Intro To Screening

Question 1

How can screening be defined?

Answer 1

• Public Health Service
• members of defined population who do not necessarily believe themselves to be at risk or already affected by specific condition
• asked a q/offered a test to identify individuals more likely to be helped than harmed by further tests/treatments to reduce risk of disease or its complications

Question 2

What are the criteria for the condition to be screened for?

Answer 2

• Should be important health problem
• Epidemiology and natural history of condition should be understood
• should be detectable disease marker, latent period and Asymptomatic stage
• all cost-effective primary prevention interventions should have been implemented as far as practicable

Question 3

What are the criteria for the screening test to be implemented?

Answer 3

• should be a simple safe precise and validated screening test
• distribution of test values in target population should be known and suitable cut off level defined
• test should be acceptable to the population

Question 4

What are the criteria for the treatment in the context of the screening tests?

Answer 4

• effective treatment/intervention with evidence of early treatment leading to better outcomes than late treatment
• evidence from high quality RCTs that screening programme is effective in reducing morbidity/mortality

Question 5

What are the criteria for implementation of a screening programme?

Answer 5

• Benefit should outweigh harm
• Cost economically balanced in relation to expenditure on medical care as a whole
• Adequate staffing and facilities for all stages should be available prior to commencement
• Info available explaining consequences of testing, investigation and treatment to enable informed choice

Question 6

What are the different types of screening?

Answer 6

• Mass (PKU)
• Selective (coal miners)
• Proactive (cervical smears)
• Opportunistic (blood pressure)

Question 7

Provide some examples of pre-clinical disease screening tests

Answer 7

• Blood pressure
• X-rays
• CT scans
• Mammography
• Cervical cytology

Question 8

What is sensitivity in the context of screening tests?

Answer 8

• The proportion of true positives correctly identified

- Probability that it will correctly classify people with preclinical disease as positive

Question 9

What is specificity in the context of screening tests?

Answer 9

• The proportion of true negatives correctly identified

- Probability that the test will classify people who are not diseased as negative

Question 10

How do you calculate sensitivity?

Answer 10

TP/(TP+FN)

Question 11

How do you calculate specificity?

Answer 11

TN/(TN+FP)

Question 12

Which is more important, sensitivity or specificity?

Answer 12

• Usually a trade off between the two
• Good sensitivity means few false negatives (fewer mistakes)
• Good specificity means few false positives (less people worried unnecessarily)

Question 13

In general, when should sensitivity be increased at the expense of specificity?

Answer 13

• when the disease is communicable (Ebola)
• when the disease is serious and an effective treatment is available
• when subsequent confirmatory diagnostic tests are inexpensive and low risk

Question 14

What are the positive and negative predictive values?

Answer 14

• PPV is the likelihood that people with a positive result have the disease
• NPV is the likelihood that people with a negative result do not have the disease

Question 15

How do you calculate PPV and NPV?

Answer 15

• PPV = TP/(TP+FP)

- NPV = TN/(TN+FN)

Question 16

What are the positive and negative predictive values determined by?

Answer 16

• By the sensitivity and specificity of the test but also the prevalence of preclinical disease in the target population
• The lower the prevalence the lower the PPV

Question 17

What does an increase in sensitivity result in?

Answer 17

Larger number of true positives and therefore a higher PPV

Question 18

What does a decrease in specificity result in?

Answer 18

An increase of false positives and a large decline in PPV

Question 19

The performance of a tool or measure can be assessed in terms of what two things?

Answer 19

• Validity: does it measure what it is supposed to measure?

- Reliability: does it give the same result on repeated application?

Question 20

How often should people be screened?

Answer 20

• Genetic or congenital conditions = once
• Most diseases need repeat screening
• frequency derived from false negative data

Question 21

What are the advantages of screening tests?

Answer 21

• Improved prognosis for some cases
• Less radical treatment
• Resource savings
• Reassurance for negative test results

Question 22

What are the disadvantages of screening tests?

Answer 22

• Longer morbidity for cases where prognosis is unaltered
• Over treatment of questionable abnormalities
• Resource costs
• False reassurance
• Adverse effects (false positives)
• hazards of the test

Question 23

Studies can be carried out to evaluate if the screening is effective. What are the potential biases in these evaluations?

Answer 23

• Volunteer bias
• Lead time bias (disease diagnosed earlier)
• Length bias (screening detects more slowly progressive disease)
• Overdiagnosis bias (would the lesion result in disease - cervical screening)

Question 24

What are the different study types involved in evaluation of effectiveness of screening tests?

Answer 24

• Ecological
• Case control
• Cohort
• RCTs

Question 25

What are the 5 disorders tested for as part of the newborn screening programme in England?

Answer 25

• Phenylketonuria (PKU)
• Congenital hypothyroidism (CHT)
• Sickle cell disorders (SCD)
• Cystic fibrosis (CF)
• MCAD deficiency (MCADD)

Question 26

What are the differences in the newborn screening programmes across the U.K.?

Answer 26

• England and Scotland: same 5
• Wales: 3/5 (PKU, CHT, CF) plus DMD
• NI: same 5 plus homocystinuria + tyrosinaemia

Question 27

What are the 9 standards for newborn blood spot screening?

Answer 27

1. Completeness of offer (notification of sample receipt in lab, result or decline of test for all 5 conditions for all babies in PCT)
2. Enhanced tracking ability (cards with NHS no)
3. Timely sample collection (95% of samples taken within 5-8 days of life)
4. Timely sample despatch (receipt within 4 working days of sample collection)
5. Quality of blood spot (avoidable rpt rate of less than 2%)
6. Timely receipt of rpt or 2nd blood spot (within 72hrs of request)
7. Timely processing of screen positive samples (within 4 days of sample receipt)
8. Timely identification of babies for whom child health records has not received notification of sample receipt, screening test result or decline (by 17 days of age)
9. Completeness of uptake (result for all conditions for all babies accepting test)