Prevention: Screening

Question 1

What is screening?

Answer 1

A key preventative strategy that enables early detection and treatment of conditions.

One high risk cases are ID’s through screening, diagnosis can confirm the outcome.

Ideally it is inexpensive and easily administered. It must correctly ID the person at risk and MUST be evidence based

Question 2

Name some methods of screening?

Answer 2

Xray
Bloods
Physical Exam
Biopsy

Question 3

Name some approaches you might take to screening?

Answer 3

High Risk - targeted
Population Approach - broad

Systematic - all women aged 50-65 in every pop
Oppurtunistic - bp reading at GP visit

Question 4

Name the first 4 WHO 1968 criteria for screening programmes/appropriateness of implementing screening programmes?

Answer 4

1. Availability of effective and reliable screening method
2. Availabiloty of effective intervention to improve outcome
3. Safety and acceptability of the test to the individ.
4. BEnefits of screening need to outweigh harm

Question 5

Screening also needs to be operationally feasible. What does operationally feasible mean in relation to implementing screening programmes?

Give an example.

Answer 5

1. Requires systems to ID and contact patients eligible for screening and to ensure high compliance
2. Requires clinics for extensive diagnositic tests and treatments if needed

Example. Increase in mammograms leads to an increase in demand for biopsies. OR increase in detection of high cholesterol leads to an increase in demand for cholesterol lower drugs etc.

Question 6

How do we determine the validity of a tool?

Whats the ideal trend in validity of a tool?

Answer 6

1. Specificity
2. Sensitivity

Ideally - High specificity, High senstivity

Question 7

What is specificity?

Give the formula.

When would having a high specificity be preferred?

Answer 7

Proportion of people who do not have the outcome crrctly ID’d

TRUE NEGATIVE

No. of true neg / no. without outcome

When subsequent confirmation of an outcome is expensive or invasive or the results may be stigmatising (prefer to avoid carrying out on false positives).

Question 8

What is sensitivity?

Give the formula

When would having high sensitivity be preferred?

Answer 8

Proportion of people correctly ID’d to have the outcome

TRUE POSITIVE

No of true pos / no with the outcome

In situations like infectios disease outbreaks where you need to ID true positives and avoid false negatives that would aid the spread of disease.

Question 9

How do we evaluate a screening method?

Answer 9

Compate the distribution of screening results to the true outcome status and use this to calculate S and S.

Question 10

What are predictive values useful for?

What are they calculated from?

Answer 10

Calculated from S and S

Useful for an individual to know how likely they are of having or not having an outcome if they tested positive or negative

AND

Also determine the prevelance of the outcome being screenes

Question 11

Define PPV?

Whats the formula?

Answer 11

PPV = the likelihood of having the outcome based on test result

No. of true positives /no who tested positive

Question 12

Define NPV?

Whats the formula?

Answer 12

Likelihood of not having the outcome based on the test result

No of true negatives / no who tested negative

Question 13

In addition to validity, what other ethical considerations need to be looked at when screening?

Answer 13

1. Individual benefit needs to be balanced with cost to society
2. PH benefit needs to be balanced with cost to the individual.
3. Enough information needs to be provided to the individual to make informed consent
4. False negs result in loss of confidence in HC and loss of life.
5. False pos results in undue anxiety and mental stress/stigmatization.

Question 14

What are some limitations to measure effectiveness of screening?

Answer 14

1. Selection Bias - only specific people are selected to come to screening, or only health centered people who accept the invitiation.

Example: more women will be screened for breast cancer than men. OR women who are low risk for cevical cancer and more likely to attend screening.

2. Lead Time Bias - If an outcome is ID early enough, but has no effect on the outcome.
3. Length- time Bias - for disease with slower progression - as above.
4. Can lead to over-diagnosis (where people with early or subclinical disease may have died from other outcomes).

Question 15

Whats the best method for evaluating a screening programme?

Why?

Answer 15

an RCT.

Most of these biases can be removed by random allocation. Some ethical considerations here would be limitations to evaluating via RCT