Screening

Question 1

what is screening

Answer 1

the practice of investigating apparently healthy individuals with the object of detecting unrecognised disease or its precursors in order that measures can be taken to prevent or delay the development of disease or improve prognosis

Question 2

what is the purpose of screening

Answer 2

When early diagnosis of disease allows early intervention which improves prognosis. When identification of high risk individuals can help target interventions to improve prognosis. When it can identify those posing a risk to others an reduce that risk

Question 3

what is a screening test

Answer 3

identifies those most likely to have the condition. excludes those least likely to have it. not the same as a diagnostic test (usually)

Question 4

what is sensitivity

Answer 4

ability to pick up true positives. it = a/a+c

Question 5

what is specificity

Answer 5

ability to exclude true negatives. it = d/b+d

Question 6

what is predictive value

Answer 6

proportion of test result that are correct

Question 7

equation for positive predictive value

Answer 7

a/a+b

Question 8

equation for negative predictive value

Answer 8

d/c+d

Question 9

challenges for screening tests

Answer 9

Generally screening low prevalence populations. Therefore need a very high specificity. This often means a lower sensitivity (ie what is the cut off for saying a test is positive or negative)

Question 10

what is the appeal of screening

Answer 10

people would prefer not to get ill. widespread belief that early detection is better.

Question 11

what are some current systematic screening programmes in the UK

Answer 11

antenatal - downs, Edwards syndromes. new born - hearing test, blood spot. adult cancer - breast and cervical

Question 12

evaluation of screening programmes

Answer 12

feasibility. effectiveness. cost. ethics of screening

Question 13

feasiblity

Answer 13

will the population attend. is the test acceptable. are facilities and resources available for screening, diagnostic follow up and treatment

Question 14

effectiveness

Answer 14

Extent to which implementing a screening programme affects subsequent outcomes. Difficult to measure due to bias

Question 15

what should an ideal screening test be

Answer 15

simple,safe acceptable, inexpendive

Question 16

Ethics of screening

Answer 16

May harm as well as benefit the individual. Risks of the test. Risks of subsequent diagnostic tests. Risks of subsequent treatment. False positive result causes unnecessary anxiety. False negative result will give false reassurance

Question 17

what is the validity of a test

Answer 17

is its ability to distinguish between subjects with the condition and those without. to asses the validity of a screening test the true disease status of the individuals must be known, usually through a definitive test which is referred to as the gold standard

Question 18

what are the receiver operator characteristics (ROC) curves used for

Answer 18

to determine a cut-off value for a diagnostic or screening test. Y axis- sensitivity vs 1-specificty (x-asis)
Choice of a particular cut-off value is informed by the attempt to maximise sensitivity and specificity

Question 19

what are the receiver operator characteristics (ROC) curves used for

Answer 19

to determine a cut-off value for a diagnostic or screening test. Y axis- sensitivity vs 1-specificty (x-asis)
Choice of a particular cut-off value is informed by the attempt to maximise sensitivity and specificity

Question 20

what are the approaches to screening

Answer 20

whole population (mass) or selected groups who are anticipated to have an increased prevalence of the condition (targeted). can be a systematic programme (people are called for screening) or opportunistic (people present to doctor for some other reason and are offered test)

Question 21

what are the different types of bias that influence effectiveness of screening

Answer 21

selection bias, lead time bias and length bias

Question 22

what is selection bias

Answer 22

people who participate in screening programmes often dffer from those who do not.

Question 23

what is lead time bias

Answer 23

screening identifies disease that would otherwise be identified at a later stage. Improvement in survival due to screening which is really due to the earlier date of diagnosis.

Question 24

what is length bias

Answer 24

As some conditions may be slower in developing to a health threatening stage, they no longer have a preclinical stage. More likely to be detected at a stage that has a more favourable prognosis leading to the false conclusion that screening is beneficial in lengthening lives of those found positive.

Question 25

cost

Answer 25

Limited health care resources therefore cost-effectiveness if screening should be considered. This includes further diagnostic tests & subsequent cost of treatment. On the other hand, the absence of screening would mean that patients incur costs due to treatment to more advanced stages of disease.

Question 26

criteria for screening (disease)

Answer 26

important health problem, well recognised and detectable pre clinical stage, natural history of disease understood, long period between first signs and overt disease

Question 27

criteria for screening (diagnostic test)

Answer 27

valid (sensitive and specific), simple and cheap, safe and acceptable, reliable

Question 28

criteria for screening (diagnosis and treatment)

Answer 28

facilities are adequate, effective, acceptable and safe treatment available, cost effective and sustainable