Typhoid fever is diagnosed by testing for antibodies produced against the bacteria Salmonella typhi, which causes the condition. In most countries where the disease is widespread, treatment is started based on symptoms while these tests are processed by the laboratory to confirm the presence of disease.

An outline for the diagnosis of typhoid fever is given below:

• Assessment to check for typhoid fever symptoms.
• Any history of similar infection within the patient's social circle or family is obtained as are the details of any recent travel to countries where typhoid is prevalent. Endemic areas include countries in Africa, the Indian subcontinent, South East Asia, and South America.
• Blood, stool and urine samples are collected so they can be checked under the microscope for the presence of Salmonella typhi. However, the bacteria may not always be detected in the early stages of disease and a series of tests may be needed to confirm the diagnosis.
• Testing the bone marrow is a more reliable method for diagnosing typhoid. However, obtaining a bone marrow sample is painful and is therefore avoided if possible.
• Blood culture is another common method of diagnosis. Blood drawn from the patient is placed on a culture media, which allows any bacteria present to grow and be detected under a microscope. For this test, around 10 to 15 ml of blood is drawn from adults and around 2 to 4 ml is drawn from toddlers and children, as children have a higher concentration of bacteria in their blood.
• A stool culture may also provide results by allowing the bacteria to grow in a culture media.
• Serum samples can be tested for antibodies against typhoid. For this test, 1 to 3 ml of blood is collected in a tube that does not contain an anticoagulant so that the clear serum portion of the blood can be separated off and tested for antibody titre.

The Widal test involves testing for agglutinating antibody levels against O antigens which usually appear 6 to 8 days after disease onset and H antigens which appear on days 10 to 12. However, the use of more recent tests such as the IDL Tubex test and the Typhidot test allow for a more rapid detection of the antibodies, leading to a faster diagnosis.