Which of the following investigation is done to diagnose typhoid in a patient after 15 days of onset of fever?

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