<prompt> Which of the following is true about the antigen titre in Widal test?O-antigen titre remains positive for several months reaction to it is rapid.H-antigen titre remains positive for several months reaction to it is rapidBoth remain positive for several months reaction to both is rapidNone  <response>: H-antigen titre remains positive for several months reaction to it is rapid. Option 1, 3 - O Abs appear early and disappear early and indicate recent infection. H Abs appear late and disappear late. Option 2 Widal test: it can be done in second week and third week. Principle: It is an agglutination test where H and O antibodies are detected in the patients sera by using H and O Antigens. Antigens used: Four antigens are used: O antigen of S.Typhi TO H antigens of S.Typhi TH S.Paratyphi A AH S.Paratyphi B BH - H agglutinins persist longer than O agglutinins. Four-fold rise in antibody titre in paired sera at 1 week interval is more meaningful than a single high titre. - After 1 week if titre rises- indicates true infection. - After 1 week if titre falls- indicates anamnestic responses. 