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Flashcards in Screening Deck (19)
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1
Q

What is screening?

A

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

2
Q

When is screening carried out?

A

Where the detection of disease at an early stage leads to improved prognosis

3
Q

What else can screening be used for?

A

Risk factors and identifying people with infectious disease

4
Q

What are limitations of screening?

A

May do more harm than good- false positives, inducing anxiety, treatment of early disease which may not have been a problem

5
Q

What is the validity of a test?

A

Its ability to distinguish between subjects with the condition and those without

6
Q

What is sensitivity?

A

a / (a+c) (See notes)

7
Q

What is specificity?

A

d/ (b+d)

8
Q

What is the positive predictive value?

A

Likelihood that a patient with a positive test will actually have the disease: a/ (a+b)

9
Q

What is the negative predictive value?

A

Likelihood that a patient with a negative test will not have disease: d/ (c+d)

10
Q

What is the predictive value dependent on?

A

Sensitivity, specificity, prevalence

11
Q

How are cut off values determined?

A

Receive operator characteristics curves. ROC curve is a graphical display of the how the proportions of true positives and false positives change for each of the possible pre-determined values. The choice of cut-off value for a test is informed by the attempt to maximize sensitivity and specificity.

12
Q

How does predictive value vary with prevalence?

A

In low prevalence population, often the case for screening programmes, a test with a “good” specificity (99%) will still lead to a low positive predictive value. Good and prompt confirmatory tests / follow-up are required in this situation to reduce unnecessary treatment or anxiety

13
Q

What are the WHO criteria for evaluating screening programmes?

A

Feasibility, effectiveness and cost

14
Q

What are 3 types of bias that affect screening?

A

Selection bias, lead time bias- screening identifies disease that would be identified later on- resulting in apparent improvement in length of survival

Length bias exists as some conditions may be slower in developing to a health threatening stage, that is, they have a longer preclinical stage. This means they are more likely to be detected at that stage but they may also have a more favourable prognosis leading to the false conclusion that screening is beneficial in lengthening the lives of those found positive.

Length bias: periodic screening is more likely to identify less aggressive cancers- cases identified likely to have better prognosis than those who present with symptoms

15
Q

What are the current systematic screening programmes in the UK- antenatal?

A

Chromosome abnormalities, syphilis, HIV, Hep B, sickle cell disease and thalassemia, physical abnormalities

16
Q

What are the current systematic screening programmes in the UK- newborns?

A

Physical exam, hearing test and blood spot

17
Q

What are the current systematic screening programmes in the UK- adult cancer?

A

Breast (f50-70), cervical (f25-64) and bowel (60-74)

18
Q

Why are other screening programmes in UK?

A

Abdominal aortic aneurysm for men over 65 and diabetic retinopathy for diabetics over 12, colorectal cancer

19
Q

Name some additional programmes not labelled under screening

A

NHS health checks, prostate cancer and chlamydia screening for people under 25, opportunistic