Which of the following investigation is done to diagnose typhoid in a patient after 15 days of onset of fever?
Widal
Blood culture
Stool culture
Urine culture
Following typhoid infection, agglutinins usually appear by the end of 1st week, so blood taken earlier will give a negative result. Antibody titers increases steadily till the 3rd or the 4th week, after which it declines gradually. So Widal test used for the measurement of H & O agglutinins for typhoid and paratyphoid bacilli is done after 2 weeks of onset of fever. A titer against the O antigen of >1:320 and against the H antigen of >1:640 is considered positive. Ref: Textbook of Microbiology By Ananthanarayan and Panicker, 8th Edition, Page 296