Can a Widal test diagnose typhoid?
Q: My son, three years old, was having fever for 2 days with a temperature of 102 even after taking paracetamol and Cefixime. The doctor has asked to do Widal test, which was found to have a titre of 1:80. The doctor said that our son has high typhoid and was admitted in the hospital and was given IV Ciprofloxacin for 2 days, three times a day. After the second dose, there was no fever. He was discharged and asked to take Chloramphenicol for 5 days. He is not having vomiting or diarrhoea. He was also vaccinated for typhoid at 2 years of age. My doubt is if this is typhoid or viral fever because many say that typhoid cannot be detected on the second day of fever itself and there may be false positives because of vaccination? Is this true?
A:It is highly unlikely that a diagnosis of typhoid (enteric fever) or any other salmonellosis can be made solely on the basis of a Widal test done on day 2 of a fever in a vaccinated child, and should ideally be based on the isolation of the organisms either from the blood (in the early stage), urine or faeces (later on in the illness). We must remember the limitations of Widal test, which is a serological test, and many times shows false positives. The interpretation of Widal test is affected by the frequency of distribution of Salmonella agglutinins (O & H) in the population. There is a certain proportion of the population whose sera will give antibodies capable of reacting to a variable titre in Widal test due partly to the occurrence of latent or after an infection with members of the enteric group like salmonella or other groups with antigenic factors similar to that used as antigen in the test. When we say diagnostic titre, we actually mean significant or suggestive. In Indian population where typhoid is endemic, single titres of only more than 1 in 200 are considered significant. The agglutinins take time to appear and will usually not show a rise in titre over the baseline before a week; therefore paired sera are usually required to interpret the Widal test. H agglutinins may persist for years after immunisation, therefore detection of agglutinins is of limited significance in the vaccinated patients with fever. An anamnestic reaction during the course of a fever not due to typhoid can often be misleading. For a vaccinated patient, the O agglutinin levels are better. They can also be elevated following immunisation but do not persist. A high O and a low H titre would suggest an active infection. A low O and a high H suggests an anamnestic reaction. The level of antibodies bear no constant relationship to the severity of the disease and relapses are equally common in those with high or low titre. The early treatment of cases of typhoid fever with antibacterials has a profound effect on antibody response and consequently the Widal test. If agglutinins have not appeared when treatment is begun, they may not do so subsequently. If they are already present, no further rise may be expected. Treatment later in the disease has little or no effect in the agglutinin response. In conclusion, Widal test is an old simple test but one needs knowledge in laboratory medicine for this test to be useful as devised. It is only an aid to the diagnosis of typhoid and paratyphoid fevers. A good medical history, proper timing of sampling, and keeping in mind that the diagnosis is aided by a significant rise in titre rather than one single sample is more helpful.