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  #1  
Old 02-06-2012
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Stats Lowering the Cut Off Value

Q. 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 02-06-2012
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Ans ,,,,,,,, D ......... will find more people with DM ......
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Old 02-06-2012
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Best answe was increase in sensitivity bt since thats nt an option i will go with D. TOO
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Old 02-06-2012
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when u lower cutoff two things happen
first the true positives increase TP
and secondly the False Positives increase too FP but more than increase in TP

i.e both TP and FP increases but FP increase>>> TP increase

so looking at all equations and applying the above changes we see that

Sensitivity= TP/ TP + FN ....increases (by virtue of TP increasing...a numerator)

Specificty=TN/TN + FP....decreases (by virtue of FP increase ..a denominator)

PPV= TP/TP+FP..decreases(cos FP increase >>TP increase)

NPV= TN/TN+ FN...increases (if FP increases very much then TN decreases much too..since FP + TN=100 hence lets say FP was initially 60 and TN was 40...If FP becomes 90 means TN will be a mere 10...and a denominator decreasing)

hence answer is increase in NPV
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Old 02-06-2012
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I would go with C.
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Ans.....D....i thought this way...since the cut off value was lowered to 126mg/dl...more ppl get diagnosed or screened and more number of ppl are added to the pool of diabetics leading to Increased prevalence.prevalence is proportional to PPV and inversely proportional to NPV,among the options increase in PPV is the right answer so i go with D.
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Old 02-06-2012
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i will go with...
D. Increase the test's positive predictive value
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Old 02-07-2012
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Quote:
Originally Posted by stfidel View Post
when u lower cutoff two things happen
first the true positives increase TP
and secondly the False Positives increase too FP but more than increase in TP

i.e both TP and FP increases but FP increase>>> TP increase

so looking at all equations and applying the above changes we see that

Sensitivity= TP/ TP + FN ....increases (by virtue of TP increasing...a numerator)

Specificty=TN/TN + FP....decreases (by virtue of FP increase ..a denominator)

PPV= TP/TP+FP..decreases(cos FP increase >>TP increase)

NPV= TN/TN+ FN...increases (if FP increases very much then TN decreases much too..since FP + TN=100 hence lets say FP was initially 60 and TN was 40...If FP becomes 90 means TN will be a mere 10...and a denominator decreasing)

hence answer is increase in NPV
very correct , thank you for your explanation this concept is there in UW and i surely missed it , i found it again in UW the id no is 1191 (378969)

well it does not need futher explanation but i am going to say where I went wrong so may be others may also know .... I did know that the test would find more people with DM but didnot know that the FP will increase more that TP the reason is there may be some one who had a high sugar diet last night may have a glucose of 126 or some one might be stressed having a sugar level of 126 but they actually dont have DM while people with 140 definitely do ........ so its clear we will have increase in sensitivity and decrease specificity but the PPV will decrease as NOT INCREASE which I failed to understand , and if you think now a very few people will have FN as most will test postive for the test but those who dont really dont have DM , so even by ruling out other option the answer would be C ...
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  #9  
Old 02-07-2012
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thanks for the explanations guys...helped to understand it better @stfidel and @hitman cheers
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Old 02-07-2012
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C
nyc explanation guys
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Old 02-07-2012
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Default one more thing

one last thing....pls do work out wat happens when cutoff is raised...

same concept....TN increases and FN increases...but FN increase>>>TN increase

guess u guys can interpret the rest

And a quick hint....

Sensitivity and Negative Predictive Value usually go together

and specificity and positive predictive value go together

just look at previous examples and work out this one and see wat i mean....cheers guys
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Old 02-08-2012
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The 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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  #13  
Old 11-29-2012
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Correct Answer I f you lower cut off

there will be more chance of finding sick people, so the posibility of a test to be positive increases( PPV ).
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