Widal Test

Widal Test

Widal test detects presence of Salmonella antibodies in the patient’s serum with Typhoid fever. Typhoid fever is caused by various species of Salmonella such as Salmonella typhi and S.paratyphi A and S. paratyphi B. Widal test is named after its inventor, Georges-Fernand Widal.

Laboratory diagnosis of Salmonella infections depends mainly on the isolation and identification of the causal salmonella from a specimen of the patient’s blood, faeces & urine. Blood culture is the gold standard method for diagnosis of salmonella infection. The sensitivity of blood/stool culture ranges from 40 - 90% if the patient has not used antibiotics. Blood and stool cultures are less frequently used in developing countries like India due to cost and requirement of highly trained professionals.
Widal test is easy, cheaper and does not need highly trained personnel. Therefore it is the most common diagnostic test used for Typhoid fever.
Widal test is based on principle of direct agglutination reaction. This test uses “O” and “H” antigens of Salmonella typhi and S. paratyphi A and S. paratyphi B to detect “O” and “H” antibodies in the serum of patients suffering from Typhoid fever.

Widal test can be performed by Rapid Slide agglutination and Tube agglutination Method. Slide agglutination may be either a qualitative or quantitative test. The tube method takes more time but is more sensitive and accurate. As Slide agglutination method is rapid, so it is the widely used method.

Widal test Results

What is a Negative Widal test?
No agglutination is a negative test result and indicates absence of clinically significant levels of the corresponding antibody in the patient serum.
In case of singular Widal test, baseline values for the Negative Widal is 1:20 – 1:80 for all the antigens (TO, TH, AO, AH, BO, BH).

What is a Positive Widal test?
The Widal test is positive if TO antigen titre* is more than 1:160 in an active infection, or if TH antigen titer is more than 1:160 in past infection or in immunized persons.
*The titre of the patient serum using Widal test antigen suspensions is the highest dilution of the serum sample that gives a visible agglutination.

Interpretation of Result:
Interpretation of Widal test is difficult though it is one of the most over-utilized diagnostic tests for typhoid fever in India.
Diagnostic titre is observed 7-10 days after the onset of fever. In first week of infection, test is negative. Antibodies appear in serum only after 6-8 days of infection and persists up to the 4th week of infection. So, the Widal test has low sensitivity and efficiency when tested before 1st week and after 4th week of infection.
In endemic areas, people usually show moderately elevated level of “O” and “H” antibodies.
Repeated subclinical infection may give high titres due to previous antibodies.

There should be a four fold rise in titre between two serum samples collected at acute phase and convalescent phase in suspected patient.
Individuals vaccinated with Typhoid vaccine (TAB) may show moderately elevated titre of all three “H” antibodies.
Treatment with antibiotics reduces antibody responses.

Infection with many non-Salmonella organisms like Malaria, Dengue, Miliary Tuberculosis, Endocarditis, Brucellosis, Influenza etc. may give anamnestic response**.
**Anamnestic response: Those individuals, who had suffered from Typhoid fever in the past, sometimes develop anti-Salmonella antibodies during an unrelated or closely related infection.

Immunological disorders such as Rheumatoid arthritis, Rheumatic fever or Nephritic syndrome demonstrate high titre of “O” and “H” antibodies.
Patients with chronic liver disease may give high titre due to failure of antigens in discriminating the specific antibodies from the Dysglobulinaemia of chronic active liver disease.
Narcotic addicts demonstrate non specific activity to the Widal test.

What is the interpretation if all values i.e. TO, TH, AH and BH agglutinins are raised?
If all agglutinins (TO, TH, AH, BH) are raised, it is most likely due to previous vaccination or an anamnestic reaction. These can be differentiated from true infection by repeat testing after a week or 10 days, which will show no rise in titre.
Theoretically, another reason for raised agglutinins could be recurrent or repeated infections due to different salmonella from the enteric fever group, but this is extremely unlikely, especially in the absence of symptoms.

In conclusion, Widal test is not reliable for diagnosis of typhoid fever since false positive and negative results are common. Widal test is considered as a presumptive test, so need further investigations to confirm an enteric fever.

A single Widal test is of little clinical relevance especially in endemic areas such as Indian subcontinent, Africa and South-east Asia. However, all laboratory results should be correlated with clinical findings to avoid misinterpretation. Other clinical features like “Rose spots”, stepladder fever, relative Bradycardia and leucopenia should also be taken into consideration while interpreting the result.

 

Dr. Hema Mathurkar
MD Microbiology (LTMMC Mumbai)