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What is Weil Felix test? 

The Weil-Felix test is a diagnostic test for rickettsial infections, which are caused by a type of bacteria called Rickettsiae. This test detects antibodies against Rickettsiae in your serum (clear liquid that remains after blood clots) and confirms if you have a rickettsial infection. Antibodies are specialised proteins produced by our immune system to fight harmful substances like microbes.

Rickettsial infections are common across the world, but a few types are more prevalent in certain places. For example, scrub typhus and Indian tick typhus are common in India. Rickettsiae can only survive inside living cells and are found in ticks, mites, fleas, lice and animals (mostly rats and mice). The infection commonly spreads to humans by the bite of infected insects.

The basis of the Weil-Felix test is that some Proteus bacteria share common antigens with Rickettsiae. Antigens are proteins through which your immune system recognises foreign substances. These are usually present on the surface of the microbes. 

When Proteus antigen suspensions are mixed with the serum of an infected person, the rickettsial antibodies in the serum react with Proteus antigens to form visible clumps (agglutination). Agglutination indicates that the person has rickettsial infection.

The following Proteus strains (subtypes) are employed to detect rickettsial antibodies in the patient’s serum:

  • Proteus vulgaris (OX-19 strain): It can strongly react with antibodies produced against spotted fever and typhus.
  • Proteus vulgaris (OX-2 strain): It strongly reacts with antibodies against spotted fever.
  • Proteus mirabilis (OXK strain): It reacts strongly with antibodies against scrub typhus.

Nowadays, tests that directly detect the bacteria, such as polymerase chain reaction (PCR), and tests that demonstrate antibodies against rickettsial antigens themselves (indirect immunofluorescence assay, latex agglutination), are preferred to the Weil-Felix test. However, Weil-Felix test is a useful rapid screening tool for rickettsial disease in places with limited laboratory facilities.

  1. Why is a Weil-Felix test performed?
  2. How do you prepare for Weil Felix test?
  3. How is Weil Felix test performed?
  4. What do Weil Felix test results mean?

Your doctor may order the Weil-Felix test if you have symptoms suggestive of rickettsial infection and a recent history of possible exposure to rickettsial agents. The test is performed five to seven days after the onset of symptoms to detect antibodies, which are mainly of the IgM (immunoglobulin M) type. IgM antibodies are the first antibodies produced by the body to fight an infection.

Rickettsial disease may present as a mild illness or a life-threatening condition, causing jaundice, pneumonia, myocarditis (inflammation of heart muscle), multi-organ failure and shock.

Symptoms of rickettsial infection are nonspecific and mimic viral infections or bacterial infections. They usually appear about one to two weeks after exposure to infection and may differ slightly based on the type of rickettsial infection, but generally include:

If diagnosed early, the infection can be readily treated with antibiotics. Certain features that can help the doctor make an early diagnosis include:

  • A history of tick bite
  • Recent travel to areas where the infection is prevalent
  • Similar illness in family members, colleagues or pets

 You do not need any special preparation before the Weil-Felix test.

A blood test is ordered to check for rickettsial antibodies. The following procedure is used to collect a blood sample:

  • A laboratory technician will tie a tourniquet (a tight band) around your upper arm and ask you to tighten your fist. This helps to make the veins in your arm more prominent.
  • The site for needle insertion will be cleaned with an antiseptic wipe.
  • The technician will then insert a sterile needle into a vein and collect a blood sample in a container.
  • Once the sample is collected, he/she will release the tourniquet and remove the needle.
  • The sample will be labelled and sent to the laboratory for testing. 

You may feel a slight pricking sensation when the needle is inserted, it subsides soon. Temporary bruises at the puncture site are also common after a blood test. Please talk to your doctor if you notice an infection at the blood withdrawal site.

Normal results:

If no agglutination is seen in the test, it indicates the absence of rickettsial antibodies in the given blood sample and may suggest that you do not have a rickettsial infection. The symptoms may be attributed to another condition.

However, since agglutinating antibodies are detectable only during the second week of infection, the result may also be negative in the early stages of the disease. Treatment in early stages of the disease may also delay or mask the immune response.

Abnormal results:

Presence of agglutinating antibodies in the given sample indicates a positive result and suggests that you have rickettsial infection.

Antibody test results are usually reported as less than or more than a certain titre. A single high titre of 1:320 or greater, or a fourfold rise in titre (starting from 1:50) between two samples taken two to four weeks apart is reported as a positive result. A single titre greater than 1:80 indicates possible infection.

Bacteria such as Salmonellae, Proteus and Streptococcus pyogenes may give false-positive results, which means that you get a positive result even though you do not have the infection.

Your doctor will interpret the results based on your symptoms, clinical history and results of other tests.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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  1. Usha K, Kumar E, Kalawat U, et al. Seroprevalence of scrub typhus among febrile patients: a preliminary study. Asian J Pharm Clin Res. 2014;7(Suppl 1):19-21.
  2. MedlinePlus Medical Encyclopedia [Internet]. US National Library of Medicine. Bethesda. Maryland. USA; Antibody
  3. Indian Council of Medical Research [Internet]. Department of Health Research. New Delhi. India.
  4. Walker DH. Rickettsiae. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 38
  5. Udayan Uttam, Dias Meena, and Machado Santosh. A hospital based study of rickettsial diseases evidenced by Weil Felix test in a tertiary care hospital. CHRISMED Journal of Health and Research. 2014;1(3):150-153.
  6. Frank SA. Immunology and Evolution of Infectious Disease. Princeton (NJ): Princeton University Press; 2002. Chapter 4, Specificity and Cross-Reactivity.
  7. Central Research Institute [Internet]. Kasauli. Himachal Pradesh. India; Proteus Antigens
  8. Danave Deepali, Kothadia SN. Role of Weil Felix Test for Rickettsial Infections. IOSR Journal of Dental and Medical Sciences. 2015 July;14(7):52-54.
  9. Lab Tests Online. Washington D.C. American Association for Clinical Chemistry; Rickettsial Diseases Testing
  10. Mahajan SK. Relevance of Weil-Felix Test in Diagnosis of Scrub Typhus in India. JAPI. 2006 Aug;54:619-621.
  11. Michigan Medicine: University of Michigan [internet]; Immunoglobulins
  12. Center for Disease Control and Prevention [internet], Atlanta (GA): US Department of Health and Human Services; Rickettsial Diseases (Including Spotted Fever & Typhus Fever Rickettsioses, Scrub Typhus, Anaplasmosis, and Ehrlichioses)
  13. National Health Service [internet]. UK; Blood Tests
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