The basis of the test is the presence of
antigenic cross-reactivity between
Rickettsia spp. and certain serotypes of non-motile
Proteus spp., a phenomenon first published by
Edmund Weil and
Arthur Felix in 1916. Weil-Felix is a nonspecific agglutination test which detects anti-rickettsial antibodies in patient’s serum. Weil-Felix test is based on cross-reactions which occur between antibodies produced in acute rickettsial infections with antigens of OX (OX 19, OX 2, and OXK) strains of
Proteus species. Dilution of patient’s serum are tested against suspensions of the different
Proteus strains. William James Wilson (1879-1954) had noticed a similar cross reaction of anti-rickettsial antobodies with other enteric bacteria. Typhus group rickettsiae (
Rickettsia prowazekii,
R. typhi) react with
P. vulgaris OX19, and scrub typhus (
Orientia tsutsugamushi) reacts with
P. mirabilis OXK. The
spotted fever group rickettsiae (
R. rickettsii,
R. africae,
R. japonica, etc.) react with
P. vulgaris OX2 and OX19, to varying degrees, depending on the species. The Weil–Felix test suffers from poor
sensitivity and specificity, with a recent study showing an overall sensitivity as low as 33% and specificity of 46%. Other studies have had similar findings. As a result, it has largely been supplanted by other methods of serology, including indirect
immunofluorescence antibody (IFA) testing, which is the gold standard. However, in resource-limited settings, it still remains an important tool in the diagnosis and identification of public health concerns, such as outbreaks of epidemic typhus. ==Procedure==