WeilFelix

Weil-Felix refers to the Weil–Felix test, a historical serological assay used to diagnose rickettsial infections. Named after Edmund Weil and Arthur Felix, the test was developed in 1916 and became a widely used screening tool in the pre-immunoglobulin era. It relies on cross-reactivity between antibodies against rickettsial antigens and certain Proteus bacterial strains.

Principle and method: The test uses killed Proteus strains OX2, OX19, and OXK as reagents. Patient serum is serially diluted and mixed with suspensions of each Proteus strain. If rickettsia-specific antibodies are present, they will cause agglutination of the Proteus antigens, producing a visible reaction. The test can be performed qualitatively or semi-quantitatively by determining the highest serum dilution at which agglutination occurs.

Interpretation and limitations: A positive Weil-Felix result is defined by agglutination at a predefined serum dilution, with higher titers suggesting a greater likelihood of infection. Reactivity to OX2 and OX19 is commonly associated with typhus group or spotted fever rickettsioses, while OXK reactivity is linked to scrub typhus. However, cross-reactions, limited sensitivity, and variable specificity mean that results must be interpreted in the clinical context. False positives and negatives are not uncommon, and the test is insufficient as a stand-alone diagnostic tool.

Current status: The Weil-Felix test has largely been supplanted by more specific methods such as indirect immunofluorescence assays, enzyme-linked immunosorbent assays (ELISA), and polymerase chain reaction (PCR) testing. It remains of historical significance and may still appear in some resource-limited settings or for educational purposes.