A test has been introduced that will detect a certain 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) -: (A) It will be a good screening test, (B) The period Prevalence will decrease, (C) The apparent 5 year survival rate will increase, (D) The incidence rate will decrease
Answer is C. It is worth 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.
Incidence will remain the same as the number of new cases diagnosed are the same.
Period Prevalence of disease will increase as the duration of clinical disease will increase as the disease is diagnosed 1 year earlier, so duration will become 6 years instead of 5 years. It is worth noting here that duration of clinical disease period will be increased not the total time from onset to death which also include subclinical period.