VDRL and RPR tests

Principle

• Non-treponemal (reagin) tests: detect IgM and IgG antibodies against a cardiolipin–lecithin–cholesterol antigen, released from damaged host cells and treponemes.
• VDRL: flocculation read under a microscope (10× objective).
• RPR: VDRL antigen modified with charcoal particles, choline chloride and EDTA; read macroscopically as black clumps on a card; no heat inactivation needed.

Specimen

• VDRL: serum heat-inactivated at 56 °C for 30 minutes; CSF (unheated) for neurosyphilis.
• RPR: serum or plasma; not used for CSF.

Procedure

• VDRL: antigen suspension prepared fresh; serum and antigen mixed on a ringed glass slide and rotated about 4 minutes at 180 rpm.
• RPR: serum and antigen on a plastic-coated card, rotated about 8 minutes at 100 rpm, read in bright light.
• Reactive samples are titrated with doubling dilutions.

Results

• Reported as reactive, weakly reactive or non-reactive, with titre.
• A fourfold (two-dilution) change in titre is clinically significant; titres fall with successful treatment and may become non-reactive.
• Reactive non-treponemal results must be confirmed by a treponemal test (TPHA, TPPA, FTA-ABS, EIA/CLIA). Many labs use a reverse algorithm (treponemal screen first).

Quality control and pitfalls

• Biological false positives (usually low titre, often ≤1:8): pregnancy, SLE and antiphospholipid antibodies, malaria, leprosy, HIV, injecting drug use, viral infections (e.g. hepatitis, infectious mononucleosis), recent vaccination, old age.
• Prozone: very high antibody (especially secondary syphilis) gives false non-reactive result on undiluted serum; test diluted serum.
• False negatives also occur in very early primary and late latent/tertiary syphilis.
• Other treponematoses (yaws, pinta, bejel) give reactive results.
• Run reactive, weakly reactive and non-reactive controls daily.

Clinical use

• Screening for syphilis, monitoring treatment response, CSF VDRL for neurosyphilis.