Screening Flashcards
What is screening
A process which sorts out apparently well people who probably have a disease (or precursors or susceptibility to a disease) from those who probably do not
- not yet symptomatic
Secondary prevention
Detective early disease in order to alter the course of the disease
Eg screening by mammography for breast cancer in order to treat it early
Primary prevention
Prevent a disease from occurring
Eg screening to identify people with risk factors and reduce risk factor levels
Purposes of screening
Reduce the risk of developing disease
Provide treatment
Provide information eg pre-natal screening for genetic disorders
Sensitivity
The proportion of people with the disease who are correctly identified by the screening test
A/a+c
Specificity
The proportion of people without the disease who are correctly excluded by the screening test
D/b+d
Positive predictive value PPV
The proportion of people with a positive test result who actually have the disease
A/a+b
Negative predictive value NPV
The proportion of people with a negative test result who do not have the disease
D/c+d
How many conditions does the NHS screen for
36 health condition
Prevalence
A+c/a+c+b+d
Criteria for screening
Wilson and Jungner criteris
Criteria for screening- the condition
- The condition sought should be an important health problem
- The natural history of the condition should be well understood
- There should be a detectable early stage
Criteria for screening- the treatment
- There should be an accepted treatment for patients with recognised disease
- Facilities for diagnosis and treatment should be available
- Adequate health service provision should be made for the extra clinical workload resulting from screening
Criteria for screening- the test
- A suitable test should be devised for the early stage
- Test should be acceptable
- Intervals for repeating the test should be determined (not a one off)
Criteria for screening- risks and benefits
- There should be an agreed policy on whom to treat
- Costs should be balanced against the benefits
- the risks (both physical and psychological) should be less than the benefits
Evaluation of screening
Ideally by RCT (individual or cluster)
Could use other methods but potential for bias
3 well-recognised biases
Selection bias
Lead-time bias
Length-time bias
Selection bias
People who choose to participate in screening programmes may be different from those who don’t
- may be at a higher risk eg women with family history of breast cancer more likely to attend
- may be at a lower risk eg women in higher socioeconomic groups (lower risk of cervical cancer) more likely to attend
Lead time bias
Disease starts
1. Patient A diagnosed after screening
2. Patient B diagnosed when symptoms develop
Patient A- lives longer
Overestimation of survival duration due to earlier detection by screening than clinical presentation.
Length-time bias
Because more aggressive diseases are asymptomatic for a shorter period, screening is more likely to detect slower progressing diseases, such as slow-growing tumours, which have a better prognosis, including longer survival.
Types of screening
Population-based screening program
Opportunistic screening
Screening for communicable diseases]
Pre-employment and occupational medical
Commercially provided screening
Population based screening
Thailand
National diabetes and hypertension screening
Opportunistic screening
Prevention and control of substance abuse
Screening for communicable diseases
Heaf test
Pre-employment and occupational medical
Vision test for commercial drivers
Examples of screening on the NHS
Abdominal aortic aneurysm (AAA)
Bowel cancer screening
Breast screening
Cervical screening
Diabetic eye screening
Newborn screening eg hearing, physical examination, blood spot
Screening in pregnancy
What affects PPV and NPV
Prevalence
Lower prevalence:
PPV - decreases
NPV - increases
