Lab

Non-Treponemal Test

Non-Treponemal Test, Indirect Syphilis Serology, Non-Treponemal Antibody Syphilis Test, Rapid Plasma Reagin, RPR, Serum Toluidine Red Unheated Serum Test, TRUST, Venereal Disease Research Laboratory, VDRL

See Also

Indications
Syphilis Diagnosis

Non-Treponemal Tests are used in Syphilis Screening and diagnosis protocols
1. Standard sequence (step 1) Syphilis Screening and confirmed by Treponemal Tests (e.g. TP-PA)
2. Reverse sequence (step 2) Syphilis Screening as a confirmation of a positive rapid immunoassay
Both RPR and VDRL use cardiolipin/lecithin Antigen extracts to detect Syphilis
1. VDRL is preferred in CSF testing for Syphilis
2. RPR Is preferred for serum testing (simple, rapid)
  1. RPR uses carbon particles, allowing it to be performed on a paper card
  2. RPR may therefore be read with the naked eye (as opposed to VDRL which requires a microscope)

Mechanism

RPR and VDRL are Non-Treponemal Test based on development of cardiolipin and lecithin antibodies in Syphilis
1. Syphilis infection and its associated cellular injury releases multiple Antigens
2. Released Antigens (e.g. cardiolipin, lecithin) trigger Antibody development in Syphilis infected patients
RPR and VDRL use an Antigen extract from beef heart (Non-Treponemal Derived Substance)
1. Combination of Cholesterol, cardiolipin and lecithin (extracts of beef heart)
2. Antigen mix is a similar for both RPR and VDRL
Antigen mix precipitates Antibody
1. Non-specific Syphilis antibodies bind Antigens, and result in observed Agglutination in either RPR or VDRL

Labs

Venereal Disease Research Laboratory (VDRL)
1. VDRL mixes serum and Antigen on a glass slide, and uses light microscopy to visualize Agglutination
Rapid Plasma Reagin (RPR)
1. RPR uses a a disposable card with impregnated carbon particles (allows for reading with the naked eye)
2. Agglutination (clumping) indicates serum contains IgG or IgM Antibody to RPR Antigen
3. Similar to VDRL Test, but faster and more simple to perform (RPR does not require a microscope)
Serum Toluidine Red Unheated Serum Test (TRUST)
1. Similar, to RPR, but impregnates paint particles into disposable cards
2. TRUST is typically associated with a higher Test Sensitivity than RPR
Other tests
1. Automated Reagin Test (ART)
2. Standard Test for Syphilis (STS)

Efficacy
Non-Treponemal Tests

Serum Test Specificity: 85-99%
1. See False Positive causes below
Serum Test Sensitivity in untreated Syphilis
1. Primary Stage (Chancre stage)
  1. False Negatives are most common in the first 4 weeks
  2. After one week: 30%
  3. After three weeks: 90%
2. Secondary Stage: 100%
3. Tertiary Stage: 90%
4. Latent Stage: may be unreactive
CSF Test Sensitivity
1. VDRL has a higher Test Sensitivity in CSF samples (Neurosyphilis) than RPR
2. Marra (2012) Sex Transm Dis 39(6):453-7 +PMID: 22592831 [PubMed]

Causes
Negative (Non-Reactive)

Normal (Syphilis not detected)
False Negative Causes
1. Testing too early after infection (esp. first 4 weeks)
2. Prozone Phenomenon
  1. High Antibody levels result in a paradoxically negative sample

Causes
Positive (Reactive)

See Syphilis False Positive Test
Syphilis RPR positive test will be returned with titer (e.g. 1:16)
1. After treatment, by 6 months, RPR should fall by a factor of 4 (e.g. 1:4)
2. On subsequent infection, expect the RPR titer to once again rise
3. Confirm all positive RPR tests with the more specific Fluorescent Treponemal Antibody Test
False Positives
1. Autoimmune disorders or Malignancy
2. Cohorts
  1. Pregnancy
  2. Advanced age
  3. Intravenous Drug Abuse (IVDA)
  4. Recent Immunizations (e.g. COVID19 Vaccine)
3. Recent infections
  1. Malaria
  2. Measles
  3. Endocarditis
  4. Infectious Mononucleosis
  5. Infectious Hepatitis
  6. Leprosy
  7. Miliary Tuberculosis
  8. Leptospirosis
  9. Lyme Disease
  10. Brucellosis
  11. Atypical Pneumonia
  12. Rat Bite Fever
  13. Typhus
  14. Other Treponemal infections (Yaws, Pinta, Bejel)
    1. False Positive in both Treponemal and nontreponemal tests

References