L12 - screening Flashcards
Define screening
The practice of investigating apparently healthy individuals with the object of detecting unrecognised disease/precursors so that measures can be taken to prevent disease and improve prognosis.
What challenges does screening face?
Low prevalence populations, high specificity of test is needed
How can lower/higher cut off points for diagnostic tests influence diagnoses made?
Higher cut off = lower sensitivity, higher specificity
Lower cut off = higher sensitivity, lower specificity
What is the appeal of screening?
People prefer not to get ill, and there is a widespread belief that early detection is better
What are the current UK systematic screening programmes?
Antenatal: chromosome abnormalities, infectious disease, sickle cell anaemia and thalassaemia, physical abnormalities
Adult cancer: breast (50-70), cervical (25-64), bowel (60-74)
New born: Physical exam, heel prick test, hearing test
Other: AAA 65(+), diabetic retinopathy 12+
What additional health checks currently occur in the UK?
NHS health checks, prostate cancer chlamydia
What are the results of UK cervical cancer screening?
67% reduction in stage 1a cancer
95% reduction in stage 3+ cancer
Prevention of 70% cervical cancer deaths
Saves 4500 lives a year
What are the results of UK HIV screening programme?
Transmission reduced from 2.1% 2000 to 0.46% 2010-11
What are the results of HIV screening in pregnant women in South Africa?
70 000 babies -> <6000 babies
1.3 million new HIV infections prevented globally
When was breast cancer screening introduced?
1988
What is the screening test for BC?
single or double mammography
What is the interval for BC screening?
Every 3 years
Approx how many women are screened for BC and how many cancers are detected per year?
2.5 mill invited, 1.9 screened, 15000 cancers detected
What are the problems of BC screening?
Over diagnosis
How is colorectal cancer screening carried out?
Faecal occult blood screening (stool)
How often does colorectal cancer screening occur?
2 yearly from age 60-74
What are the criteria for screening programmes set out by WHO?
Disease - important health problem, well recognised and detectable pre-clinical stage, natural history understood, long period between first signs and overt disease
Screening test - valid, simple, cheap, safe, acceptable, reliable
Diagnosis and treatment - adequate facilities, effective, acceptable, safe, cost effective, sustainable
What is the 5 year survival rate of prostate cancer?
71%
What are the types of bias affecting evaluation of screening effectiveness?
Selection bias - people participating in screening often differ from those who don’t
Lead time bias - screening identifies a disease that would often be identified at a later stage, which may present an apparent ‘improvement’ in survival
Length bias - some conditions develop at a slower rate (longer preclinical stage)
Define validity of a screening test
Ability to distinguish between subjects with and without the condition
Describe the approaches to screening
Mass - Applies to whole population (usually defined by age or gender)
Opportunistic - The population is approached when they make contact for another reason
Targeted - Select sub-groups thought to be at increased risk
Systematic - The population is called for screening using a register
