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Old 08-05-2012
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Default This reference interval change

The American Diabetes Association (ADA) recently lowered the cutoff value for fasting glucose used in
diagnosing diabetes mellitus from 140 mg/dL to 126 mg/dL. This reference interval change would be expected to
produce which of the following alterations?


A. Decrease the test's sensitivity

B. Increase the test's false negative rate

C. Increase the test's negative predictive value

D. Increase the test's positive predictive value

E. Increase the test's specificity
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Old 08-05-2012
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Quote:
Originally Posted by Novobiocin View Post
The American Diabetes Association (ADA) recently lowered the cutoff value for fasting glucose used in
diagnosing diabetes mellitus from 140 mg/dL to 126 mg/dL. This reference interval change would be expected to
produce which of the following alterations?


A. Decrease the test's sensitivity

B. Increase the test's false negative rate

C. Increase the test's negative predictive value

D. Increase the test's positive predictive value

E. Increase the test's specificity
C..................
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Old 08-05-2012
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Explanation:

The correct answer is C. The negative predictive value of a test (PV-) refers to the percent chance that a
normal test result is a true negative (TN; a healthy person with a normal test result) rather than a false negative
(FN; a diseased person with a normal test result). It is expressed by the following formula:

PV- = TN/(TN + FN)

Similarly, the positive predictive value of a test (PV+) reflects the probability that an abnormal test result
represents a true positive (TP) rather than a FP (a TP is an abnormal test result in an individual with disease,
while a FP is an abnormal test result in a healthy person). PV+ is calculated using the following formula:

PV+ = TP/(TP + FP)

There is a relationship between the test's sensitivity (the ability of a test to detect disease in those who truly
have the disease), specificity (ability of the test to correctly identify those without disease) and the PV- and
PV+, respectively. Tests with 100% sensitivity (no FNs) and tests with 100% specificity (no FPs) automatically
have a PV- and PV+ of 100%, respectively.

Changing the reference interval of a test alters its sensitivity, specificity, PV+, and PV-. In this question,
lowering the upper limit of the reference interval of a fasting glucose from 140 mg/dL to 126 mg/dL increases
the test's sensitivity, since a lower glucose cut-off approaches the normal value for glucose in the normal
population (70-110 mg/dL). Furthermore, increasing the test's sensitivity automatically increases the test's PV-,
since there are fewer FNs.


The test's sensitivity is increased (choice A) rather than decreased by the given change in the reference
interval.

Since the test's sensitivity is increased, the FN rate at the new reference interval is decreased (choice B).

The test's positive predictive value (choice D) decreases, since the test's specificity, which ultimately
determines its FP rate, decreases as the test's sensitivity increases. Stated another way, a greater number of
normal individuals will have FP fasting glucose levels when the test's upper limit is decreased to 126 mg/dL.

A test's sensitivity is inversely related to its specificity. For example, changing the reference interval of a test to
increase its sensitivity automatically lowers its specificity (choice E), since the number of FPs will increase.
Similarly, when altering a reference interval to increase a test's specificity, its sensitivity is reduced because of
an increase in the number of FNs.

In summary, lowering a test's upper limit of normal increases the test's sensitivity, which decreases the FN rate,
and increases the PV-. Increasing the test's sensitivity also decreases the test's specificity. Decreasing a test's
specificity increases the FP rate and decreases the PV+.
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