In epidemiology, screening involves testing people without known disease to
determine whether they have a disease. Screening is not a diagnostic tool. It
is used to reduce morbidity and mortality in a population. Early detection of
disease allows for early entry into the healthcare system with the idea that early
treatment will lead to more favorable outcomes. For example, newborns are
routinely screened for a variety of genetic diseases, such as phenylketonuria
(PKU) and sickle cell disease.
In articles involving screenings, it is important to assess the validity of a
screening test. Investigators often report measures for sensitivity, specificity, and
positive predictive value of the test. Sensitivity describes the ability of the test
to correctly identify people with the disease by positive test results. Specificity
describes the ability of the test to correctly identify people without the disease
by negative results. Positive predictive value is the probability that a person
who screens positive actually has the disease.
To calculate sensitivity, specificity, and positive predictive value, investigators
use a variation of the 2 × 2 table as follows:
Disease No Disease
Positive Screen True positives (TP) False positives (FP)
Negative Screen False negatives (FN) True negatives (TN)Match the following:- Case-control study a. OR
- Cohort study b. RR
TEST YOUR KNOWLEDGE 8-6
How did you do? 1. a, 2. b8.7 Screening
By the end of this section, you will be able to:
‹ Define screening
‹ Calculate sensitivity, specificity, and positive predictive value
‹ Explain the relationship between sensitivity and specificityKEY TERMS
screening: Testing
people without
known disease
to determine
whether they have
a disease; used to
reduce morbidity
and mortality in
populations
sensitivity: The
ability of the test to
correctly identify
people with the
disease by positive
test results
specificity: The
ability of the test to
correctly identify
people without the
disease by negative
results8.7 Screening 213