Session 6 - Screening Flashcards
Define screening
-Systematic attempt to detect an unrecognised condition which can be done rapidly to distinguish those who are likely to have disease from those who haven’t
Does a positive screening test mean you have the disease?
-No, it means you are high risk and diagnostic tests must be performed.
With reference to test validity, what is sensitivity?
-Proportion of people with the disease who test positive (how good the test is at getting a positive result from those who have the disease)
With reference to test validity, what is specificity?
-The proportion of people who do not have the disease which test negative (how good the test is at getting a negative result if you do not have disease)
How do you calculate sensitivity of a screening test?
-Disease present +ve/ (disease present +ve and -ve)
How do you calculate the specificity of a screening test?
-Disease free -ve /(disease free +ve and -ve)
What is the positive predicted value? How do you work this out?
- How likely a person is to have the disease when they have tested positive
- Disease +ve /(disease +ve + disease free +ve)
What is the negative predicted value? How do you work this out?
- How likely a person is to be disease free when they have tested negative
- disease free -ve/(disease free -ve + diseased -ve)
What 3 groups of criteria are there when referring to screening criteria?
- The disease
- The test
- The treatment
Outline the screening criteria for the disease aspect
- Must be an important health problem
- Epidemiology and natural history must be well understood
- Must have an early detectable stage
Outline the screening criteria for the test aspect
- Simple and safe
- Precise and valid with an agreed cut off
- Acceptable to population being screened
- Agreed policy on who to investigate further
Outline the screening criteria for the treatment aspect
- Effective evidence-based tx available
- Early treatment is advantageous
Which aspect of test validity does prevalence effect and how?
- PPV
- The higher the prevalence the higher and more accurate the PPV
State 2 advantages of screening
- Reduces number of deaths from a certain condition
- Earlier detection of diesease prevents morbidity
What are the disadvantages of screening?
- Many people have to be screeded to sace 1 life -> costly, invasive and causes unneccesary worry and anxiety
- Some people detected by screening die anyway
- People diagnosed with disease without harm or symptoms may be subjected to reduced QoL due to checkups
- Refers well people for investigation -> false positives
- Fails to refer people who have the disease -> false negatives -> ess likely to present if symptoms occur as been told low risk
- Overdiagnosis of diseases which wouldnt have caused harm
Explain how screening alters the dr-pt contract
- Normally patients self present asking for help
- Screening targets apparently health people who have not sought help and offers help for something they havent thought about
Describe some limitations of screening
- Cannot guarantee protection
- Always false +ves and -ves
- Need for informed choice on whether to have screening or not
- Always unneccessary investigations
What is lead time bias?
- Early diagnosis falsely appears to prolong survival as screened patients appear to survive longer but were only diagnosed earlier
- Patients live the same length of time but a longer period knowing they have the disease
What is length time bias?
- Screening programmes are better at detecting slow growing, unthreatening cases vs fast agressive cases
- Therefore diseases detected by screening are already more likely to have a favourable prognosis
What is selection bias?
-Skewed by ‘healthy volunteer’ effect. Those who attend screening are more likely to be doing other things to protect themselves from disease
Describe some structural sociological critiques of screening
- Victim blaming -> individuals encouraged to take responsibility for own health - can everyone do this?
- Individualising pathology -> What about addressing underlying material causes of disease
Describe some surveillance sociological critiques of screening
-Individuals and populations increasingly subject to surveillance
Describe a social constructionist critique to screening
-Health and illness practices can be seen as moral - given meaning through particular social relationships
Describe a feminist critique of screening?
-Is screening aimed more at women than men?
Outline the aspects of the cervical screening programme
- Aims to decrease the number of invasive cervical cancers and deaths
- Women aged 25-64 years -> 25-49 3 yearly: 50-64-> 5 yearly; 65+ only those who had recent abnormalities
- under 25s not screened due to lots of false negatives as invasive cervical cancers rare under 25
- normal results -> continue screening
- Abnormal result -> analysed -> either no tx or laser ablation LLETZ
- Incorporates HPV test -> +ve referred to colposcopy
