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 

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 Diseased Negative Test positive True PositiveFalse Positive Positive Predictive Value TP / (TP + FP) Test negative False NegativeTrue 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