This test measures somatic O and flagellar H antibodies against Typhoid and Paratyphoid bacilli. The agglutinins usually appear at the end of the first week of infection and increase steadily till third / fourth week after which the decline starts. A positive Widal test may occur because of typhoid vaccination or previous typhoid infection and in certain autoimmune diseases. Non specific febrile disease may cause this titre to increase (anamnestic reaction). The test may be falsely negative in cases of Enteric fever treated with antibiotics in the early stages. The recommended test specially in the first week after infection is Blood Culture.