Session 6 - Screening

Question 1

Define screening

Answer 1

Systematic attempt to detect an unrecognised condition by use of tests, exams etc which can be applied rapidly to distinguish between persons with and without the disease

Question 2

What are the 3 ways a disease can be detected?

Answer 2

1. Spontaneous presentation
2. Opportunistic case finding
3. Screening

Question 3

What must the disease be for a screening program to be implemented?

Answer 3

• Must be an important health problem
• Epidemiology and natural history must be well understood
• Must have an early detectable stage
• Cost-effective primary prevention interventions must have been considered and where possible implemented

Question 4

What must the test be for the screening program to be implemented?

Answer 4

• Simple and Safe
• Precise and valid
• Acceptable to the population
• There must be an agreed policy on whom to investigate further

Question 5

What must the treatment be for a screening program to be implemented?

Answer 5

• Effective evidence based treatment must be available
• Early treatment must be advantageous, must not just bring forward the date of diagnosis

Question 6

What must the programme be for the screening to be implemented?

Answer 6

• Other options considered, e.g. improving treatment
• Benefit should outweigh physical and psychological harm
• Facilities for diagnosis and treatment

Question 7

What is a false positive?

Answer 7

Screening programme refers well people for further investigation

Question 8

What is a false negative?

Answer 8

Failure to refer people who do actually have early disease

Question 9

What is sensitivity?

Answer 9

The proportion of the people with the disease who are correctly identified by screening test

Correctly Positive screening test/ (CPST+Negative screening test but has disease)

Question 10

What is the specificity?

Answer 10

The proportion of the people without the disease who are test negative

“If I don’t have the disease, will I test negative?”

Question 11

What is positive predictive value?

Answer 11

The probability that someone who has tested positive actually has the disease

PPV = True positives / (true + false positives)

Question 12

What is negative predictive value?

Answer 12

Proportion of the people who are test negative who actually do not have the disease

NPV = True negatives / (false + true negatives)

Question 13

What are the advantages and disadvantages for screening for disease?

Answer 13

Adv:

• Early detection may improve outcome
• True negatives reassure patients

Disadv:

• False positives expose patients to invasive diagnostic tests
• False negatives falsely reassure patients
• False negatives not offered diagnostic testing that could benefit
• Expensive interventions could divert money away from treatments

Question 14

What is lead time bias??

Answer 14

• Screened patients appear to survive longer, but only because they were diagnosed earlier
• Patients live the same length of time, but longer knowing they have the disease

Question 15

What is length time bias?

Answer 15

• Screening programmes are better at picking up slow-growing, unthreatening cases than aggressive, fast-growing ones
• Diseases that are detectable through screening are more likely to have favourable prognosis, and may indeed never have caused a problem

Question 16

What is selection bias?

Answer 16

• Those who have regular screening are also likely to engage in other health behaviours that protect them from disease
• Similar to ‘healthy worker’ bias

Question 17

Give 3 examples of screening programmes in the Uk

Answer 17

• PKU
• downs syndrome
• breast cancer

Question 18

Give 2 sociological critiques of screening

Answer 18

• Surveillance critiques
• Victim blaming - not all individuals can take responsibility for their own health

Question 19

What is the illness iceberg? Why is it relevant to STIs?

Answer 19

Numerb of cases identified vastly outnumbered by unidentified cases

Relevant to STIs as many are asymptomatic or present transient non specific symptoms

Question 20

What is the difference between sensitivity and PPV?

Answer 20

100% sensitive is everyone with the disease is picked up

100% PPV is everyone who tests positive has the disease

Question 21

What is the difference between specificity and NPV?

Answer 21

100% specific is everyone who doesnt have the disease tests negative

100% NPV is everyone who tests negative doesnt have the disesae