what is the chance - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums what is the chance
 Register FAQs Members List Search Today's Posts Mark Forums Read

 USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam

#1
02-06-2012
 USMLE Forums Master Steps History: --- Posts: 1,203 Threads: 180 Thanked 1,251 Times in 441 Posts Reputation: 1265
what is the chance

Q. The blood supply used by a local hospital for transfusions during surgical procedures is discovered to be infected with hepatitis C. To protect the blood supply from future contamination, all new donors are required to be screened for hepatitis C. The test used for this screening has a sensitivity of 95%, and a specificity of 90%. If this test is used on a sample of donors in which 10% are known to have hepatitis C, what is the chance that a donor who tests negative is actually free from the disease? .
 The above post was thanked by: Amenah (02-07-2012), jinni (02-07-2012)

#2
02-06-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 538 Times in 375 Posts Reputation: 548

ans.............D.......
#3
02-06-2012
 USMLE Forums Addict Steps History: Not yet Posts: 191 Threads: 8 Thanked 121 Times in 76 Posts Reputation: 131

It will be negative predictie value for the test B. 95%
#4
02-06-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 538 Times in 375 Posts Reputation: 548

Quote:
 Originally Posted by mohitkmc It will be negative predictie value for the test B. 95%
hey how did you calculate NPV ??

yes you are right its NPV , but could not get it , so thought that sensitivity is the best test to predict a person without a disease and since a few false negative will be present which i take as 5% (100 - 95 % of sensitivity ) so 90% - 5% = 85% ......

thanks ..
#5
02-07-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 160 Threads: 15 Thanked 77 Times in 57 Posts Reputation: 88

u r doing right in thinking
FN=1-sensitivity
1-0.95=0.05
FP=1-specifity
1-0.9=0.1
then u have 0.1 people with the dis so u left with 0.9 without dis (fp+TN) so ur TN is )0.8
now just apply NPV=TN\FN+TN
0.8\0.8+0.05
u ll get around 95%
 The above post was thanked by: Hitman (02-07-2012)
#6
02-07-2012
 USMLE Forums Addict Steps History: Not yet Posts: 175 Threads: 18 Thanked 63 Times in 48 Posts Reputation: 73

Took me 3 minutes for the calculation
Just pray questions like this dont come in the real test
#7
02-07-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 538 Times in 375 Posts Reputation: 548

Quote:
 Originally Posted by jinni A is the answer Took me 3 minutes for the calculation Just pray questions like this dont come in the real test
how did you do it ????????
#8
02-07-2012
 USMLE Forums Addict Steps History: Not yet Posts: 175 Threads: 18 Thanked 63 Times in 48 Posts Reputation: 73

Sensitivity =TP/TP+FN => 95/100 = 10/10+FN

so FN= 10/19
TN +FN = 90 => TN = 90 - 10/19 => TN = 1700/19
NPV = TN/TN+FN
=1700/19/90
1700/1710
=99%
#9
02-07-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 160 Threads: 15 Thanked 77 Times in 57 Posts Reputation: 88

[QUOTE=jinni;92799]Sensitivity =TP/TP+FN => 95/100 = 10/10+FN

so FN= 10/19
TN +FN = 90 => TN = 90 - 10/19 => TN = 1700/19
NPV = TN/TN+FN
=1700/19/90
1700/1710
=99%[/ Hi u can't assume the 0.1 is a TP value The q is saying the disease people not people who have a test positive .
 The above post was thanked by: jinni (02-07-2012)
#10
02-07-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 538 Times in 375 Posts Reputation: 548

[QUOTE=Amenah;92888]
Quote:
 Originally Posted by jinni Sensitivity =TP/TP+FN => 95/100 = 10/10+FN so FN= 10/19 TN +FN = 90 => TN = 90 - 10/19 => TN = 1700/19 NPV = TN/TN+FN =1700/19/90 1700/1710 =99%[/ Hi u can't assume the 0.1 is a TP value The q is saying the disease people not people who have a test positive .
yes Jinni its 10 % of ppl who have Hep C it contains both TP and FN (a+c) , thats correct , the answer is 95 %
the only difference i found in Amenah your and my calculation id that TN is 0.9 not 0.8 I put the value of FP = 0.1 in the equation d/b+d = specificity and got the ans as 0.9 .. still the final ans doesnot change as 0.9/0.9+.05 and 0.8/0.8 + 0.05 both equal to 95% ....
 The above post was thanked by: jinni (02-07-2012)
#11
02-07-2012
 USMLE Forums Addict Steps History: Not yet Posts: 175 Threads: 18 Thanked 63 Times in 48 Posts Reputation: 73

Thanks Amenah and Hitman. You both are correct.

