12 Screening

Question 1

Q: What is screening? Different to?

Answer 1

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

Further diagnostic tests are then done to confirm diagnosis

A screening test is not the same as a diagnostic test.

Question 2

Q: When is screening not generally indicated?

Answer 2

A: If earlier detection does not offer any hope of improved outcome then screening is generally not indicated ( e.g. earlier detection of breast cancer allows treatment (surgery, radiotherapy and chemotherapy) that can reduce mortality (leading to increased survival)

Question 3

Q: What are the limitations of screening? (3)

Answer 3

A:  By definition screening tests are carried out on apparently healthy individuals and it is always possible that screening may, inadvertently, do more harm than good.
 This could include false alarms, inducing anxiety, and the treatment of early disease which would not otherwise have become a problem.
 When considering population screening programmes the benefits and harms must be carefully assessed, and the benefits should always outweigh the harms.

Question 4

Q: An ideal screening test should be? (6)

Answer 4

A: simple, safe, acceptable, inexpensive, repeatable and valid (sensitive and specific).

Question 5

Q: What is the validity of any test? How is it described? Additional parameter?

Answer 5

A: its ability to distinguish between subjects with the condition and those without

Validity is described in terms of sensitivity and specificity of the test

An additional test parameter is the predictive value. This is particularly useful in clinical practice

Question 6

Q: What is the gold standard

Answer 6

A:a definitive test

Question 7

Q: Describe the sensitivity of a screening test.

Answer 7

A: The sensitivity is the ability of the test to correctly identify people with the disease

sensitivity = a ÷ (a+c)

a= disease status + and test result +
c= disease status + and test result -

Question 8

Q: Describe the specificity of a screening test.

Answer 8

A: The specificity is the ability of the test to correctly identify people without the disease

specificity = d ÷ (b+d)

d= disease status - and test result -
b= disease status - and test result +

Question 9

Q: What is the the positive predictive value (PPV)? is

Answer 9

A: the likelihood that a patient with a positive test result will actually have the disease

positive predictive value = a ÷ (a+b)

a= disease status + and test result +
b= disease status - and test result +

Question 10

Q: What is the negative predictive value (NPV)?

Answer 10

A: the likelihood that a patient with a negative test result will not have the disease

negative predictive value = d ÷ (c+d)

d= disease status - and test result -
c= disease status + and test result -

Question 11

Q: What is the predictive value of a test dependent on?

Answer 11

A: – sensitivity
– specificity
– prevalence of the condition in the population

Question 12

Q: What are Receiver Operator Characteristics (ROC) curves? Used to? How?

Answer 12

A: graphical display of the how the proportions of true positives and false positives change for each of the possible pre-determined values

used to determine a cut-off value for a diagnostic or screening test (continuous variable)- proportion of true-positives and false-positives are calculated for possible values

The choice of cut-off value for a test is informed by the attempt to maximize sensitivity and specificity

Generally, there is a trade-off between sensitivity and specificity, and the decision must be based on their relative importance

Question 13

Q: What can occur in very low prevalence population with screening programmes? What’s needed in those situations?

Answer 13

A: 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

Question 14

Q: Screening can either involve? (2) Programmes and exmaples? (2)

Answer 14

A: the whole population (mass), or selected groups who are anticipated to have an increased prevalence of the condition (targeted)

In either of these there may be a systematic programme where people are called for screening (e.g. cervical cancer, breast cancer) or an opportunistic programme when a person presents to the doctor for some other reason and they are offered a test (e.g. Chlamydia screening in young people, blood pressure screening in older people)

Question 15

Q: List 5 major screening programme types in the UK?

Answer 15

A: Antenatal screening
Neonatal and childhood
Cancers 
Infections 
Cardiovascular disease

Question 16

Q: List 6 examples antenatal screening. Offered to?

Answer 16

A: – Syphilis, HIV, hepatitis B, rubella, chromosome abnormalities, foetal growth
– Some of these are offered to all pregnant women, others are based on risk assessments

Question 17

Q: What are newborn babies screened for? (4) Checked for? Later?

Answer 17

A: – phenylketonuria, hypothyroidism, haemoglobinopathies and sickle cell disease (in some geographical areas where these conditions are more common).
– Babies are also checked for congenital hip dislocation. Routine checks in later childhood screen for problems with hearing and development.

Question 18

Q: Give 3 examples of cancer programmes.

Answer 18

A: – There are systematic programmes for breast cancer and cervical cancer in women.
– A screening programme for bowel cancer for all men and women aged 60 – 69.

Question 19

Q: What are people attending sexual health services offered? What screening is mandatory for health care workers? What’s a new national opportunistic screening programme rolled out across the country?

Answer 19

A: – screening for HIV
– Hepatitis B
– chlamdyia in young people (under 25)

Question 20

Q: Give 2 examples of CVD screening. What type of screening is carried out in primary care?

Answer 20

A: – Abdominal aortic aneurysm screening for men aged 65.
– Diabetic retinopathy screening for people aged 12 and over with diabetes.
– Targeted and opportunistic screening is carried out for blood pressure, high cholesterol, diabetes in primary care.

Question 21

Q: Even after a disease is determined to be appropriate for screening and a valid test becomes available, it does not necessarily follow that a widespread screening programme should be implemented. Evaluation of a potential screening programme involves which 3 issues?

Answer 21

A: 1)Feasibility

2) Effectiveness
3) Cost

Question 22

Q: Describe feasibility when evaluating potential screening programmes. Depend on?

Answer 22

A: Feasibility will depend on how easy it is to organise the population to attend for screening, whether the screening test is acceptable, whether facilities and resources exist to carry out the necessary diagnostic tests following screening.

Question 23

Q: Describe effectiveness when evaluating potential screening programmes. How? Difficult why? (3)

Answer 23

A: Effectiveness is evaluated by measuring the extent to which implementing a screening programme affects the subsequent outcomes. This is difficult to measure because of a number of biases that affect most of the study designs used:
– Selection bias exists
– Lead time bias exists
– Length bias exists

Question 24

Q: Why does length bias exist?

Answer 24

A: 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.

Question 25

Q: Why does lead time bias exist?

Answer 25

A: because screening identifies disease that would otherwise be identified at a later stage. This may result in an apparent improvement in the length of survival due to screening which is really due to the earlier date of diagnosis.

Question 26

Q: Why does selection bias exist?

Answer 26

A: as people who participate in screening programmes often differ from those who do not.

Question 27

Q: Describe cost when evaluating potential screening programmes. Value? Resources? Includes? Positives?

Answer 27

A: – The cost of screening programmes is important.
– Resources for health care are limited and there are many competing demands for available money, health care professionals and facilities.
– The relative cost-effectiveness of a screening programme compared with other forms of health care should therefore be considered. Costs relate not just to the implementation of the screening programme but also to the further diagnostic tests and the subsequent cost of treatment.
– On the other hand, in the absence of screening, costs will be incurred by the treatment of patients in more advances stages of disease

Question 28

Q: Why are ethics considered in screening? Result can be? Risks? False positive? False negative?

Answer 28

A screening test is a medical intervention that is done to a person who is not ill and usually to someone who has not initiated the request for the test. For this reason the ethics of carrying out screening must be carefully considered.

 For the individual the screening test can do harm as well as giving benefit
 There may be a risk attached to the screening test or subsequent diagnostic test
 A false positive result can cause unnecessary anxiety
 A false negative result will give false reassurance

Question 29

Q: Define gold standard.

Answer 29

A: a recognised way of determining who really has the disease

Question 30

Q: Define prognosis.

Answer 30

A: is the outcome of an illness, including duration of disease, mortality and morbidity