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What is Widal test?

Widal test is one of the oldest tests devised for the diagnosis of typhoid fever, an enteric disease caused by the bacteria Salmonella typhi.

It checks for the presence of anti-typhoid antibodies in the blood, specific proteins produced against S. typhi. The test procedure involves checking a patient's blood samples, which are taken about 10 to 14 days apart. If a rise in the number of antibodies is observed, it can indicate the presence of bacteria in the body.

This test does not differentiate between various types of enteric fevers and tends to give false-positive results, so, it is no longer used as a confirmatory test in most developed countries. Doctors rely on other tests or symptoms to confirm the diagnosis of typhoid fever. 

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

Widal test is mainly used as a diagnostic method to check for typhoid fever. Doctors order this test if the following conditions are seen in a patient:

Before the test, it is important that you inform your doctor if you have had any previous vaccination or immunisation to typhoid fever. It is also crucial to let the doctor know about the presence of any other infections, like malaria, or if you have been in close contact with a person infected with the bacteria. These factors would be helpful in the correct diagnosis of typhoid fever

The testing procedure is performed on blood serum. Therefore, a blood sample needs to be collected from the person. Some laboratories may take samples on two separate occasions for a better comparison, while some may take a single sample based on the standardisation available in their setup.

A lab technician will first clean the area on your arm with a cotton swab and tie a tourniquet to make a vein prominent. Using a sterilised needle, they will withdraw the blood in a small closed test tube; after which, they will cover the area with cotton gauze to stop bleeding. Finally, they a bandage will be applied to the area. 

You might experience slight discomfort due to the prick of the needle, but it will subside soon. There could be some bruising on the skin after the test, which will also fade away by itself.

Normal results:

The test results are scored according to the amount of reaction that occurs between the person’s antibodies and the test solution. The list given below shows the correlation between test score and result:

  • 0 means no reaction
  • 1+ means 25% reaction
  • 2+ means 50% reaction
  • 3+ means 75% reaction
  • 4+ means 100% reaction

When the score is 0, it is considered a normal result. It means that the person is not suffering from typhoid fever.

The test also involves the dilution of blood serum to check for the occurrence of a reaction. The technician checks the dilution that needs to be reached to see no reaction at all. Therefore, for the simplicity of interpretation, the dilution may also be stated in the test reports. However, a negative result does not completely remove the chance of infection.

Abnormal results:

A value other than 0 is considered abnormal, and it could mean that the person may have typhoid fever.

Since the sensitivity of the test has been questioned time and again, the doctor may order further tests to ascertain the diagnosis.

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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References

  1. Andualem G et.al. A comparative study of Widal test with blood culture in the diagnosis of typhoid fever in febrile patients.. BMC Res Notes. 2014 Sep 17;7:653. PMID: 25231649
  2. Olopoenia LA et.al. Widal agglutination test - 100 years later: still plagued by controversy. Postgrad Med J. 2000 Feb;76(892):80-4. PMID: 10644383
  3. Omuse G. Diagnostic utility of a single Widal test in the diagnosis of typhoid fever at Aga Khan University Hospital (AKUH), Nairobi, Kenya. Trop Doct. 2010 Jan;40(1):43-4. PMID: 19850607
  4. Center for Disease Control and Prevention [internet], Atlanta (GA): US Department of Health and Human Services; What are the signs and symptoms of typhoid fever and paratyphoid fever?
  5. National Heart, Lung, and Blood Institute [Internet]. U.S. Department of Health and Human Services; Blood Tests