Weil-Felix is a nonspecific agglutination test that detects anti-rickettsial antibodies in patient’s serum. Weil-Felix test is based on cross-reactions that occur between antibodies produced in acute rickettsial infections with antigens of OX (OX 19, OX 2, and OXK) strains of Proteus species. Dilution of the patient’s serum is tested against suspensions of the different Proteus strains.
Several rickettsiae, such as Rickettsia prowazekii, Rickettsia tsutsugamushi, and R. rickettsii, posses antigens that cross-react with antigens of OX strains of Proteus vulgaris.
Proteus OXK strain agglutinins are produced in scrub typhus and OX 2 and OX 19 agglutinins in other rickettsial diseases, see the table below
|OX 19||OX 2||OXK|
|R. prowazekii (Epidemic typhus)||+ + +||+/-||–|
|R. typhi (Murine typhus)||+ + +||+/-||–|
|R. tsutsugamushi (Scrub typhus)||–||–||+ + +|
|R. rickettsii (Rocky Mountain Spotted fever)||+/ + + +||+/ + + +||–|
Limitation of Weil-Felix test:
Both sensitivity and specificity of the Weil-Felix test are low, but its predictive value can be increased by testing both acute and convalescent-phase samples and observing a rise in antibody titre.
Weil-Felix test has low sensitivity, i.e. it gives a high percentage of false-negative results. This is common in the case of Scrub Typhus.
It also shows low specificity, i.e. false-positive results are obtained in other diseases such as leptospirosis, and relapsing fever (diseases which require differentiating from rickettsial infections), in Proteus infections, brucellosis and acute febrile illness.