Cancer screening Flashcards
What key principles need to be considered when designing a screening test?
Population to screen
Frequency
Cost-effectiveness
Sensitivity and specificity
Feasibility
Population compliance
Is there an interventions for positive findings?
Type of cancer
Any potential adverse consequences.
What is meant by the sensitivity of a test?
Ability to rule out a disease
Aka high sensitivity means few false negatives.
Is positive in all actual cases of disease aka D-Dimer test.
What is meant by specificity of a test?
Ability to rule in a disease
Aka high specificity means few false positives.
Is only positive in that specific disease context aka HbA1c>48mmol for diabetes.
How effective is D-dimer as a screening test for a PE?
Highly sensitive - SNOUT sensitive test as a negative result rules OUT the PE
Not very specific - lots of other conditions can also cause a raised D-Dimer for example infection, cancer, pregnancy, VTE
How effective is HbA1c for screening for diabetes?
Nighly specific - SPIN, Specific positive rules IN disease
Not very sensitive - can be diabetic and below the threshold
However lowering the threshold would decrease the specificity of the test.
What tends to be the relationship between sensitivity and specificity as properties of screening test?
Inversely proportional
As sensitivity increases, specificity tends to decrease.
What are some ideal screening characteristics for cancers?
1.Need to have a clear intervention which can reduce mortality if cancer is diagnosed early
2.Needs to be a relatively common cancer otherwise not cost effective.
3.Feasability - have we got the staff/resources to carry out the screening.
4. Clear epidemiological factors to identify target population
5. Needs to be accepsible with minimal downsides to increase compliance
6. Needs to balance efficacy with cos effectiveness - often frequency
What are the two different types of bias with cancer screening?
Length time
Lead time
What is meant by length time bias in a screening programme?
Screening programs tend to identify more slow growing longer asymptomatic tumours, compared to fast growing tumours with a smaller asymptomatic period.
The prevalence of unidentified asymptomatic cancer is higher in the population. (note incidence may be same a fast growing)
These slow growing tumours inherently have a better prognosis and longer survival rates compared to aggressive tumours, appears the screening has better patient outcomes - this might not be the reality.
What is lead time bias in the context of a screening test?
When screening test enables an earlier diagnosis of a condition compared to when patients would present/be diagnosed symptomatically.
By screening and diagnosing patients closer to the biological onset of disease their perceived survival time may increase, despite them not actually living any longer.
What are the three national screening programes in the UK?
Breast cancer
Bowel cancer
Cervical cancer
What population is offered breast cancer screening?
Offered to women (trans men also automatically invited, trans women taking hormonal treatment for >2yrs can qualify may need to call GP), aged between 50-70 yrs in the UK.
First invite between 50-53yrs, then every three years until turn 71yrs.
What cancer is the most common cancer in women?
Breast cancer
Why are younger women not screened for breast cancer?
Over 80% of breast cancers occur in women over 50
Not an effective use of resources to screen under 50s
Younger women have denser breast tissue - x-ray images less clear and harder to identify so more false positives/negatives.
How successful is breast cancer screening?
20% relative reduction in breast cancer mortality
AKA one death averted for every 200 women who attend screening, prevents 1300 deaths per year.
