hea 855 principles of epidemiology screeningcourses.justice.eku.edu/hea855/ppt/hea855 lesson...

HEA 855

Principles of

Epidemiology

Screening

HEA 855: Principles of Epidemiology 2

The application of a test to people who are asymptomatic for the purpose of classifying them with respect to their likelihood of having a particular disease

Objective: To delay the onset of symptomatic or clinical disease or to improve survival

What is Screening?


Natural History of Disease Age (years)

40 60

Preclinical Phase

Detectable

Preclinical

Phase


Timing of Prevention Efforts &

Natural History of Disease Age (years)

40 60

PRIMARY SECONDARY TERTIARY


Screening

For screening to be successful you need a:

Suitable disease

Suitable test

Suitable screening program


Characteristics of a Suitable

Screening Test

Inexpensive or economical

Easy to administer or convenient

Acceptable to a large number of individuals

Free of risk and impose minimal discomfort on clients

Highly valid

Reliable

Validity is more important than reliability!!


How is validity measured?

By comparing results of screening with

those from a more definitive test known as

a “gold standard”

The extent to which the results agree with

the gold standard is measured by the

sensitivity and specificity

Set up a 2 X 2 table

Disease Status (TRUTH)

Results

of

Screening

Test

Positive

Negative

Total

Positive

a

b

a + b

Negative

c

d

c + d

Total

a + c

b + d

How to Calculate

Sensitivity & Specificity



Enables you to pick up the cases of disease

Probability of testing positive if the outcome or health problem really exists

Expressed as a percentage (%): % of people with the disease correctly labeled by the test as diseased.

# of people with disease who test positive

Total # of people with disease

___TP__ x 100 or ___a___ x 100

TP + FN a + c

Measure of Test Validity:

Sensitivity

X 100


Enables you to pick out the no diseased people

Probability of testing negative if the health problem

really does not exist

Expressed as a percentage (%): % of people without

the disease correctly labeled by the test as not

diseased

# of people without disease who test negative

Total # of people without disease

___TN__ x 100 or ___d___ x 100

FP + TN b + d

Measure of Test Validity:

Specificity

X 100


Example: Prostate Cancer

Suppose that 100,000 men were screened for

prostate cancer

4000 of these men had a positive result on

screening blood test

Of those that screened positive, 800 had a

biopsy (i.e. the gold standard) indicating

diagnosis of prostate cancer

Of the 96,000 who screened negative, 100

developed prostate cancer within a year and are

assumed to be false negatives



Set up a 2 x 2 table

What is the prevalence of prostate cancer

in this population?

Calculate and interpret the sensitivity of

this screening test

Calculate and interpret the specificity of

this screening test


Prostate Cancer (Truth)

Results

of

Screening

Blood

Test

Positive

Negative

Positive

800

3,200

4,000

Negative

100

95,900

96,000

900

99,100

100,000



Answers to Example

Prevalence: 900 people with disease/100,000 total people= .9%

Sensitivity: 800/900 or 88.9%

Means that 88.9% of men who had prostate cancer tested positive on the screening test

Specificity: 95900/99100 = 96.8%

Means that 96.8% of men who did not have prostate cancer tested negative on the screening test


Suitable Screening Programs

More than just a screening test

Includes:

Screening test

Follow-up evaluations for people with positive test results

Diagnostic and therapeutic components

To measure the feasibility of a screening program must measure the yield or predictive value


Yield – the amount of previously

unrecognized disease which is

diagnosed & brought to treatment as a

result of screening.

Predictive value of a test – the likelihood

that an individual with a positive test has

disease (positive) or the likelihood that

an individual with a negative test does

not have the disease (negative).

Measures of Screening Program

Feasibility


Predictive value of a positive test =

PV+ = __TP__ x 100 or __a__ x 100

TP + FP a + b

Number who test + with disease/

number with positive result

Predictive Value Positive

(Note: the denominator is everyone with a positive test)


Predictive value of a negative test =

PV- = __TN__ x 100 or __d__ x 100 FN + TN c + d

Number who test - without disease/

number with negative result

Predictive Value Negative

(Note: the denominator is everyone with a negative test)



Using the previous example of prostate

cancer screening

Calculate and interpret the PV+

Calculate and interpret the PV-


Predictive value positive= 800/4000 or

20%

Means that 20% of the men who screened

positive truly did have prostate cancer

Predictive value negative= 95900/96000 or

99.9%

Means that 99.9% of the men who screened

negative truly did not have prostate cancer



Influence of Prevalence

PREVALENCE HAS A PROFOUND

EFFECT ON USEFULNESS OF A

SCREENING TEST

PV will increase when sensitivity, specificity,

and as disease prevalence increases

This is demonstrated in the following

example of screening for HIV in a “Low

Prevalence” and “High Prevalence”

population


HIV infection

Results

of EIA

Screening

Test

Yes

No

Positive

78

80

158

Negative

2

39,840

39,842

80

39,920

40,000

Results of HIV Screening in

“Low Prevalence” Population


HIV infection

Results

of EIA

Screening

Test

Yes

No

Positive

9,800

60

9,860

Negative

200

29,940

30,140

10,000

30,000

40,000

Results of HIV Screening in

“High Prevalence” Population

Comparison of HIV Screening Results in

Low vs. High Prevalence Populations

Low Prevalence

(0.2%)

High Prevalence

(25%)

Sensitivity 78/80 =

97.5%

9800/10000 =

98%

Specificity 39840/39920 =

99.8%

29940/30000 =

99.8%

PV+ 78/158 =

49.4%

9800/9860 =

99.4%

PV- 39840/39842 =

99.9%

29940/30140 =

99.3% HEA 855: Principles of Epidemiology 24


There is a much higher yield from

screening the high-risk population, even

with identical tests.

Also, a high-risk individual with a positive

test result is more likely to be infected than

a low-risk person with a positive result

So, screening in a high risk population is

more feasible.

Comparison of HIV Screening

Results in Low vs. High

Prevalence Populations


Evaluating Screening Programs

Criteria for establishing and evaluating the

effectiveness of a screening program

should address

The health problem to be screened

The screening test to be used

The group to be screened

hea 855 principles of epidemiology screeningcourses.justice.eku.edu/hea855/ppt/hea855 lesson...

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