Widal

Widal

Widal Test: An Overview

The Widal test is a serological test used for diagnosing typhoid fever, an infection caused by Salmonella enterica serotypes Typhi and Paratyphi. It detects the presence of agglutinating antibodies against Salmonella antigens in a patient’s serum. This test was developed by Georges-Fernand Widal in 1896 and remains widely used in regions where typhoid fever is endemic.

Principle of the Widal Test

The Widal test is based on the principle of agglutination, wherein specific antibodies in the patient’s serum react with Salmonella antigens (O and H antigens). The O antigen represents the somatic antigen, whereas the H antigen corresponds to the flagellar antigen. The presence of these antibodies in significant titers suggests a current or recent infection.

Procedure of the Widal Test

The test is performed using either a slide agglutination or tube agglutination method:

  1. Slide Agglutination Test: A drop of the patient’s serum is mixed with a drop of antigen solution on a glass slide. If agglutination occurs, it indicates the presence of antibodies.
  2. Tube Agglutination Test: Serial dilutions of the serum are mixed with antigen suspensions in test tubes. After incubation at 37°C for 18-24 hours, the degree of agglutination is observed.

Interpretation of Results

  • A significant antibody titer (generally 1:160 or higher) in a single test suggests an infection.
  • A fourfold rise in antibody titers in two samples taken 7-10 days apart is considered diagnostic.
  • Lower titers can be due to past infection or vaccination and should be interpreted cautiously.

Limitations of the Widal Test

  • False Positives: Previous infections, vaccinations, or cross-reacting infections can lead to false-positive results.
  • False Negatives: Early-stage infections may not produce detectable antibodies.
  • Lack of Standardization: Variability in antigen preparations and cutoff values affects reliability.
  • Low Sensitivity and Specificity: Newer diagnostic methods like blood culture and polymerase chain reaction (PCR) offer greater accuracy.

Conclusion

Despite its limitations, the Widal test remains a commonly used diagnostic tool, particularly in resource-limited settings. However, it should be supplemented with clinical findings and other laboratory tests for a definitive diagnosis.

References

  1. Parry, C. M., et al. (2002). “Typhoid fever.” The New England Journal of Medicine, 347(22), 1770-1782.
  2. Olopoenia, L. A., & King, A. L. (2000). “Widal agglutination test – 100 years later: Still plagued by controversy.” Postgraduate Medical Journal, 76(892), 80-84.
  3. House, D., et al. (2001). “Typhoid fever: Pathogenesis and disease control.” Microbes and Infection, 3(8), 602-615.
  4. World Health Organization. (2018). “Typhoid vaccines: WHO position paper.” Weekly Epidemiological Record, 93(13), 153-172.
By
0