The chemical test that is used for diagnosis of typhoid is the Widal test.

Explanation:
Typhoid fever is a bacterial infection caused by Salmonella typhi. The symptoms of typhoid fever include a high fever, headache, abdominal pain, and constipation or diarrhea. The diagnosis of typhoid fever is usually made by blood culture or stool culture, but these tests may take several days to produce results. In some cases, a rapid diagnostic test such as the Widal test may be used to provide a quicker diagnosis.

The Widal test is a serological test used to detect antibodies against Salmonella typhi in the blood. The test measures the levels of agglutinins (antibodies) in the blood that are produced in response to the bacterial infection. The test is performed by mixing a sample of the patient's blood with a suspension of killed Salmonella typhi bacteria. If the patient has been infected with the bacteria, their blood will contain antibodies that will cause the bacteria to clump together (agglutinate). The test results are reported as a titer (e.g. 1:160), which indicates the highest dilution of the patient's serum that still causes agglutination.

The Widal test is not always reliable, as it can produce false-positive results due to cross-reactivity with other bacterial infections or previous immunization against typhoid fever. Therefore, it is usually used in combination with other diagnostic tests to confirm the diagnosis of typhoid fever.

Widal test is a serological test for typhoid or enteric fever.ESR is done to check out the concentration of Erythrocytes in blood.PCR is done to amplify the amount of DNA molecules.ELISA is done in AIDS .