View source for 29. Basic principles of screening

* 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

[[Category:Public health 2 - General epidemiology and demography]]