Screening program = comprehensive program on identification of persons with unrecognized disease or risk factors for disease
Screening test = a test used for the identification of persons with unrecognized disease or risk factors for disease
Screening is a simple process which classifies persons into one of two groups
The first group is for those with a positive screening test. They should undergo further testing or initiate treatment
The second group is for those with a negative screening test. The probability that they have the disease is too low to justify further testing or treatment
Requirements for screening
Early detection must lead to a favourable diagnosis (i.e., the disease can be treated in the early stages)
The disease should be serious
The disease should be relatively common
If left undiagnosed, the disease will progress
Important properties of a screening test
Must be simple to perform
Must be inexpensive
Must be safe
Must be accurate
The results must be reliable
Evaluation of a screening program
Reliability – the ability of a test to give consistent results when repeated on the same person under the conditions
Feasibility – for a screening program to be feasible it must be quick, easy to do, safe, cost-effective, not very uncomfortable and must not cause side-effects
Validity – the screening test must have high sensitivity and ideally high specificity as well
Performance – the screening test must have high positive and negative predictive values
Predictive values depend on the disease prevalence in addition to the specificity and sensitivity
Yield – the screening program must cause a significant decrease in unrecognized disease cases
Effectiveness – the screening program must be effective in reducing disability, morbidity, and mortality from the disease
This is obviously the most important point
There are four possible outcomes of a screening test
A person is a true positive – i.e. they test positive and they have the disease
The screening was good for this person
A person is a true negative – i.e. they test negative and they don’t have the disease
The screening was good for this person, as it reassures them that they don’t have the disease
However, they are still exposed to the costs, inconvenience, and hazards of screening
A person is a false positive – i.e. they test positive but don’t have the disease
Upon further testing they will test negative, but for this person the screening only serves to cause unnecessary psychological and emotion distress, in addition to the costs, inconvenience, and hazards
A person is a false negative – i.e. they test negative but do have the disease
The screening gives this person a false sense of reassurance, while exposing them to costs, inconvenience, and hazards
