Weil-Felix Test: OX-19, OX-2, OX-K Patterns and How to Interpret Them

The Weil-Felix test is a rapid, low-cost agglutination test used as a presumptive screen for rickettsial infections. It works on a coincidence of nature: certain strains of Proteus carry surface antigens that happen to cross-react with antibodies the body makes against rickettsiae. So instead of using rickettsial antigen (which is hard to culture safely), the test uses suspensions of Proteus OX-19, OX-2, and OX-K strains, and reads which ones the patient's serum agglutinates. Because it relies on cross-reaction rather than a specific antigen, it is a heterophile agglutination test, and its pattern of reactivity, not a single result, is what points toward a diagnosis.

Several rickettsiae, including Rickettsia prowazekii, Rickettsia rickettsii, and the scrub typhus agent Orientia tsutsugamushi (formerly Rickettsia tsutsugamushi), carry antigens that cross-react with the OX strains of Proteus.

How the test works

The Weil-Felix reaction detects cross-reactive antibodies (predominantly IgM) produced early in rickettsial infection. These antibodies do not reach detectable levels until roughly 5 to 10 days after symptoms begin, which is the main reason a test done too early is falsely negative. The patient's serum is serially diluted and tested against each Proteus OX suspension; the highest dilution still producing visible agglutination is the titer for that strain. Because a single titer is hard to interpret against background reactivity, a fourfold or greater rise between an acute-phase and a convalescent-phase sample (7 to 14 days apart) is the most reliable evidence of infection.

Procedure

The Weil-Felix test can be run as a rapid slide test (for screening) or a tube test (for a titer). Both test the patient's serum against each Proteus antigen suspension separately: OX-19, OX-2, and OX-K.

**Slide method (rapid screening)**

1. Bring reagents and serum to room temperature (22–30°C); cold reagents give false results.
2. Place a drop of undiluted patient serum (about 50 µL) in each of three circles on a glass slide or tile, one per antigen.
3. Add one drop of the OX-19, OX-2, or OX-K antigen suspension to its respective circle.
4. Mix and rock the slide gently for one minute.
5. Read for visible agglutination. Clumping within one minute is a presumptive positive for that antigen. A saline negative control confirms the antigen is not autoagglutinable.

A positive slide result should be confirmed and titered by the tube test.

**Tube method (semi-quantitative titer)**

1. Prepare doubling dilutions of the patient's serum in 0.25% phenol saline, typically 1:20 through 1:320, in a row of tubes for each antigen. Include a saline negative control tube per antigen.
2. Add a fixed volume of the OX-19, OX-2, or OX-K antigen suspension to every tube in the corresponding row.
3. Mix and incubate in a water bath. A common protocol is 50–55°C for about 4 to 6 hours, or 37°C overnight, following the manufacturer's insert.
4. Read each tube for granular agglutination. The titer is the reciprocal of the highest serum dilution still showing definite agglutination.

Because normal sera can carry background agglutinins (notably up to about 1:80, sometimes higher, with OX-K), a presumptive positive is generally taken as roughly 1:160 to 1:320 or above, and a fourfold rise across paired sera is more reliable than any single titer.

The OX-strain agglutination patterns

Different rickettsial groups produce antibodies that agglutinate different Proteus OX strains. The pattern across the three antigens is what suggests the disease group.

The key patterns to hold onto: the typhus group drives OX-19 (with OX-2 variable), the spotted fever group reacts with both OX-19 and OX-2, and scrub typhus is the odd one out, reacting only with OX-K. Q fever and rickettsialpox are Weil-Felix negative across the board, which is itself a useful discriminator.

Interpretation

A presumptive positive is usually taken as a single titer of about 1:160 to 1:320 or higher against the relevant OX strain, in a patient with a compatible illness and exposure history. A fourfold rise across paired sera is stronger evidence. The pattern matters as much as the number: an OX-K response points to scrub typhus, while an OX-19 (with or without OX-2) response points to the typhus or spotted fever groups.

Crucially, a negative Weil-Felix test does not exclude rickettsial infection, especially early in illness or in scrub typhus, where the test misses a high proportion of true cases. Where available, specific tests such as indirect immunofluorescence (IFA) for the relevant organism or PCR are now preferred for confirmation, and the Weil-Felix test is best regarded as a resource-limited presumptive screen rather than a definitive diagnosis.

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 titer.

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.

How to Remember

1. 19 and 2 for typhus and spots, K for scrub. OX-19 anchors the typhus group; add OX-2 and you cover the spotted fever group (which hits both 19 and 2); OX-K stands alone for scrub typhus. Think "K for chigger" if it helps, since scrub typhus is the chigger-borne one and the only one that lights up OX-K.
2. Cross-reaction, not detection. The test never sees a rickettsia. It sees antibodies bumping into look-alike Proteus antigens. That single fact explains both its usefulness (cheap, no rickettsial culture needed) and all its faults (nonspecific, cross-reacts with Proteus UTIs and other febrile illnesses).

Key exam facts in one table

Where students get confused

"The Weil-Felix test detects rickettsiae." It does not. It detects the patient's antibodies by their cross-reaction with Proteus OX antigens. No rickettsial antigen is used at all. This is why it is called a heterophile test.

"A negative result rules out rickettsial disease." No. Antibodies take 5 to 10 days to rise, so early tests are often negative, and scrub typhus in particular is frequently missed. A negative test in a compatible illness does not exclude the diagnosis.

"OX-19 positive means typhus, definitely." The pattern is suggestive, not definitive. The spotted fever group also reacts with OX-19 (alongside OX-2), and false positives occur with Proteus infections and other febrile illnesses. It is the pattern plus clinical and epidemiological context that matters.

"Scrub typhus should show up on OX-19." It should not. Scrub typhus (Orientia tsutsugamushi) reacts only with OX-K. If you are looking for scrub typhus on OX-19, you will miss it. This is the single most exam-tested discriminator in the test.

References

• Cox, A. L., Zubair, M., & Tadi, P. (2023). Weil-Felix test. In StatPearls. StatPearls Publishing.
• Woodward, T. E. Rickettsial diseases and the Weil-Felix reaction. In CRC Handbook of Viral and Rickettsial Hemorrhagic Fevers.
• Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
• Procop, G. W., et al. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.