Screening and Outbreak

Question 1

What is screening and what’s the purpose of it?

Answer 1

Examination of asymptomatic people to classify them as likely or unlikely to have a disease

Aim to detect disease before clinically apparent to improve outcome for people with the disease

Question 2

Is it appropriate to screen for every type of health outcome? and why

Answer 2

NO

→ unethical to screen if no treatment is available
→ treatment must be effective and early treatment must improve disease outcome (benefit vs cost)

Question 3

What are the 2 measures of accuracy?

Answer 3

Sensitivity - Ability of a test to correctly identify those WITH the disease
(HIGH SENSITIVITY = NO FALSE NEGATIVES)

Specificity - Ability of a test to correctly identify those WITHOUT the disease
(HIGH SPECIFICITY = NO FALSE POSITIVES))

Question 4

Can a screening test have an 100% accuracy for both sensitivity and specificity?

Answer 4

NO

If sensitivity is at 100% - meaning no false negatives (e.g. for COVID testing) specificity would have to be < 100% - meaning there would be some false positives

Question 5

What are the 4 biases associated with screening

Answer 5

Lead time Bias

Length Bias

Selection Bias

Over-diagnosis Bias

Question 6

What is Lead time bias

Answer 6

Extends the interval between diagnosis and death (survival time) even if time of death is unchanged

The earlier we diagnose a disease, the longer patients will appear to survive, when in fact we just started counting earlier.

Survival time shouldn’t be used to assess the efficacy of screening

Question 7

What is Length Bias

Answer 7

Fast-growing tumours will progress rapidly and be less likely to be detected by screen in the preclinical phase.

Screening will detect a disproportionate number of slow-growing tumours with good prognosis

Question 8

What is Selection Bias

Answer 8

People who choose to be screened may differ from people who choose not to, in risk of developing the outcome or dying from it.

Question 9

What is Over-diagnosis Bias

Answer 9

Some diseases will never cause problems and wouldn’t be diagnosed without screening

Question 10

What is the only study that can avoid screening biases?

Answer 10

The only study design that can avoid these biases is a randomised controlled trial of screening - intervention and control groups measuring disease mortality (suffering from a disease or medical condition)

Question 11

Benefits of Screening?

Answer 11

Improved prognosis (course of a medical condition) for some people diagnosed by screening

Less radical treatment which cures some people with early disease

Reassurance for those with negative test results

Question 12

Downsides of Screening?

Answer 12

Longer morbidity for people whose prognosis is unaltered

Over-treatment of questionable abnormalities

False reassurance for people with false negatives

Anxiety and sometimes morbidity for people with false positive results

Question 13

What is population screening?

Answer 13

Initiated by health system

People are well (asymptomatic)

Offered with expectation of benefit

Question 14

What is an outbreak?

Answer 14

Two or more cases of a specific health condition occurring in a location over a period of time in excess (local epidemic)

Question 15

What is a Epidemic

Answer 15

Occurrence in a community or region of cases of illness excess of normal

Question 16

What is a Pandemic

Answer 16

worldwide epidemic (or across several countries)

• think GLOBAL PANDEMIC

Question 17

What are the 4 stages of an approach to an outbreak?

Answer 17

Surveillance

Descriptive Investigation

Analytical Investigation

Action

Question 18

What is Surveillance

Answer 18

clinicians and labs notify the Medical Officer of Health whenever they diagnose specified diseases

Detect clusters or outbreaks

Inform public health intervention

Monitor the effect of intervention

Monitor the introduction (and long term effect) of new drugs or treatments

Support disease elimination

e.g. covid tracer

Question 19

What is Descriptive Investigation

Answer 19

Distribution of disease in a population can identify risk factors

Time
epidemic curve - can suggest possible means of transmission (point source or person to person - propagated)

Place
Geographic location e.g. water supply location, region, suburb, work address vs home address

Person
Age, occupation, workplace/school

Question 20

What is Analytical Investigation

Answer 20

Test hypotheses about epidemic source (case-control - comparing OR in 2x2 table, Environmental inspection, microbiological samples)

Question 21

What is Action

Answer 21

Examples: 
Health education advice
Cleaning and disinfection
Exclusion or isolation
Modification of procedures
Closure (e.g. food premises or gyms)

Question 22

What is a Point Source Outbreak

Answer 22

Many susceptible people exposed to a common exposure → epidemic has rapid onset (at the same time) and usually localised to one group of people

Question 23

What is a Propagated Epidemic

Answer 23

• Person to Person

Disease is spread through contact between infected and susceptible persons

Epidemic has slower onset and resolution than point source

Question 24

What are 3 types of Biases?

Answer 24

Recall Bias
Selection Bias
Measurement Bias

Question 25

What is Recall Bias

Answer 25

participants do not remember previous events or experiences accurately or details

Question 26

What is Selection Bias

Answer 26

volunteers for a study will be different compared to people who do not wish to be part of the study or do not go out of their way to volunteer

Question 27

What is Measurement Bias

Answer 27

systematic or non-random error that occurs in the collection of data in a study.