Screening Flashcards
Screening is
Systematic application of a test or inquiry to identify individuals at sufficient risk of a specific disorder to warrant further investigation or direct preventative action , amongst persons who have not sought medical attention on symptoms of that disorder
Examples …
- ultrasound to identify a abdominal aorta
- measuring serum AFP , bhCG , oestriol during pregnancy for downs
- heel prick Tests for congenital hypothyroidism , PKU
Screening isn’t
Same as making a diagnosis
Positive screening tests Dosent normally give you a diagnosis
We use screening tests when more definitive tests are more dangerous or unpleasant
Most screening is probably thought of as secondary prevention
Symptom iceberg
Most healthcare ( interpreting , treating, alleviating of symptoms ) is carried out by ordinary people
Many people self treat I.e for high temp, runny nose, achy joints , headaches
Only small proportion of illness is seen by Gp’s and around 1% reaches hospital
So doctor has a distorted view of illness
Why screen ?
- opportunities for primary prevention are limited
- opportunities for treatment are limited
- screening gives potential for early and more effective treatment
Criteria for appraising
- viability
- effectiveness
- appropriateness of a screening programme
Condition - test - treatment - programme
The condition
An Important health problem
Epidemiology and natural history of the condition should be adequately understood and there should be a detectable risk factor and a latent period
Cost effective primary prevention should have been implemented
The test
Should be a simple , safe and precise validated screening test
Distribution of test values should be known and a suitable cut off agreed
The test should be acceptable
There should be an agreed policy on further management
The treatment
Should be an effective treatment with evidence of early treatment leading to better outcomes
Should be agreed policies covering who should be offered treatment
Clinical management of the condition should be optimised prior to a screening. Programme
The programme
Most be RCT evidence that programme is effective in reducing mortality and morbidity
Should be evidence that whole programme is suitable to professionals and public
Benefit from programme should outweigh the harm
Opportunity cost of programme should be economically balanced in relation to health care spending
Must be a plan for quality assurance and adequate staffing and facilities
Why do we need trials ?
bias is a major problem
- selection bias in particular
- healthy screenees
- screening detects more slowly progressing disease ( lengthy bias )
Endpoint of trials must not be survival from diagnosis
Consequences of length bias
Disease with longer sojourn time are easier to catch in the screening net
On average individuals with disease detected thru screening automatically have a better prognosis than people who present with symptoms/ signs
If we simply compare individuals who choose to be screened with those who don’t we will get a distorted picture
Basing our RCT on intention to screen we will include the full range of outcomes and be able to assess the impact of the screening
Lead time bias
Point detected by screening in the presymptomatic phase before symptoms present
Consequences :
- survival is inevitably longer following diagnosis thru screening because of extra lead time
- because of this the appropriate measure of effectiveness is deaths prevented not survival
2 examples :
- colorectal cancer - current policy is for a national screening. Programme to be rolled out from 2006
- prostrate cancer - currently policy - no screening programme
Ethics of screening
- issue of benefit and harm are at heart of screening ( what are the harms and benefits )
- screening follows a utilitarian logic
- autonomy - truly informed choice
- what are our objectives - birth prevalence / information
Screening infants and children
What is good and bad about ‘good screening’
Good
- early detection of disease means the risk of death or illness can be reduced for some people
Bad
- some people get tests , diagnosis and treatment with no benefit
- some people get ill or die despite a negative screening test
