Chapter 9: Epidemiology and Screening for Disease

Screening

The presumptive identification of unrecognized disease or defects by the application of tests, examinations, or other procedures that can be applied rapidly.

• Occurs during pathogenesis phase

• Positive screening test → diagnostic test to confirm actual disease presence

Details of screening (how its different than diagnostic)

Asymptomatic people/subclinical cases of disease

Positive result indicates suspicion of disease

Detects disease indicators, not the disease itself

Done first

Ex. Mammogram, positive screening doesn’t guarantee you have breast cancer

Details of diagnostic (how its different than screening)

Establish the presence or absence of the disease Targets symptomatic people or asymptomatic people with positive screening test Usually more invasive/expensive, don’t want to put people through them if you don’t need to Done second Ex. biopsy following positive mammogram screening

Mass (population) screening

Screen entire population regardless of risk status

Ex. taking peoples temps before entering school in covid

Selective (targeted) screening

Screen specific groups who are at high risk for disease.

Ex. HIV screening for sex workers

Criteria for appropriateness to screen (3)

1. Social

2. Scientific

3. Ethical

Criteria for appropriateness to screen - social

health problem should be important for the community

• Follow-up and interventions must be available

• Favorable cost-benefit ratio

• Public acceptance must be high

Criteria for appropriateness to screen - scientific

sufficient understanding of the disease in question (cause, natural history - when symptoms arise/when best to detect for)

• Prevalence is high

Criteria for appropriateness to screen - ethical

can’t freak people out and leave them high and dry

• Need to have interventions available

• Don’t screen if treatment doesn’t change outcome

Characteristics of a good screening test (5)

1. Simple: easy to administer → especially if can complete at home

2. Rapid: quick to administer, fast results

3. Inexpensive: favorable cost-benefit ratio

4. Safe: don’t cause harm

5. Acceptable: people must be willing to use test

Why is screening important?

Catch disease early = better prognosis

Reliability

aka precision

consistently repeated results

Validity

aka accuracy

ability of the measuring device to give a true statement

Gold standard

Diagnostic test to say if you have the disease, not perfect but best available test that has a standard with known results

True positve

Screen +

Gold standard +

False positive

Screened +

Gold standard -

False negative

Screened -

Gold standard +

True negative

Screened -

Gold standard -

Sensitivity

Characteristic of the test —> how good is the screening test at its job

examining the true positives, how many people who test positive actually have the disease

x% sensitivty = x% of people who screen positive have the disease

Specificity

Characteristic of the test —> how good is the screening test at its job

examining the true negatives, how many people who test negative actually don’t have the disease

x% specificty = x% of people who do not have the disease will test negative

Positive predictive value

Clinical relevance/applications of a test

 true positives and all positives, Probability that following a positive test result, that individual will truly have that specific disease

Negative predictive value

Clinical relevance/applications of a test

Probability that following a negative test result, that individual will truly not have that specific disease

Accuracy

The degree of agreement between the test result and the gold standard

all the true negatives and positives divided by grand total

Prevelance

The number of existing cases of a disease or health condition in a population at a point or during a period of time

true positives + false negatives over grand total