Screening PHIL THIRKELL

What is screening?  A process of identifying apparently healthy people who may be at risk of a disease or condition  Identify  Apparently healthy  Increased risk of a disease/condition

Give 4 screening programmes undertaken in the UK.  Antenatal screening  Postnatal screening – hearing, heel prick, neuroblastoma  Cervical smear  Mammography  Chlamydia screening  Bowel Cancer – FOBT  Prostate cancer  Abdominal Aortic Aneurysm  Depression – PHQ-9 questionnaire  etc.

Criteria for a Screening Programme  Wilson + Jungner criteria 1. Important health problem 2. Treatment available 3. Facilities available for diagnosis and treatment 4. Latent stage of the condition 5. Test available to detect the condition 6. Test is acceptable to the population 7. Natural history of the disease is known 8. Policy of who gets treatment has been made 9. Financially viable 10. Case-finding is a continual process, not just a one off

Neonatal screening  Which conditions are screened for with blood spot testing?  Phenylketonuria  Sickle cell disease  Cystic fibrosis  Congenital hypothyroidism  Medium-chain acyl-CoA dehydrogenase deficiency

Antenatal Screening  What is a pregnant woman screened for?  Pre-eclampsia  Rhesus antigen status / blood group  Anaemia  Diabetes  Syphilis  Hepatitis B/C  HIV

Anomaly Scan – USS between weeks  What is an anomaly scan used for?  Spina bifida  Down’s syndrome  Hydrocephalus  Cleft lip/palate  Date the pregnancy  Sex of the baby  Multiple pregnancy  Organ development  Abdominal wall

Test Outcomes DiseasedNon-Diseased Test positive True PositiveFalse Positive Test negative False NegativeTrue Negative

Sensitivity  The number of people who have the disease who get a positive test result  True positive / (True positive + False Negative)  e.g. 50 people with known Rheumatoid Arthritis. RhF blood test is positive in 42 of the patients.  Sensitivity is 84%

Specificity  The number of people who don’t have a disease who are correctly told they don’t have it  True negatives / (True negatives + False positives)  E.g. 30 patients with no evidence of rheumatoid arthritis have a blood test for RhF. 2 patients have a positive result.  Specificity = 93%

Positive Predictive Value  The number of people who have a positive test result who actually do have the disease  True positives / (True positives/False positives)  e.g PSA blood tests performed on men >65yr. 800 are raised above normal levels. Biopsy reveals that 95 of these have prostate cancer.  PPV = 95/(95+800) = 11%

Negative Predictive Value  The number of people who have a negative test result who definitely don’t have the disease  True negatives / (true negatives + false negatives)  e.g PSA blood tests on men >65yrs have normal PSA results. 20 of these turn out to currently have prostate cancer despite a normal PSA.  1680/ ( ) = 98.8%

Diseased Non- Diseased 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)

Screening Bias  Healthy screenee  Length time  Lead time  Overdiagosis

Healthy screenee  Proactive patients who turn up to screening opportunities take better care of themselves are less likely to have a positive result  Less likely to smoke, drink too much, have low income  More likely to exercise, eat healthily, attend healthcare at other times  Internal locus of control

Length time  Screening appears to improve prognosis because slow-forming conditions are detected and treated earlier than they would compared to waiting for symptoms to start  e.g. 500 slow forming and 500 fast forming cancers happen each year  Slow forming – no symptoms and better prognosis  Fast forming – obvious symptoms and poor prognosis  Screening can detect lots of slow forming, but not many fast cancers  Because slow has better prognosis, it appears that screening helps outcome, but actually just selects a high proportion of slow cancers

Lead time  A screening test diagnoses something earlier but has no impact on outcome  Appears to increase survival time, but doesn’t Screening detects a disease Symptoms start Death Screened patients Non-screened patients Lead time

Overdiagnosis  Patients are diagnosed with a condition which isn’t going to affect their life expectance  e.g. prostate cancer diagnosis in old men  Get a PSA blood test done, high result but managing with symptoms ok  Now told they have cancer – anxiety, health insurance etc.

A new blood test is developed for rheumatoid arthritis. What is the sensitivity, specificity, PPV and NPV? DiseasedNon-Diseased New test positive New test negative 3150 Sensitivity = 250 / (250+3) = 98.8 % Specificity = 150 / (150+26) = 85.2 % PPV = 250 / (250+26) = 90.5 % NPV =150 / ( ) = 98 %