Question
Enumerate the agents causing enteric fever. Name the serological test indicated for the above condition. In which week of illness should the test be done. What is the gold standard method for diagnosis and in which week of illness is it highly sensitive. (1+1+1+0.5 +0.5)
Answer
Agents causing enteric fever: Salmonella enterica serovar Typhi, and serovars Paratyphi A, B, and C.
Serological test indicated: the Widal test — detects agglutinating antibody against Salmonella O and H antigens.
Week of illness for the test: the Widal test is meaningfully positive/interpretable from around the second week of illness (a rising titre in paired sera taken 7–10 days apart, ideally acute plus convalescent, gives the most reliable evidence).
Gold standard method for diagnosis: Blood culture (with bone marrow culture being even more sensitive at any stage of illness).
Week of illness with highest sensitivity: blood culture is most sensitive in the first week of illness, during the bacteraemic phase, with sensitivity declining thereafter.