Weil - Felix Test

The Weil–Felix test is an older heterophile agglutination assay that detects anti‑rickettsial antibodies by measuring cross‑reactivity with Proteus strains (OX2, OX19, OXK). It’s used to support diagnosis of rickettsial infections (typhus, spotted fever, scrub typhus) but has limited sensitivity and specificity and may give false negatives or cross‑reactions; confirmatory tests (IFA/ELISA or PCR) are preferred.

No. A positive Weil–Felix test does not indicate typhoid fever. Weil–Felix is an old, nonspecific agglutination test used mainly for rickettsial infections and has low sensitivity and specificity; it can cross-react with other febrile illnesses, producing false positives. Typhoid is diagnosed by blood culture, PCR, or targeted serology plus clinical assessment—confirm with specific laboratory tests.

False-positive Weil–Felix results are usually due to cross-reacting antibodies from other infections (e.g., typhoid, leptospirosis, brucellosis, viral illnesses, malaria) or nonspecific agglutinins from autoimmune conditions (rheumatoid factor, hypergammaglobulinemia) and recent vaccination. Poor technique or contaminated reagents can also cause spurious agglutination. Overall, low specificity and cross-reactivity limit the test’s accuracy.

Widal test detects agglutinins (O and H) to Salmonella typhi for typhoid diagnosis; it relies on rising titers and has cross‑reactivity and limited specificity. Weil‑Felix detects anti‑rickettsial antibodies via cross‑reaction with Proteus OX strains (OX2, OX19, OXK) and is a crude screening for rickettsial infections. Both are serologic, variably sensitive/specific, and often need paired sera or confirmatory testing.

Rickettsial infections typically start with sudden high fever, severe headache, muscle aches and malaise. A characteristic rash—maculopapular or petechial—often appears, sometimes with a dark scab (eschar) at the bite site. Other symptoms can include nausea, vomiting, abdominal pain, cough and confusion. Symptoms usually develop days to two weeks after an infected tick, mite, flea or lice bite.

Read Weil–Felix results by checking agglutination titers for OX2, OX19 and OXK. Note the cutoff used (commonly ≥1:160) and whether there’s a fourfold rise between acute and convalescent samples. OXK suggests scrub typhus; OX2/OX19 indicate spotted fever or typhus group. Low or single titers may be nonspecific; correlate with symptoms and confirm with specific tests (IFA, PCR).