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
The 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 the first week but still done to make a baseline of antibody titre. 
In the second week if there is four times rise in titre from the baseline 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, and Vi,
• Then aspire blood from the 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 into each test tube.
• That diluted serum 1/160, 1/180, 1/360 which caused 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 the 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 the 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 the result of the test is negative. It can occur in the following conditions

• Hyperproteinemia
• Immune deficiency