The Weil–Felix test is an agglutination test for the diagnosis of rickettsial 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of its simplicity, it has been one of the most widely employed tests for rickettsia on a global scale, despite being superseded in many settings by more sensitive and specific diagnostic tests.
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History and basis for test
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. The serum of patients diagnosed with epidemic typhus was found to agglutinate in the presence of bacteria now known as Proteus vulgaris. Ensuing work elucidated that it was in fact the somatic (O) antigen that cross-reacted with anti-rickettsial antibodies, and furthermore, that different Proteus O antigens would cross-react with different species of Rickettsia.
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
The Weil–Felix test can be done as either a slide or a tube test. The antigens necessary (OX2, OX19, and OXK) can be obtained commercially.
Slide method
On a solid surface (glass slide, tile, card), a small amount (50–100 μL) of the patient’s serum is placed. A single drop of the desired antigen is added, and the resulting suspension is mixed and then rotated for one minute. Visible agglutination is indicative of a positive result, and corresponds roughly to a titer of 1:20. Positive results can be further titrated using the tube method, which is more labour-intensive.
Tube method
Using 0.25% phenol 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.