Widal test in Typhoid Fever Diagnosis – Procedure And Results

o Repeated exposure of a person to a small amount of the salmonella bacteria and its related species.

o Immunization developed from a previous episode of typhoid fever

o False results from other infections such as malaria.

• Two blood tests: The role of two samples for diagnosis is also not used regularly due to inconsistent results and delay in the treatment because of the second blood sample.

Role of Widal test vs. Role of Blood culture

The gold standard test for the diagnosis of typhoid fever is blood culture. The test is

• Costly

• Not available everywhere

• Takes about 3-5 days to confirm the results.

Sometimes with the use of antibiotics, the blood culture results may also get affected.

However, the Widal test in typhoid fever is cheaper, commonly available, easy to perform, and needs minimal equipment. Thus, it is continued to be widely used in the endemic areas for giving a probable diagnosis of typhoid and paratyphoid fever. This can be further supported by a strong clinical suspicion and results of other tests.

During the Widal test procedure, a special liquid containing “O” and “H” antigens is mixed with an equal amount of serum (filtered from your blood). If you have antibodies in your blood for the two antigens, then the mixing of the two liquids will form visible clumps. This is called agglutination.

The IgM antibodies against O antigen appears first during the acute typhoid fever while the IgG antibodies against H antigen forms later but stays longer.

There are two types of Widal tests- slide agglutination and tube agglutination tests. The slide test is used for initial screening, and the tube test is used to confirm the results of the slide test.

In both the tests, the liquid agent containing antigens is mixed with equal amounts of serially diluted serum obtained from your blood. The results are scored from 0 to +4 according to the degree of agglutination:

• 0 (no agglutination),

• +1 (25% agglutination)

• +2 (50% agglutination),

• +3 (75% agglutination)

• +4 (100% agglutination).

The least quantity of serum exhibiting a 50% or 2+ agglutination is denoted as the titer (endpoint of serum activity).

The results are stated as:

• No agglutination- in the absence of clumping, or

• Agglutination presents with titers of such as 1:20, 1:40, 1:80 or 1:160, and so on.

There are several factors that sometimes make the test positive even when you don’t have typhoid fever. This is called a False-positive test, and the causes are as follows:

Other group D salmonella bacteria: Apart from the bacteria (salmonella typhi) that causes typhoid fever, there are many other salmonella bacteria grouped from A to E types. Group D consists of the salmonella typhi with many other bacteria that don’t cause typhoid fever but has the “O” antigen.

Group A and B salmonella bacteria: There are other bacteria from the group A and B that has “O” antigen similar to salmonella typhi.

The Widal test detects and quantifies the antibodies against the O antigen irrespective of type of Salmonella- whether typhoid or non-typhoid Salmonella.

Other microbes: Moreover, there are other microbes such as malaria parasites that may show similar antigen cross reactivity.

Previous infection or immunization: There are also significant chances that a person with a prior history of typhoid infection or repeated trivial exposures may develop antibodies in the blood. This is especially common in endemic areas where people have high chances of a past history of disease.

False-negative

There are also several reasons when your Widal test comes negative even when you are suffering from typhoid fever. This is called a False-negative test, and the causes are as follows:

Use of antibiotics: if you have already taken antibiotics for your condition prior to getting the Widal test, it may lead to already decreased levels of antibodies in the blood. Thus, leading to reduced titers in the test and false-negative result.

Carrier state: in this state, you may carry salmonella typhi bacteria but may not develop the infection with significant antibodies to be detected in the blood.