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Blood banks will shortly start to screen all donors for NANB [#permalink]
 07 Nov 2006, 02:59
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Blood banks will shortly start to screen all donors for NANB hepatitis. Although the new screening tests are estimated to disqualify up to 5 percent of all prospective blood donors, they will still miss two-thirds of donors carrying NANB hepatitis. Therefore, about 10 percent of actual donors will still supply NANB-contaminated blood.
The argument above depends on which of the following assumptions?
(A) Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed.
(B) Donors carrying NANB hepatitis do not, in a large percentage of cases, develop the disease themselves at any point.
(C) The estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate.
(D) The incidence of NANB hepatitis is lower among the potential blood donors than it is in the population at large.
(E) The donors who will still supply NANB-contaminated blood will donate blood at the average frequency for all donors.
Ps select this psace for OA & my doubts:
OA is A. Okay A is assumed here ‘coz some donors might be filtered for carrying other infections in their blood, hence total will be less than 10%. But what is wrong with E? If same donor frequency isn’t assumed than even A doesn’t holds further. Thanks 
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Re: CR: Challenge7: Blood Donors [#permalink]
07 Nov 2006, 03:18
themaddy2002 wrote: Blood banks will shortly start to screen all donors for NANB hepatitis. Although the new screening tests are estimated to disqualify up to 5 percent of all prospective blood donors, they will still miss two-thirds of donors carrying NANB hepatitis. Therefore, about 10 percent of actual donors will still supply NANB-contaminated blood. The argument above depends on which of the following assumptions? (A) Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed. (B) Donors carrying NANB hepatitis do not, in a large percentage of cases, develop the disease themselves at any point. (C) The estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate. (D) The incidence of NANB hepatitis is lower among the potential blood donors than it is in the population at large. (E) The donors who will still supply NANB-contaminated blood will donate blood at the average frequency for all donors. Ps select this psace for OA & my doubts: OA is A. Okay A is assumed here ‘coz some donors might be filtered for carrying other infections in their blood, hence total will be less than 10%. But what is wrong with E? If same donor frequency isn’t assumed than even A doesn’t holds further. Thanks The question could have been more interesting, without OA staring at me like this 
In this argument the concern is about whether about 10 percent of actual donors will still supply NANB-contaminated blood. We are not bothered about frequency of their supply. Hence not E.
It is A because, Donors carrying NANB hepatitis are not caught through other screening tests, and they keep donating the blood. These donors are part of donor pool. The focus is the number/percentage of donors. Not their frequency of donation.
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This question blew the living lights out of me.. can some one please help me understand the question. After knowing the answer, I could understand what the answer is saying.. but all those statistics and percentages in the stem made no sense at all..
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dwivedys wrote: This question blew the living lights out of me.. can some one please help me understand the question. After knowing the answer, I could understand what the answer is saying.. but all those statistics and percentages in the stem made no sense at all..
In total, 15 % of the blood donors have NANB hepatitis.
5% of these total blood donors (not 5% of 15%:wink: )can be caught with screening tests. Remain 10% will keep supplying blood inspite of they having NANB hepatitis.
10% = 2/3 of 15%
All the percentages have total blood donors as base (100%)
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Quote: 10% = 2/3 of 15%
Wonderful...this is what I was missing.. May God Save Me!
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I can see how the ANSWER is A,
I'm pleased I did not see the OA to start with since I went for E as a best fit, but when I read A knowing it is the answer I see where I went wrong!
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Set arbitary numbers:
- # of donars = 300
- # of donars disqualifed = 15
- Since 2/3 of infected donars are not caught, the means 15 = 1/3 (infected donars) and # of infected donars = 45
- 2/3 of infected donars = 30 which is 30/300*100% = 10%
So:
B --> Not important as we're concerned only with what is the catch-rate at this current moment
C --> does not require this assumption as the working above clearly shows it's not an under-estimate
D --> not required to help the conclusion stand
E --> This figure is not affected by who donates blood often. It's the make-up of the donars that affect the percentage given in the conclusion.
A is the best choice because the make-up of the donars will change once someone is disqualified from some other tests.
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I went straight for E but after reading explaination above now realize that correct answer is A:)
U guys are brillent...
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To me, its clear (A).
The NANB test only catches 5% and all the other test do not catch NANB positives so a large number of blood donors are NANB positive (10%).
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ywilfred, ak_idc these really are great explanations. Thanks 
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for me A
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A is an assumption.
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