Session 6 Screening

• Disease Detection - three methods:

  • Spontaneous presentation
  • Opportunistic case finding
  • Screening
• Systematic attempt to distinguish between apparently well persons who are considered likely to have the disease in question, from those considered not likely to - screening
• Advantages of Screening?

  • Early detection and treatment
  • Prevention and Risk Reduction
  • Public health benefits
  • health education and awareness
  • Cost-effective
• Disadvantages of screening?

  • False positives and negatives
  • Overdiagnosing and overtreatment
  • Ethical considerations
  • Resource allocation
  • Logistical challenges
• NHSPopulation Screening Programmes :Screening for Adults

  1. Diabetic eye screening (from age 12 and above annually)
  2. Cervical cancer
  3. Breast cancer
  4. Bowel cancer
  5. Abdominal Aortic Aneurysm (AAA)
• • Breast cancer screening is offered to women aged 50 to 70 -Women aged 70 and over can self-refer.
  • Cervical screening is offered to women/people aged 25-64 - Every 3 years between the ages of 25-49 years, and - Every 5 years between the ages of 50-64 years.
  • NHS bowel cancer screening is offered to people aged 50 or over - 50 to 74, you'll automatically be invited to do a home testing kit every 2 years (FIT test) - Over 75, you can ask for a kit every 2 years by calling.
  • AAA - all men at 65 offered a single ultrasound screening scan.
• Screening in pregnancy?

  • Infectious diseases (hepatitis B, HIV and syphilis)
  • Fetal Anomaly screening (Down's syndrome T23, Patau's syndrome T13 and Edwards' syndrome T18)
  • Sickle cell disease and thalassaemia
  • 20-week scan to check the physical development of the baby
  • Diabetic eye screening if you are pregnant and have type 1 or type 2 diabetes
• Screening for newborn babies?

  • Physical examination, which includes the eyes, heart, hips and testes.
  • Hearing test
  • Blood spot test to check if the baby has any of 9 rare conditions (e.g Sickle cell disease, Cystic fibrosis, Congenital hypothyroidism, phenylketonuria, medium-chain acyl-CoA dehydrogenase deficiency, maple syrup urine disease, isovaleric acidaemia, glutaric type 1, homocystinuria).
• Criteria for screening programmes: Wilson & Jungner criteria

  • Disease/condition
  • Test
  • Treatment
  • Programme
  • Implementation criteria
• • Test validity represents the extent to which a test measures what it intends to measure.
  • Can be thought of as the accuracy of the test.
  • Measured by sensitivity and specificity.
  • Determined by comparing a new test with the gold standard test.
• Sensitivity: The ability of a test to correctly identify an individual as having a disease. These are individuals who actually have the condition (e.g., a disease) and test positive for it.
• Sensitivity= true positives / (true positives + false negatives)
• Specificity: The ability of a test to correctly classify an individual as ‘disease-free’.
• Specificity= true negatives / (true negatives + false positives).
• a highly specific test can help to rule in a disease when positive.

  A highly sensitive test can help to rule out a disease when negative.
• Positive predictive value (PPV): The proportion of test-positive individuals who actually have the disease. A measure of how accurate a positive test result is likely to be.
• PPV =truepositives/ (true positives + false positives)
• Negative predictive value (NPV): The proportion test-negative individuals who reallydo not have the disease. A measure of how accurate a negative test result is likely to be.
• NPV =true negatives / (true negatives + false negatives)
• Where earlydiagnosis falsely appears to prolong survival: lead time bias.
• Screening programmes better at picking up slow growing, unthreatening cases than aggressive, fast growing ones : Length time bias
• Studies of screening often skewed by ‘healthy volunteer’ effect : selection bias
• Evaluation difficulties?

  • Lead time bias
  • Length time bias
  • Selection bias
  • Morbidity vs mortality
  • Population diversity and equal access.