# [SAR HS 300] - Midterm Exam Guide - Everything you need to know! (43 pages long)

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6 Feb 2017
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SAR HS 300
MIDTERM EXAM
STUDY GUIDE
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Measure of Validity of Screening Tests
1.!Sensitivity: the extent to which a test identifies as
positive all individuals who have a given disease.
a/(a+c)
2.!Specificity: the extent to which a test identifies as
negative all individuals who are free of a given
disease. d/(b+d)
Example: 500 elderly individuals are screened for diabetes
with a fasting blood glucose. 265 screened positive. The 265
positive screened then underwent a glucose tolerance test
(GTT) and 240 were positive. The 235 who screened negative
underwent GTT and 15 tested positive. Calc sensitivity and
specificity of screening test
!Prevalence: 255/500
!Sensitivity (a/a+c) = 240/255 x 100 = 94.1.
Interpretation: Among those tested, 94.1% of persons with diabetes screened positive by fasting
blood glucose screening test.
SO 5.9% WERE FALSE NEGATIVES
!Specificity (d/b+d) = 220 / 245 x 100 = 89.8. Interpretation: Among those tested, 89.8% of those
without diabetes screen negative by fasting blood glucose screening test.
SO 10% WERE
FALSE POSITIVES
Normal Range: decided as 2 standard deviations from the mean
Relationship between sensitivity and specificity
A successful screening test will balance cut point criteria so that both over-referrals and under-referrals
for diagnostic tests are minimized.
Measure of Accuracy of Screening Tests
Accuracy: of a screening test is the degree of agreement between the screening test and the gold standard,
and is calculated by: (a+d)/(a+b+c+d)
Predictive Value:
!Positive Predictive Value: the proportion of individuals screened positive by the test who actually
have the disease. a/(a+b)
among this screened population, _ % of those who screened positive for diabetes by fasting
blood glucose screening test truly have diabetes.
!Negative Predictive Value: the proportion of individuals screened negative who are actually free
of the disease. d/(c+d).
Among this screened population, 93.6% of those who screened negative for diabetes by fasting
blood glucose are truly free of the disease
SUMMARY
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Prevalence: effects of prevalence of disease on screening test results
!Positive and negative predictive value are affected by prevalence of disease.
!When the prevalence of a disease falls, the positive predictive value falls, and the negative
predictive value rises.
If prevalence RISES:
!Number of false negatives DECREASES
!Positive predictive value INCREASE
!Would NOT CHANGE that sensitivity and specificity
Screening program feasibility: Once acceptable sensitivity and specificity levels of screening test are
attained
!The predictive value is the main way to measure a screening program’s feasibility
Characteristics of Diseases Appropriate for Screening
1)!Serious disease with serious consequences
2)!Treatment more effective at earlier stages
3)!Has a detectable preclinical phase
4)!Preclinical phase is long (> year)
Characteristics of a good screening test
!Simple
!Rapid
!Inexpensive
!Safe
!Acceptable
!Valid
Example of good screening tests: Serology - HIV and Hepatitis B
!Mammogram breast cancer
!Pap smear cervical cancer
!BP & cholesterol heart disease
!Stool guaiac colorectal cancer
!Eye examsglaucoma
!(Prostate cancer blood PSA (now under
debate)
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