nontreponemal

Nontreponemal tests are serologic assays used in diagnosing syphilis. They detect antibodies to cardiolipin-lecithin-cholesterol complexes, which arise from tissue damage rather than against Treponema pallidum itself. Because they are not specific to treponemal antigens, these tests are useful for initial screening and for monitoring treatment response, rather than for definitive infection status.

The main non-treponemal tests are the rapid plasma reagin (RPR) and the venereal disease research laboratory (VDRL) test. RPR uses carbon particles to visualize flocculation on a card; VDRL typically uses a slide with cardiolipin antigen. Both provide qualitative results (reactive or nonreactive) and quantitative titers, which reflect antibody levels.

Interpretation and follow-up: A reactive result suggests current or recent infection but can occur with other conditions; confirmation with a treponemal test (such as TPPA or FTA-ABS) is required for a definitive diagnosis. Titers are tracked over time; a fourfold decline after treatment indicates response, while rising titers may indicate relapse or reinfection. Non-treponemal tests are often used to monitor therapy because titers correlate with disease activity.

Limitations: false positives can occur in autoimmune disease, pregnancy, infections, or vaccination; false negatives may occur in very early primary syphilis or late latent infection; the prozone phenomenon can cause falsely negative results at very high antibody concentrations and may require sample dilution. They do not distinguish past treated infection from active disease, hence the need for treponemal testing for confirmation and for long-term surveillance.