Typhoid fever is a systemic disease caused by Salmonella typhi and is the major cause of morbidity and mortality worldwide. WHO reports that about 21 million cases and over 600,000 deaths occur due to Typhoid fever annually. Developing countries bear the highest burden due to increased urbanization, unhygienic disposal of faeces and limited access to safe water supply.
Accurate diagnosis of typhoid fever at an early stage is important not only for etiology, but also to identify individuals who might serve as potential carriers and may be responsible for future outbreaks of Typhoid fever. Clinical diagnosis of typhoid fever is difficult, as the presenting symptoms are diverse and similar to those observed with other febrile illnesses. Serodiagnosis of typhoid fever has been attempted since late nineteenth century by Widal and Secard. The test is based on demonstrating the presence of agglutinins (antibodies) in the serum of an infected patient against the flagellar (H) S.typhi , S.paratyphi A , S.paratyphi B and somatic (O) antigens of S.typhi.
In the developed world, Widal Test is no longer used as a diagnostic tool due to low prevalence of typhoid, access to safe drinking water and better laboratory facilities to isolate the offending pathogen from blood culture.
A review of all published reports on the diagnosis of typhoid in the world between 1994 and 2015 was made by reviewing all publications against a set criteria for objective diagnosis, in all 16 studies were reviewed and they has sample size ranging from 50 to 1735 and used blood culture isolation of S typhi as the gold standard.
Sensitivity is the probability that a truly infected individual will test positive whereas specificity is the probability that a truly uninfected individual will test negative. Positive Predictive Value (PPV) is the probability that those positive by the test are truly positive and Negative Predictive Value (NPV) is the probability that those testing negative by the test are truly uninfected.
The mean sensitivity of Widal test was 73.5 + 12.6 %. The probability of a true typhoid patient to be positive by Widal Test ranges from 60.9% to 86.1% (in other words, 13.9 to 39.1% of true typhoid patients may be falsely negative by Widal Test. The lowest sensitivity of Widal Test was 45.2% and highest 98% (the mean ability of Widal Test to detect salmonella uninfected febrile patient as negative falls between 55.5 to 95.9%. This means that 4.1 to 44.5% of true negative test false positive by this method compared to blood culture method. The lowest specificity of Widal Test was 60 + 29% and mean NPV of Widal Test was 75.2 + 24.8%.
The overall conclusion is that the reliability of Widal test is poor, the mean sensitivity, specificity, PPV and NPV all remain below 80%,. Therefore, Widal Test shouldnot be used for diagnosing typhoid fever unless supported by invasive clinical picture and other confirmatory tests.