Screening

Question 1

What are the three ways patients can be discovered to have a disease?

Answer 1

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

Question 2

What is screening and what must take place afterwards?

Answer 2

Screening is a systematic attempt to detect an unrecognised condition by the application of tests, examinations, or other procedures, which can be applied rapidly (and cheaply) to distinguish between apparently well persons who probably have a disease (or its precursor) and those who probably do not.

After screening, if it comes back positive then a more formal diagnostic test is done to confirm it is a true positive result.

Question 3

What is the purpose of screening?

Answer 3

• To give a better outcome compared with finding something in the usual way (having symptoms and self-reporting to health services)
• If treatment can wait until there are symptoms, there is no point in screening
• Finding something earlier is not the primary objective

Question 4

What are the 5 aspects of screening that must be considered before screening can take place?

Answer 4

The condition
The test
The intervention
The screening programme
The implementation

Question 5

Describe what must be considered about the condition?

Answer 5

• An important health problem (frequency/severity) with epidemiology, incidence, prevalence and natural history understood
• All the cost-effective primary prevention interventions should have been implemented as far as practicable

Question 6

What must be considered about the test involved in the screening?

Answer 6

• Simple, safe, precise and validated screening test
• Distribution of test values in the population must be known and an agreed cut-off level must be defined and agreed
• Acceptable to target population
• Agreed policy on further diagnostic investigation of those who test positive and choices available to them

Question 7

Define Sensitivity and Specificity

Answer 7

• Sensitivity (detection rate)
Is the proportion of the people with the disease who are test positive, also known as the detection rate. The proportion of the people who really have the disease who are identified correctly by the test as having the disease. Sensitivity is the probability a case will test positive

Sensitivity = True positive/(true positives + False negatives)

• Specificity
Is the proportion of the people without the disease who are tested negative? The proportion of the people who really do not have the disease who are identified correctly by the test as not having the disease. Probability a non-case will test negative

Specificity = True Negatives/(True Negatives + False Positives)

Question 8

Define Positive predictive value and negative predictive value?

Answer 8

• Positive predictive value
Proportion of people who tested positive who truly has the disease. This value is strongly influenced by the prevalence of the disease.

Positive Predictive Value = True Positives/(True Positives + False Positives)

• Negative predictive value
Proportion of people who tested negative who truly do not have the disease.

Negative Predictive Value = True Negatives/(True Negatives + False Negatives)

Question 9

What must you consider about the possible interventions?

Answer 9

There must be an affective intervention for patients identified through screening, with evidence that intervention at a presymptomatic phase leads to better outcomes for the screened individual compared with usual care.

There should be agreed evidence based policies covering which individuals should be offered interventions and the appropriate intervention to be offered.

Question 10

What must you consider about the screening programme overall?

Answer 10

Proven effectiveness in reducing mortality or morbidity (high quality RCT data).

Evidence that the complete screening programme is clinically, socially and ethically acceptable to health professionals and public.

Benefit gained by individuals should outweigh any harms for example from over diagnosis, overtreatment, false positives, false reassurance, uncertain findings and complications.

Opportunity cost of the screening programme should be economically balanced in relation to expenditure on medical care as a whole

Question 11

What must you consider about implementing the screening prgramme?

Answer 11

All other options for managing the condition should have been considered.

Management and monitoring programme – quality assurance.

Adequate staffing and facilities for programme.

Evidence-based information available to potential participants (informed choice).

Public pressure should be anticipated - decisions should be scientifically justifiable to the public

Question 12

What is lead time bias?

Answer 12

Early diagnosis falsely appears to prolong survival. 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 13

What is length time bias?

Answer 13

Screening programmes better at picking up slow growing, unthreatening cases than aggressive, fast-growing ones. Diseases that are detectable through screening are more likely to have a favourable prognosis, and may indeed never have caused a problem. Could lead to false conclusion that screening is beneficial in lengthening the lives of those found positive – curing people that didn’t need curing?

Question 14

What is selection bias?

Answer 14

Studies of screening often skewed by ‘healthy volunteer’ effect. Those who have regular screening likely to also do other things that protect them from disease. An RCT would help deal with this bias.