Question
Weil felix test.
Answer
The Weil-Felix test is a heterophile agglutination test historically used for the serodiagnosis of rickettsial infections, exploiting a cross-reaction between certain rickettsial antigens and antigens of specific Proteus strains that are much easier to culture in the laboratory.
Principle: patients infected with Rickettsia species develop antibodies that cross-react with the somatic (O) antigens of certain non-motile Proteus strains (OX-19, OX-2, OX-K) due to a shared polysaccharide antigenic determinant — patient serum is tested for agglutination against these standard Proteus antigen suspensions, and a rising titre (≥4-fold) in paired sera, or a single significantly elevated titre, supports the diagnosis.
Pattern of reactivity:
- Typhus group (epidemic typhus — R. prowazekii; endemic/murine typhus — R. typhi) — agglutinate OX-19 strongly, OX-2 weakly.
- Spotted fever group — agglutinate both OX-19 and OX-2.
- Scrub typhus (Orientia tsutsugamushi) — agglutinates OX-K only.
- Coxiella burnetii (Q fever) — Weil-Felix is negative.
Limitations: only moderately sensitive/specific — false positives can occur with other infections (Proteus UTI, leptospirosis) and the test is negative early in illness; largely superseded by more specific serological methods (IFA, ELISA) where available, but remains useful as a low-cost screening test in resource-limited settings.