7) Screening Flashcards
Define screening.
Screening is a systematic attempt to detect an unrecognised condition by the application of tests, examinations, or other procedures, which can be applied rapidly (and cheaply) to distinguish between apparently well persons who probably have a disease (or its precursor) and those who probably do not.
Suggest three ways we might detect disease?
- Spontaneous presentation (find something, go to GP)
- Opportunistic case finding (looking for something else and “hey that’s a bit weird, let’s investigate”)
- Screening (you look finnneeee, however, let’s do this test so we can just make sure)
List the criteria for implementing a screening programme relating to the disease/condition
- Must be serious condition (QoL or likely to cause death)
- Must understand natural history/epidemiology
- Early detectable stage - no point if you can do nothing about it
- Cost-effective primary prevention interventions must have been considered and where possible implemented
List the criteria for implementing a screening programme relating to the test
- Simple (doing lots)
- Precise and valid (tells truth)
- Acceptable
- Distribution of test values in the population (which value you draw the line between negative and positive)
List the criteria for implementing a screening programme relating to the the treatment
- Effective evidence based treatment must be available (find out have it and can’t do anything)
- Early treatment must be advantageous, must not just bring forward the date of diagnosis (lead time bias)
- Agreed policy on whom to treat
List the criteria for implementing a screening programme relating to the the programme.
- Proven effectiveness (see RCT)
- Benefit should outweigh physical and psychological harm
- Facilities for diagnosis and treatment
- Facilities for counselling
What two types of error do screening programmes make? What are some of the issues for the patient with these errors?
- False positives (stress, anxiety, inconvenience, cost from doing diagnostic test on them)
- False negatives (inappropriate reassurance leading to them presenting late)
What is the sensitivity of the test? How is it calculated?
The proportion of people with the disease who test positive (also known as detection rate) This is calculated by the true positives divided by (true positives + false negatives)
What is the specificity of the test? How is it calculated?
The proportion of the people without the disease who test negative. This is calculated by true negatives divided by (true negatives + false positives)
What is the positive predicted value of the test? How is it calculated?
Probability that someone who has tested positive actually has the disease. This is calculated by true positives divided by (true positives + false positives)
What is the negative predicted value of the test? How is it calculated?
The proportion of the people who test negative who actually do not have the disease. This is calculated by true negatives divided by (true negatives + false negatives)
How do you calculate the prevalence of a disease?
The (true positives + false negatives) divided by (true positives + false positives + true negatives + false negatives)
Suggest some advantages of screening.
- Early detection of disease may improve outcome
- True negatives reassure patients
Suggest some disadvantages of screening.
- False positives expose patients to invasive diagnostic tests which can lead to worse quality of life
- False negatives falsely reassure patients
- False negatives not offered diagnostic testing they may benefit from
- Expensive interventions that divert money away from treatments
Give examples of screening programmes in the UK
- Abdominal Aortic Aneurysm
- Bowel cancer
- Breast cancer
- Cervical cancer
- Diabetic Retinopathy
- Down’s syndrome
- Foetal anomalies
- PKU
- Sickle Cell and Thalassaemia
