Widal Test-Method-False Positive and False negative Widal Test

Widal test.

(tube dilution Agglutination test)

In widal test antibody titre against different salmonella antigen (known) is measured by dilution of serum

Body produces antibodies against H, O, and Vi antigens of Salmonella typhi and paratyphi A, B, C during 2nd and 3rd week.

The widal test is negative during first week but still done to make a base line of antibody titre. 

In the second week if there is four times rise in titre from base line then widal test is positive. (Indication of typhoid).

Method

  • Put known antigens of salmonella into three different test tubes and mark them as O, H, nad Vi,
  • Then aspire blood from patient and make its serum.
  • Then this unknown serum is diluted as 1/180 , 1/160, 1/360
  • Then look for agglutination by putting drops one by one starting from 1/160 in to each test tube.
  • That diluted serum 1/160, 1/180, 1/360 which casued agglutination is the antibody titre for that specific antigen (o, H, Vi)

Results:

  • Raised antibody titre against O antigen shows current infection
  • Raised titre against H antigen shows previous infection
  • Raised antibody titre against Vi antigen shows Carrier state

False positive Widal Test:

False Positive means that the disease is not there, still the result is positive. It occurs in following conditions:

  • Immunized patients
  • Old infection
  • Cross reaction with other bacterial antibodies

False Negative Widal Test:

False negative means that the disease is present, still result of the test is negative. It can occur in the following conditions

  • Hyperproteinemia
  • Immune deficiency

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