Clinical management | How to Interpret Syphilis Test Results

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).

RPR Interpretation:

(De-identified cases have been provided below for training purposes)

Syphilis Serology Specimen: Serum, Collected: 11/11/2022 13:00 Received: 12/11/2022 16:30 Date of serology test: 13/11/2022 Test name Result Flag Ref-Range Units T. pallidum Total Ab (EIA) Detected RPR 4
Patient syphilis testing history
13/04/2022	RPR 1:16	TPPA Pos (+3)	T. pallidum Total Ab detected
13/06/2022	RPR 1:128
13/08/2022	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:

Client presented early in syphilis infection returning an RPR of 1:16 on 13/04/2022 – treatment was not initiated at this stage as the patient had left prior to receiving results. Client was recalled on 14/04/2022, but failed to return at this point

Client returned to clinic, where the patient management system prompted subsequent testing on 13/06/2022. As indicated in the results the titre had increased to RPR 1:128 – treatment was initiated in this consultation.

Client was asked to return for subsequent testing to make sure treatment was effective. As of the 13/08/2022 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 13/11/2022, shows a RPR of 1:4, indicating the titre had returned to baseline or close to baseline.

Client did not return for 12-month check-up.

Serum Treponemal Serology
Request number	Date	RPR	TPPA	EIA
12345678	13/04/2018			Not detected
12548451	13/06/2020			Not detected
15488784	13/02/2022	Reactive (1:256)	Pos	Detected
14984541	19/02/2022	Reactive (1:256)		Detected
25451521	22/05/2022	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. 13/02/2022 – Nucleic Acid Amplification Test (NAAT) – swabbing of chancre: reactive 11/02/2022 - PoCT on presentation in clinic: non-reactive

Explanation/interpretation:

Sexual health nurse was made aware of a sore on client, identified as a possible syphilis chancre. Swabbed and sent sample away for NAAT testing. Additionally, ran a PoC test at the time of presentation, which returned a non-reactive result. PoC test was done in attempt to reduce time to diagnosis. In this case the PoC test was a false negative, emphasising the importance of swab and/or serology when there is a high clinical suspicion of syphilis.

Name:	xxxxx	DOB:	39
Sex:	Male	Indigenous Status:	Aboriginal
Community:	xxxxx	HRN:	123456789

Date Tested	Treponemal Test	Result	Treponemal Test 2	Result 2	RPR	Treatment
21/11/2019	EIA	Negative				:
5/07/2022	EIA	Positive	TPPA	Positive	1:64	:
27/07/2022	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 2022) 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 x 3 doses each dose at weekly intervals for treatment.

Their treatment serology showed EIA positive with RPR 1:256. The clients 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 client was advised they did not need the further 2 doses. Client was pleased to know that they did not have to go through the painful treatment again. Was advised to return on 3 December 2022 for repeat syphilis serology check to monitor treatment response.

References:

¹Miller P, Skov S, Knox J. How to Interpret Syphilis Results: a Manual for Nursing and Medical Staff in Remote Communities. 2nd edn. Nganampa Health Council Inc., 1999. Available from: http://www.nganampahealth.com.au/