screening revision! by ilona blee. what are some uk screening programmes? antenatal & newborn...
TRANSCRIPT
Screening revision!By Ilona Blee
What are some UK Screening programmes?
Antenatal & newborn screening
Newborn Blood Spot
Newborn Hearing Screening
Chlamydia screening
Screening for bowel cancer – fecal occult blood
Abdominal aortic aneurysm
Breast cancer
Have an idea of what these conditions are screening for!
What are pregnant women screened for?
High blood pressure
Diabetes mellitus
Pre-eclampsia
Infectious diseases like hepatitis B, HIV, syphilis & rubella
What conditions does the newborn blood spot screen for?
Sickle cell disease
Cystic fibrosis
Congenital hypothyroidism
Inherited metabolic diseases maple syrup urine disease or phenylketonuria
http://newbornbloodspot.screening.nhs.uk/professionals
What is the name of the criteria for a screening programme?
Wilson & Jugner criteria
What is the purpose of the Wilson & Jugner criteria?
To assess the effectiveness and appropriateness of screening programmes
What are the Wilson & Jugner criteria?
WILSON & JUGNER CRITERIA
1. Condition should be an important health problem
2. Condition should have an existing treatment for it
3. Condition should have a latent stage
4. The natural history of the condition should be known and understood
5. A test for the condition/disease should exist
6. The test available should be accepted by the population
7. Test should be cost effective/financially viable
8. Facilities for diagnosis & treatment should be available
9. Case-finding should be a continuous process, not just a one-off
10. Agreed upon policy on whom to treat
Classifying Test Results
What is sensitivity?
The proportion of people who have the disease (true positive) that the test correctly detects
Mathematically = true positive / (true positive + false negative)
So if you have the disease, sensitivity tells you the probability of the test picking up the disease
What is specificity?
The proportion of people who do not have the disease (true negatives) that the test correctly identifies as not having the disease
Mathematically = true negatives / (true negatives + false positives)
Sensitivity & specificity measure test performance
Positive predictive value The probability that a person has the disease (true positive) given that that
have had a positive test result (all the positives)
Mathematically = true positives / (true positives + false positives)
With lower disease prevalence, there will be a lower positive predictive value
Define prevalence
The number of people in a defined population with a disease at a given time
What are TWO types of prevalence?
1. Point
2. Period
Negative predictive value
The probability that a person does not have the disease (true negative) given that they have a negative test result (all the negatives)
Mathematically = true negatives / (true negatives + false negatives)
Positive & negative predictive value tell us about disease prevalence and test performance
Disease Positive
DiseasedNegative
Test positiv
eTrue Positive False Positive
Positive Predictive Value
TP / (TP + FP)
Test negati
veFalse Negative True Negative
Negative Predictive Value
TN / (TN + FN)
Sensitivity
TP / (TP + FN)
Specificity
TN / (TN + FP)
List the FOUR types of screening bias
1. Healthy screenee
2. Length time effect
3. Lead time effect
4. Overdiagnosis
The healthy screenee – what does this mean?
Patients who are active about looking after themselves and their health; these people are more likely to attend screening programmes and are less likely to have a positive result. People who don’t attend are more likely to smoke, drink and have
low income
Patients like this could be described to have an “internal locus of control”
What THREE things do people with an internal locus of control believe?
1. That they are responsible for their own health
2. Illness can be avoided by good health behaviours
3. Ill health is due to poor health behaviour
Length time effect – what is this?
Screening is better at detecting disease that develops more slowly and so the prognosis is better for these conditions.
Therefore, because the slower conditions have a “better” prognosis, it looks like screening helps with their outcome
In reality, screening just isn’t catching the fast forming conditions that have a poor prognosis and result in death
Lead time effect – what is this?
Screening is able to detect a disease earlier and can make it look like, as a result of the screening the survival time has been increased ie. The prognosis looks like it is longer but it actually doesn’t have an impact on the outcome
For example we have two people with cancer. The first one is screened and given a 10 year prognosis.
The second isn’t screened, goes 5 years with no symptoms, and is then given a prognosis time of 5 years. The person who was screened looks like they have a better prognosis.SCREENING SYMPTOMS
Lead Time
Overdiagnosis – what does this mean?
Patients are diagnosed with a condition that isn’t going to cause symptoms in the patients lifetime.
E.g. prostate cancer. Become more diagnosed following prostate specific antigen (PSA) screening
Ended up just making patients anxious and shouldn’t be done in patients with a limited life expectancy
Thank-you!
Any questions?
Good luck with your revision