Ans. is 'a' i.e., Lead time o In respect to screening test and diagnostic test, a disease passes through following phase, sequentially. i) Disease onset --> Disease process just staed. ii) First possible point of diagnosis The earliest point in disease process where a diagnosis can be made, i.e., before this point diagnosis can not be made by any test. iii) Final critical point of diagnosis --> The point after which even diagnosis and treatment will not change the outcome. iv)Usual time of diagnosis --> Time in disease process when most of the people are diagnosed by usual diagnostic procedure (not screening test). o So, by screening, the intention is to diagnosis the disease between : ? i) First possible point Before that diagnosis cannot made by any test. ii) Final critical point After that treatment will not change the outcome. o The time period between these two points is called screening time and all screening tests are aimed to detect the disease during this period. Lead time o The lead time is the time period between diagnosis by early detection (i.e., earliest point of diagnosis by screening test) and diagnosis by standard diagnostic tests. o Lead time is the advantage gained by screening. Implications of lead time & Screening time o If the person is diagnosed in screening time (from first possible point to final critical point), the outcome can be improved/changed by early treatment. o If the person is diagnosed after screening time (after final critical point), the outcome will not change even with treatment. o So, though the lead time is the advantage gained by screening, only the earlier poion of lead time, i.e., screening time is impoant for screening tests. o For example if the usual outcome of a disease is death (without any intervention). Following may occur : - i) Person is diagnosed before final critical point in screening time ---> Person may survive or death may be postponed by treatment (outcome is altered/improved). ii) Person is diagnosed after final critical point, even if in lead time (i.e., poion of lead time after screening time) ----> Person will die even with treatment, at the same time (the time of death without any intervention). iii) Screening is not useful for the diseases, which do not have lead time (screening time is a poion of lead time) --> Because there is nothing to be gained by screening as the disease will be diagnosed at the usual time of diagnosis (as by standard diagnostic tests).