A test has been introduced that will detect a ceain disease 1 year earlier than it is usually detected. Which of the following is most likely to happen to the disease within 10 years after the test introduced? (Assumed that 1 year early detection has no effect on the natural history of disease) ?

Ans is 'c' i.e., The apparent 5 year survival rate will increase It is woh noting, that though the new test is detecting the disease one year earlier than the usual diagnostic test, it is not affecting the final outcome (as 1 year early detection has no effect on natural history). That means the test is detecting the disease after final critical point (the point after which even diagnosis and treatment will not change the outcome). So, the test in question cannot be considered as screening test. The test in question will result in increase in the apparent 5 years survival rate. But this is a potential bias: For example, consider a disease in which 50% of people die 5 year after the diagnosis by usual diagnostic test Now a new test is used which diagnoses the disease 1 year earlier then usual diagnostic test. So all these 50% patient will be surviving 5 years after the diagnosis because the time period after the onset of disease and death is same, but the disease is diagnosed one year earlier.