In the following natural history of disease. What is B-X duration -
Ans. is 'c' i.e., Screening time o In respect to screening test and diagnostic test, a disease passes through following phase, sequentially. i) Disease onset - Disease process just started. 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. 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. Lead time is the advantage gained by screening. o Thus in the given question - Screening time - B-X Lead time - B-Y
Lead time
Lag time
Screening time
None
Concept of disease causation