Widal test – Interpretation, Normal Values, Procedure


The Widal test is used in the diagnosis of Typhoid fever.

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The Widal test is a test procedure that involves mixing of the bacteria which cause typhoid fever to the blood consisting of certain antibodies taken from an infected person. It is an assumptive serological test used for undulant fever or enteric fever. When the Widal test is used to check of the presence of Salmonella infections, then it is a presentation of the occurrence of O-soma false-positive result. It is important to carefully and correctly interpret the test results as per the past history of enteric fever, the typical composition of antibodies in the general population living in the endemic regions of the world and typhoid vaccination.

Another test which is employed to detect and diagnose typhoid fever is the Typhidot test. As is the case with every serological test, the increase in the antibody quantities required to conduct the diagnosis takes seven to fourteen days, which restricts its relevance/use in early detection. The other procedures that can be used to diagnose Salmonella typhi or paratyphi include options like urine, blood and stool cultures. It may be noted that such microbes make H2S from thiosulfate which can be effortlessly recognized on differential media like Bismuth sulfite agar.

Occasionally 2-mercaptoethanol is combined to the Widal test. This chemical has the ability to easily denture the IgM class of antibodies. Thus when a reduction in the tier is observed after utilizing this agent, then it indicates that the input of IgM has been eliminated with the IgG constituent left behind. This distinguishing of antibody classes is vital, as it permits for the differentiation of a recent infection (IgM) from an older infection (IgG).

The Widal test result is positive:

  • If the TO antigen titre is greater than 1:160 in an infection that is active, OR
  • If TH antigen titre is greater than 1:160 in older infection or in immunized individuals

A single Widal test holds little clinical applicability due to the several cross reacting infections, which include malaria. If other tests, such as a more precise serology or bacteriologic culture, are unavailable, then inferences such as a fourfold rise in the titre {For example from 1:30 to 1:120} during the course of an infection, or a change from an IgM reaction of a specific titre to an IgG reaction of the same titre in the least, would signify a typhoid infection.

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Widal test procedure

The Widal test can be conducted via the two below listed methods:

  • Tube agglutination
  • Slide Agglutination

It may be noted that tube agglutination has more accuracy as compared to the slide agglutination technique. It is possible to go up to 1:1280 titres in the tube agglutination method, whereas the slide method limits the test to 1:320 titres only. However, the slide agglutination is faster with the results being produced inside five minutes.

The Widal test procedure is carried out in the following way:

  • Prepare a sequence of serum dilutions for every antigen that is to be tested. Take tubes with 0.5 ml saline for regulation of all antigens that are to be used.
  • Dry and thoroughly cleansed test tubes have to be used. Make dilutions starting with 1:10 and doubling-up through 1:320
  • Mix 0.1 ml of serum with 0.9 ml of physiological saline and later thin serially by adding 0.5 ml diluted serum to 0.5 ml saline and removing 0.5 ml from the final tube.
  • Make a series of ten dilutions that end with 1:5120 so as to detect possible increase in titre in the submitted sample.

In simple words, the procedure involves:

  • Preparation of the blood plate with black background
  • Addition of a single drop of serum
  • Addition of antigens, i.e., o, AH, H, and BH
  • Mixing of serum and antigen
  • Rotation of the sample for one minute
  • The results may be either negative non-reactive or positive reactive

Widal test interpretation

  • The antibodies develop at the end of the first week and the titres rise during 2nd, 3rd and 4th week, after which they slowly decrease. Hence a Widal test performed early in the first week may give negative results.
  • A single Widal test is not sufficient for correct results and at least two tests should be conducted.
  • The level of antibodies of a population in a given area indicates the baseline titre. One has to know this before interpreting a Widal test. O antigen with a titre of 100 or more and H antigen with a titre of 200 or more are deemed important
  • Patients who have already undergone antibiotic treatment may not show any increase in titre on a Widal test.
  • Patients vaccinated against Salmonella may show false positive reactions which can be verified with another Widal test in the next week. Then vaccinated people will not show any increase in titre, while untreated cases will show rise in titre in the Widal test results
  • Patients with past history of enteric fever may have Salmonella antibodies and hence may show false negative reactions. This can be verified with a second test in the next week, which shows rise in titre.
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