L17 Screening : a prevention strategy

Question 1

What is screening in health?

Answer 1

Identifying risk factors for disease or unrecognised disease or complication of a disease by applying tests on a large scale to a population

Question 2

At what levels does screening function as a prevention strategy?

Answer 2

Primary - before biological onset (identification of risk factors)
Secondary - before diagnosis (early diagnosis)
Tertiary - after diagnosis (identifying further complication)

Question 3

Explain how a screening programme is designed

Answer 3

A large population who are deemed high risk to a certain disease and show no symptoms are defined as the eligible population

All members of this population undergo screening

Those who test positive undergo a diagnostic test (gold standard test) and those who test positive from this are treated

Those who test negative in both the diagnostic test and the screening test are re-screened after a period of time

Question 4

What is the objective of screening?

Answer 4

Improve health outcome - reduce morbidity, mortality, disability

Question 5

What are the criterias to consider when designing an appropriate screening programme?

Answer 5

Suitable disease
Suitable test
Suitable treatment
Suitable screening programme

Question 6

What is a suitable disease for screening?

Answer 6

1. Important public health problem
   - A relatively common disease
   - A relative uncommon disease in which if detected early can lead to better outcomes
2. Knowledge of the natural history of the disease
   - Detectable early
   - Long duration of pre-clinical phase (higher chance that screening test will help detect the disease)

Question 7

What is a suitable test for screening?

Answer 7

Reliable - consistent results
Safe
Simple
Affordable
Acceptable
Accuracy - sensitivity and specificity

Question 8

What are the two measures of accuracy of a screening test?

Answer 8

Sensitivity and specificity

Question 9

What is a diagnostic test accuracy study?

Answer 9

A test designed to determine the accuracy of a screening test by comparing it to a diagnostic test (gold standard test)

Question 10

What is sensitivity?

Answer 10

The likelihood a person who actually has the disease shows a positive result from the screening test

True positive / All from study with disease (determined by diagnostic test)

Question 11

What is specificity?

Answer 11

The likelihood a person who does not have the disease shows a negative result from the screening test

Question 12

What is the positive predictive value PPV?

Answer 12

The proportion of people who actually have the disease from all those who show a positive result from the screening test

i.e. the probability of having the disease if the test shows a positive result

Question 13

What is the negative predictive value NPV?

Answer 13

The proportion of people who actually do not have the disease from all the people in the study who show a negative result from the screening test

i.e. the probability that the person actually does not have the disease when they test negative in the screening test

Question 14

What is the difference between fixed and non fixed characteristics of diagnostic test accuracy studies?

Answer 14

Fixed - sensitivity and specificity
Non-fixed - PPV and NPV

PPV and NPV values reflect both the test accuracy and the prevalence of the disease

Question 15

What is suitable treatment for a screening programme?

Answer 15

Evidence that early treatment will lead to better outcomes

Effective, acceptable and accesible

Evidence-based policies identifying who should be offered treatment and what treatment should be offered

Question 16

What is a suitable screening programme?

Answer 16

The benefits outweigh harm

RCT evidence that screening programme will result in reduced mortality and increased survival time

Adequate resourcing and agreed policy for testing, diagnosis, treatment and programme management

Cost effective

Able to be supported by health care system along all stages

Targeted towards those who are more likely to benefit from the programme

Question 17

What is lead time bias?

Answer 17

Because screening allows earlier diagnosis of a disease the survival time will natural be longer than if it were measured from the usual clinical diagnosis possibly giving the false impression of success

Question 18

What is length time bias?

Answer 18

When considering rapidly and slowly progressing diseases in a population, a screening test will naturally be more likely to identify disease in slower progressing diseases -> this would increase the mean survival time giving false impression of success