Screening

Question 1

What is screening?

Answer 1

testing of apparently healthy individuals to detect unrecognised disease/its precursors so measures can be taken to prevent or delay the development of disease or improve prognosis

Question 2

When is screening carried out?

Answer 2

when detection of disease at early stage leads to improved prognosis
- breast cancer (allows treatment - surgery, chemotherapy) to reduce mortality

also used for risk factors (to identify those at increased risk that can take measures to reduce this)
- screening for high BP/cholesterol to reduce CVD risk

used to identify people with infectious disease where treatment will improve outcome (chlamydia screening) or prevent transmission (screening food handlers for salmonella or health workers for hep B)

Question 3

Limitations of screening?

Answer 3

• more harm than good via false alarms, treating early disease that would not have been problem, anxiety
• benefits don’t always outweigh risks
• wasted funds

Question 4

What is the validity of a test?

Answer 4

ability to distinguish between subjects with the condition and those without

DESCRIBED BY SPECIFICITY AND SENSITIVITY

Question 5

How can validity of screening test be assessed?

Answer 5

need to know true disease status of individuals, through definitive testing known as gold standard

Question 6

What is sensitivity?

Answer 6

ability to correctly identify those with disease

a/a+c

Question 7

What is specificity?

Answer 7

to correctly identify those without disease

b/b+d

Question 8

What is positive predictive value?

Answer 8

likelihood that patient with positive test result has disease

a/a+b

Question 9

What is negative predictive value?

Answer 9

likelihood that patient with negative results doesnt have disease
d/c+d

Question 10

What does predictive value depend on?

Answer 10

sensitivity
specificity
prevalence of condition in population

Question 11

What determines cut-off value for a diagnostic or screening test?

Answer 11

Receiver Operator Characteristics curves (ROC)

• first proportion of true positive/false negatives determined (sensitivity and specificity)
• ROC curve displays how proportions of true positives and false positives change for each pre-determined value and cut off is determined by attempting to maximise sensitivity/specificity

Question 12

How does predictive value value vary with prevalence?

Answer 12

low prevalence –> test with high specificity lead to low PPV

prompt confirmatory tests/follow up needed to reduce anxiety/unnecessary treatment

Question 13

How is screening targeted?

Answer 13

whole population (mass)
selected group anticipated to have increased prevalence (targeted)

may be a systematic programme where people called for screening or opportunistic programme where person offered it by doctor

Question 14

Antenatal screening?

Answer 14

offered to all pregnant women/based on risk assessments

HIV, HEP P, rubella, foetal growth, chromosomal abnormalities

Question 15

Neonatal and childhood screening?

Answer 15

Screen for phenylketonuria, hypothyroidism, haemoglobinopathies, SCD
For congenital hip dislocation
Routine checks for hearing/development

Question 16

Cancer screening?

Answer 16

Systematic programme for women breast/cervical

Bowel cancer for men/women 60-69

Question 17

Infection screening?

Answer 17

• new national opportunistic chlamydia screening in young people under 25
• sexual health service attendees offered HIV screening
• all health workers screened for hep B

Question 18

For CVD?

Answer 18

• abdominal aortic aneurysm screening for men aged 65
• diabetic retinopathy screening for aged 12+ with diabetes
• targeted/opportunistic screening for BP, high cholesterol and diabetes in primary care

Question 19

What 3 issues must be evaluated when evaluating potential screening programme?

Answer 19

1. Feasibility - how easy to organise population for screening, if acceptable test, facilities to carry out diagnostic test post screening
2. Effectiveness - how much does test influence subsequent outcomes? But there is bias.
   - selection bias of those who participate in screening vs those who do not
   - lead time bias
   - length bias
3. Cost
   - resources limited for healthcare and competing demands for funds
   - consider cost effectiveness compared with other health care forms including subsequent diagnostic tests/treatment

Question 20

What is lead time bias?

Answer 20

screening identifies disease that would otherwise be identified at later stage showing apparent improvement in length of survival but really due to earlier diagnosis date

Question 21

What is length bias?

Answer 21

some conditions slower to develop to life threatening stage so more likely to be detected at that stage with more favourable prognosis leading to false conclusion that screening is beneficial in lengthening life

Question 22

Why would cost still increase without screening?

Answer 22

treating patients in more advanced stages of disease

Question 23

Ethics of screening?

Answer 23

• may do harm and benefit
• risk of test, diagnostic test after
• false positive cause anxiety
• other unplanned effects of positive test
• false negative give false reassurance

Question 24

Ethics of screening?

Answer 24

• may do harm and benefit
• risk of test, diagnostic test after
• false positive cause anxiety
• ## other unplanned effects of positive test