Ivonne wrote:It can not be D. If the cut off value decreases, there are more false positives(agree?). The positive predictive value PPV means given a positive test how sure we are that the patient has the disease and if we know that there are more false positives then the PPV is decreased. This is the idea if the cut off value decreases then: - More sensitivity(more ruling out possibility)
- More Negative predictive value
- less Positive predictive value
- More false positives
- less false negatives
I think it is not increasing the false positives. According to ADA, fasting plasma glucose is the diagnostic criteria for diagnosing DM. It alone is sufficient to diagnose a patient as having diabetes. That means once cut-off point is lower down more people in the population will be treated as diabetic patients. that means it increased the prevalence. The real change is diagnosing some previous diabetes (-) as diabetes (+). In my opinion, it is not about true or false positive since that is not the screening test. changing the cut-off point in screeing can have that kind of false-true change in the test performance..but not in diagnostic one like fasting plasma glucose.
Edited by drgho on 01/10/08 - 12:55 AM
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| SmokyWaters
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C
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| angel23
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plz explain this ques....i dint get this.........
in kaplan book on pg 2 it is written prevalance can be changed by simply moving the cut off point that defines a disease.........
so in this case....moving cutoff point shud increase prevalance.........
this ques was given i think in high yield but i dint get it.........
ivonne why did we assume there are more false positives there can be more true positives also..........
plz explain
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| Ivonne
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The explanation is better with a graphic but you are right there are more false positives when the cut off value is decreased because we will catch more healthy people with a positive test(false test result) and more true positives(we will grab more diseased people with this new cut off-------->our test is more sensitive)
This a Kaplan Qbank question you may want to read their explanation. Hope i didn't confuse you more.
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| angel23
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ok i got that.....wen we decrease the cutoff we reach more closer to the mean value....in this case it is 70-110.....so there will be more of true positives...i.e people with disease....this will increase senstivity.....
and it will hav less of false negative...i.e people who hav the disease but are tested negative....this means NPV increases coz denominator decreases.....
thanks ivonne
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| drgho
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 not so sure....hi, SURVIVOR could you please post the answer and full explanation? thanks a lot.
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| angel23
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drgho....
i hav seen in kaplan q bank biostats ques....
the explanation is wat i gave.....
got tht 4rm qbank....
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| kpmle2
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C. Increase in its negative predictive value
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| Ig F
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decreasing the cut off value will produce less false pasitives....which will eventually increase NPV....which means that people with the negative test are truly disease free.....so correct answer is C....
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