Take the sample to be 100. So

95/100=TP/10
TP = 9.5

NOW FN = 10-9.5 = .5
TN= 100-10=90
NPV = TN/TN+FN
NPV = 90/90+.5 = 90/90.5 = 99%
 The above post was thanked by: Hitman (02-08-2012)
#12
02-08-2012
 USMLE Forums Master Steps History: --- Posts: 1,203 Threads: 180 Thanked 1,251 Times in 441 Posts Reputation: 1265

To answer this question you must construct a 2 x 2 table. Start with a sample of 1,000 because it is a nice, round number and, because screening tests are about ratios, it does not matter what sample size you use. The Table is then completed by computing 10% of 1,000 for the total number of diseased in the marginal (100), taking 95% of this 100 and 90% of the remaining 900 non-diseased part of the sample. With the table complete, Negative predictive value can be computed using the given numbers. The complete table and the negative predictive value calculation are presented below: Negative predictive value = 810/815 or just over 99%. Note that you should be able to estimate that without even actually completing the calculation. To complete the full set of calculations: Note: the test has excellent negative predictive value, but a positive predictive value just slightly better than chance (50%)
 The above post was thanked by: Amenah (02-08-2012), anomali (02-08-2012), docord (02-08-2012)
#13
02-08-2012
 USMLE Forums Master Steps History: Not yet Posts: 928 Threads: 17 Thanked 538 Times in 375 Posts Reputation: 548

Quote:
 Originally Posted by jinni Thanks Amenah and Hitman. You both are correct. But i again calculated and the answer is 99%.Please correct me. Take the sample to be 100. So 95/100=TP/10 TP = 9.5 NOW FN = 10-9.5 = .5 TN= 100-10=90 NPV = TN/TN+FN NPV = 90/90+.5 = 90/90.5 = 99%
wow ! now it looks so simple , nice one , should have taken a sample , i guess i made a mistake by taking FP = 1 - specificity , otherwise i did the same calculation upto finding FN then i found FP from the above formula and placed it in d/b+d then found d ie TN as 0.9 ..... but have made a mistake in decimal point , should have taken a sample ... thanks ..
#14
02-08-2012
 USMLE Forums Scout Steps History: Not yet Posts: 36 Threads: 5 Thanked 8 Times in 6 Posts Reputation: 18

\%
Quote:
 Originally Posted by jinni Thanks Amenah and Hitman. You both are correct. But i again calculated and the answer is 99%.Please correct me. Take the sample to be 100. So 95/100=TP/10 TP = 9.5 NOW FN = 10-9.5 = .5 TN= 100-10=90 NPV = TN/TN+FN NPV = 90/90+.5 = 90/90.5 = 99%
dear jinni,
however the answer is 99%,but it finally comes from 81/81.5 not from 90/90.5.the problem with your calculation is TN#100-10.why?
assume there are 100 people(no matter whatever you take the sample size though).now 10 people are diseased but 90 are nondiseased,not necessarily TN.in other words,this doesnt give us TN=90,rather tells us TN+FP=90.
in the other hand,TN=9FP(can be extracted from specifity).now,we know two things about the TN&FP relationship:
a.TN=9FP
b.TN+FP=90
from these two===>TN=81 FP=9

the complete table assuming n=100 would be :
TP=9.5 FN=0.5 FP=9 TN=81
NPV=0.993
PPV=0.513

Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
User Name:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
 AMG IMG ---
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

 Not yet Step 1 Only CK Only CS Only 1 + CK 1 + CS 1+CK+CS CK+CS 1+CK+CS+3 ---
Favorite USMLE Books
 What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
 Where you live. Leave blank if you don't want to tell.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

 Similar Threads Thread Thread Starter Forum Replies Last Post rickygbv IMG Residency Match Forum 5 01-16-2012 02:35 PM tenderhearts30 IMG Residency Match Forum 1 07-26-2011 04:13 PM mg2222000 IMG Residency Match Forum 7 05-05-2011 06:19 AM abii USMLE Step 1 Forum 3 11-03-2009 03:35 PM yellowsand IMG/FMG Forum 4 10-11-2009 12:15 PM

 Home - Contact Us - Forum Rules - Advertise - Mobile Site - Archive - Privacy Statement - Top
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2014)