The Weil–Felix test is an agglutination test for the diagnosis of

rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "rickett ...

l infections. It was first described in 1916. By virtue of its long history and of its simplicity, it has been one of the most widely employed tests for rickettsia on a global scale, despite being superseded in many settings by more sensitive and specific diagnostic tests. The Weil-Felix

antibody An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

was recently found to target rickettsia LPS O-antigen.

History and basis for test

The basis of the test is the presence of

antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

ic cross-reactivity between ''

Rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "rickett ...

'' spp. and certain serotypes of non-motile ''

Proteus In Greek mythology, Proteus (; Ancient Greek: Πρωτεύς, ''Prōteus'') is an early prophetic sea-god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" ''(hálios gérôn)''. ...

'' spp., a phenomenon first published by

Edmund Weil Edmund Weil (16 April 1879 – 15 June 1922) was a German Bohemian bacteriologist. He is best known for the Weil–Felix test used in the diagnosis of Rickettsia, rickettsial infections. He conducted this research during World War I and died from a ...

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. Typhus group rickettsiae (''

Rickettsia prowazekii ''Rickettsia prowazekii'' is a species of gram-negative, alphaproteobacteria, obligate intracellular parasitic, aerobic bacillus bacteria that is the etiologic agent of epidemic typhus, transmitted in the feces of lice. In North America, the ...

'', '' R. typhi'') react with ''P. vulgaris'' OX19, and scrub typhus (''

Orientia tsutsugamushi ''Orientia tsutsugamushi'' (from Japanese ''tsutsuga'' meaning "illness", and ''mushi'' meaning "insect") is a mite-borne bacterium belonging to the family Rickettsiaceae and is responsible for a disease called scrub typhus in humans. It is a ...

'') reacts with '' P. mirabilis'' OXK. The

spotted fever A spotted fever is a type of tick-borne disease which presents on the skin. They are all caused by bacteria of the genus ''Rickettsia''. Typhus is a group of similar diseases also caused by ''Rickettsia'' bacteria, but spotted fevers and typhus are ...

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 ''Sensitivity'' and ''specificity'' mathematically describe the accuracy of a test which reports the presence or absence of a condition. Individuals for which the condition is satisfied are considered "positive" and those for which it is not are ...

, with a recent study showing an overall sensitivity as low as 33% and specificity of 46%. Other studies have had similar findings.Kaplan, JE, and LB Schonberger. 1986. The sensitivity of various serologic tests in the diagnosis of Rocky Mountain spotted fever. Am J Trop Med Hyg 35:840–844. As a result, it has largely been supplanted by other methods of serology, including indirect

immunofluorescence Immunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on microbiological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to specif ...

(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

The Weil–Felix test can be done as either a slide or a tube test. The

s necessary (OX2, OX19, and OXK) can be obtained commercially.

Slide method

* Place 50–100µl of patient serum on the slide * Add a drop of desired antigen (Proteus OX19 or OX2 or OXK) * Mix the suspension by rotating the slide for 1 minutes * Visible agglutination indicates the positive test.

Tube method

Using 0.25%

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it req ...

saline as a diluent, a series of tubes containing twofold dilutions of patient serum are made with a final volume of 1 mL. A drop of antigen suspension is added to each tube, and the mixture is incubated at 50–55 °C for 4–6 hours. A positive tube would show visible flocculation or granulation, which is accentuated when the tube is gently agitated. The titer corresponds to the most dilute tube in the series that still shows positivity. Generally, a titer of ≥1:320 is considered diagnostic.

References

{{DEFAULTSORT:Weil-Felix Test Infectious disease blood tests