2024) and any clinical signs and symptoms relating to primary or secondary syphilis, it was categorized as <2 years probable case. As per jurisdictional guidelines, it means they still required to receive benzathine benzylpenicillin x3 doses each dose at weekly intervals for treatment.
Their treatment serology showed EIA positive with RPR 1:256. The patients RPR increased by 2 titres (four-fold) – which proved it was confirmed <2 years infection case. This meant it was a recent infection requiring only ONE stat dose of Benzathine benzylpenicillin 2.4 million units.
The patient was advised they did not need the further 2 doses. Patient was pleased to know that they did not have to go through the painful treatment again. Was advised to return on 03 December 2024 for repeat syphilis serology check to monitor treatment response.
Reference: Miller P, Skov S, Knox J. How to interpret syphilis results: a manual for nursing and medical staff in remote communities. 2nd ed. South Australia: Nganampa Health Council Inc.; 1999. Available from: http://www.nganampahealth.com.au/