Widal test should be performed after first week of fever. The reason being,the antibodies against “O” and “H” antigens of Salmonella, start appearing in the blood of patients by the end of 1st week, with sharp rise in 2nd and 3rd week and the antibody titre remains steady till 4th week after which it starts declining. It is preferable to test two blood samples at an interval of 7 to 10 days to demonstrate a rising antibody titres.