drplacebo
Forum Senior
Topics: 6
Posts: 138
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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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| Palaniappan
Forum Senior
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Posts: 148
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It is D..??Increasing the PPV??
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| robin082006
Forum Hero
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Decrease CUT OFF--> increaseD SENSITIVITY, INCREASED NEGATIVE PREDICTIVE VALUE, DECREASED POSITIVE PREDICTIVE VALUE, DECREASED SPECIFICITY.
ANSWER IS C
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| drplacebo
Forum Senior
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Posts: 138
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robin is right,
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)
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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| Ancylostoma
Forum Guru
Topics: 42
Posts: 636
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agreed, the answer is c
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| fabrilosu
Forum Newbie
Topics: 8
Posts: 18
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I would go for C.
I have a less rigourous and more intuitive explanation, for the right side of the brain...
Lowering the low end of a range it's gonna make less likely for a "boarderline person" to result negative. So if you a get a negative also now that is so easy to be positive ('cos the lower end is now 126 instead of 140) this negative that you get must be much more belivable than before (i.e. a negative got having 140 as low end)
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