Diagnosis of typhoid fever: Revisiting the Widal test
By Dr Amrish Kamboj, MD (Pathology)
Typhoid fever is a bacterial disease, caused by Salmonella typhi. It is transmitted by the ingestion of food or water contaminated by the faeces or urine of infected people.
Symptoms usually develop 1–3 weeks after exposure, and may be mild to severe. They include high fever, malaise, headache, poor appetite, constipation or diarrhoea and the appearance of skin rashes. A similar but often less severe disease is caused by Salmonella paratyphi A and, less commonly, by S paratyphi B and S paratyphi C.
Typhoid fever remains a major cause of morbidity in the developing world. There are about 16 million cases of typhoid reported around the world each year.
Typhoid fever can be treated with antibiotics. However, resistance to common antibiotics is widespread. Improvement occurs in the third and fourth week in those without complications. About 10% of patients have recurrent symptoms (relapse) after feeling better for one to two weeks.
What type of laboratory tests are available to help in the diagnosis
of typhoid fever?
Laboratory tests for typhoid fever can be subdivided into microbiological tests, serological tests, and molecular diagnostics
Microbiological tests
Blood culture Blood cultures usually become positive in the first week of illness in patients who have not taken antibiotics.
Stool and urine cultures become positive after the first week of infection.
Serological Diagnosis
Laboratory diagnosis of S. typhi infection in large parts of world still relies on serological tests such as the Widal test. The Widal test has been in use for more than a century as an aid in the diagnosis of typhoid fever.
Other tests like Typhidot, Typhidot-M, Tubex and urine antigen detection have given promising initial results but have yet to be evaluated in larger trials in community settings
When does the Widal test become positive in a patient with typhoid
fever?
The traditional view is that the test becomes positive only in the second week of the illness. However some studies in endemic areas have found the test to be positive (titer to O and/or H antigen of 1:100) in 90% of patients with blood culture positive typhoid fever and admitted during the first week of their illness.
Concluding messages on typhoid diagnosis and the Widal test
► Isolation of S. typhi by culture is the gold standard for the diagnosis of typhoid.
► Although new serological tests for the diagnosis of typhoid fever are becoming available, they must be carefully validated in each region before being used widely.
► In Guyana, elevated levels of agglutinating O and H antibodies as measured in the Widal test can be helpful in making a presumptive diagnosis of typhoid fever if interpreted with care.
► Use of the Widal test should therefore be restricted to those who have a reasonable probability of having typhoid fever.
► Reliance on the Widal test alone may lead to overdiagnosis of typhoid fever
► A negative Widal test result in a patient with a clinical history compatible with typhoid does not exclude the disease.
The Widal test is not a very specific test, and should not be used for diagnosing typhoid unless it is repeated and shows a rising concentration of the antibodies against the typhoid bug.
