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