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#1
03-28-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,406 Threads: 118 Thanked 647 Times in 409 Posts Reputation: 657
The probability that both first and repeated tests are correct?

NOTE: this is a USMLE Consult-"inspired" question, so look away if you plan on using USMLE Consult as a measure of your progress.

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The result of a diagnostic test is negative. The physician orders a retest, which also is negative. The characteristics of the test used in the physician's practice population are shown below. What is the probability that both tests are correct?

Test characteristics
Sensitivity    = 85%
Specificity    = 60%
Positive predictive value  = 68%
Negative predictive value  = 80%

A. 4%
B. 46%
C. 48%
D. 64%
E. 72%
F. 80%
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#2
03-28-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,684 Threads: 213 Thanked 1,600 Times in 660 Posts Reputation: 1610

Wouldn't it just be the Negative Predictive Value which is 80% .. That means that the probability of a Negative test actually being a true negative is 80% OR the probability of the patient not having the disease if she gets a negative test result would be 80%.

I would go with F) 80% ...
#3
04-09-2012
 USMLE Forums Addict Steps History: Not yet Posts: 138 Threads: 1 Thanked 46 Times in 26 Posts Reputation: 56

interesting question. I would also go with E), but a part of me thinks that performing the test twice would change something..

#4
04-09-2012
 USMLE Forums Master Steps History: 1 + CS Posts: 675 Threads: 84 Thanked 428 Times in 207 Posts Reputation: 438

isnt this sort of a probability question?

"BOTH are correct", so X AND Y are correct, so you MULTIPLY the probabilities

X is specificity, and Y is NPV
so 60% * 80% = 48% (ANSWER C)

I have no clue what i just did, but that number came up in my head and its on the answer list, so i thought i'd throw it in here...

Last edited by Renaissance; 04-09-2012 at 12:58 AM.
#5
04-09-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 529 Threads: 57 Thanked 284 Times in 152 Posts Reputation: 294

I think its D 64%

they are asking about the negative predictive value of the test. since it was done 2 times .... so the probability of them both being correct would be 0.8*0.8 = 0.64 or 64%

i may be wrong here
 The above post was thanked by: Dr.NickRiviera (04-09-2012)
#6
04-09-2012
 USMLE Forums Master Steps History: 1 + CS Posts: 675 Threads: 84 Thanked 428 Times in 207 Posts Reputation: 438

Quote:
 Originally Posted by numbndumb I think its D 64% they are asking about the negative predictive value of the test. since it was done 2 times .... so the probability of them both being correct would be 0.8*0.8 = 0.64 or 64% i may be wrong here
that makes sense
#7
04-09-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,406 Threads: 118 Thanked 647 Times in 409 Posts Reputation: 657

NPV x NPV
0.8 x 0.8
0.64
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"inflammable means flammable!? What a country."
 The above post was thanked by: dr.morphine (04-09-2012)

 Tags Biostatistics-Epidemiology, Step-1-Questions

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