Understanding sensitivity and specificity

Question 1

How is the sensitivity of a test determined?

Answer 1

By dividing the number of people who test positive in the diseased group by how many are actually diseased x100 to get a percentage of diseased correctly classified.

Question 2

How is the specificity of a test determined?

Answer 2

By dividing the number of people who test negative in the non-diseased group by how many are actually non-diseased x100 to get a percentage of non-diseased correctly classified.

Question 3

When using a highly sensitive test with low specificity, is it more helpful if the result is negative or positive?

Answer 3

Negative

High sensitivity means there is a very low risk of false negatives so diseases can be ruled out if a negative result comes from a highly sensitive test.

Question 4

When using a highly specific test with low sensitivity, is a negative or positive result more useful?

Answer 4

Positive.

High specificity tests have a low level of false positive results. Therefore a positive result means there is a high chance the patient has the disease.

Question 5

What is more useful when using a test in diagnosis, specificity and sensitivity of tests or predictive values?

Answer 5

Predictive values

When a patient presents, we do not know if they have the disease or not so don’t know if they will receive a true or false result upon testing. Predictive values give a percentage that the outcome is a positive or false result.

Question 6

What is a positive predictive value?

Answer 6

The proportion of people with a positive test who are actually have the disease.

Question 7

What is a negative predictive value?

Answer 7

The proportion of people who test negative who don’t have the disease

Question 8

What is the difference between a negative predictive value and a post-test probability of disease given a negative result?

Answer 8

They are opposites. The negative predictive value is the probability you do not have the disease if you test negative and the post-test probability is the probability you do have the disease despite a negative result. If one was 20% the other would be 80%.

Question 9

What factor is the usefulness of a test results for a patient dependent on? Why?

Answer 9

The prevalence of the disease amongst the population being tested because predictive values will change between populations with a high prevalence of disease and low prevalence of a disease so the probability of the individuals result being true is dependent on disease prevalence.

Question 10

What is the role of screening?

Answer 10

To detect disease early to minimise emergence of symptoms +/- complications and to screen for asymptomatic disease.

Question 11

What category of prevention is screening?

Answer 11

Secondary category of prevention

Question 12

What are the 11 current screening programmes in the UK funded by the NHS?

Answer 12

1.Abdominal aortic aneurysms
2.Bowel cancer screening
3.Breast screening
4.Cervical screening
5.Diabetic eye screening
6.FASP (foetal anomaly screening programme)
7.IDPS (infectious diseases in pregnancy screening)
8.NIPE (newborn and infant physical exam)
9.NHSP (newborn hearing screening programme)
10.NBS (Newborn blood spot test for CF and other metabolic conditions)
11.SCT (sickle cell and thalassaemia)

Question 13

What is the difference between screening and diagnostic testing?

Answer 13

Screening only identifies risk of disease, diagnostic testing helps determine the presence of disease.

Question 14

What is the screening sieve?

Answer 14

A diagrammatic representation of screening programmes separating those at risk from those at low risk in order to provide further testing and support if necessary to high risk individuals.

Question 15

What are some benefits of screening programmes?

Answer 15

Earlier diagnosis
Earlier treatment (can be more effective +/- less invasive)
Often better prognosis preventing deaths
Reduce risk by removing risk factors e.g. removing polyps

Question 16

Who is offered abdominal aorta screening? What does it involve?

Answer 16

Men over 65 undergo a non-invasive ultrasound scan

Question 17

What are risks/harms of screening?

Answer 17

Not 100% accurate = false + and - results causing over treatment/unnecessary harmful treatments like surgery in false + or no treatment in false - cases. Also leads to unnecessary anxiety or false reassurance respectively.
In foetal screening can lead to difficult decisions

Question 18

Why is prostate cancer screening not funded by the NHS in the UK?

Answer 18

As the level of false positives and false negatives were found to be high (75% and 15% respectively) and the consequences of this were deemed to outweigh the positives of funding the screening programme in low risk populations.

Question 19

How often is the decision about whether the NHS will fund a prostate cancer screening programme reviewed?

Answer 19

Every 3 years

Question 20

What are the 7 criteria a screening programme has to meet to pass through the UK screening committee?

Answer 20

1. The condition has to be frequent +/- severe enough to be an important health problem
2. The screening test has to be simple, safe, precise and validated
3. There should be an effective intervention for patients identified as at risk
4. High quality research needs to evidence the screening programme will effectively reduce mortality or morbidity
5. Benefit of screening must outweigh harms
6. Cost needs to be economically balanced
7. Must be clinically, socially and ethically acceptable to health professionals and the public

Question 21

What is a screening programme that people question if we should screen for just because we can screen for it?

Answer 21

Down’s syndrome/trisomy 21

Question 22

What is the most recent Down syndrome screening test that is to be rolled out soon in the UK?

Answer 22

The non-invasive prenatal test (NIPT)