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#1
08-06-2014
 USMLE Forums Addict Steps History: Step 1 Only Posts: 115 Threads: 16 Thanked 54 Times in 32 Posts Reputation: 64
Specificity and Sensitivity Question

An academic medical center is studying the use of fecal occult blood (FOB) screening for the detection of colon cancer versus the gold standard of colonoscopy. Their study includes 100 patients who were given a fecal occult blood test followed by a colonoscopy 3 weeks later to look for signs of malignancy. After obtaining data for all patients, the researchers report that FOB screening has a sensitivity of 86% and a specificity of 70%. Stem Close Based on these data, which of the following statements is correct?

A.If a patient has colon cancer, their probability of having a positive FOB screen is 86%

B.If a patient has colon cancer, their probability of having a negative FOB screen is 86%

C.86% of patients with a positive FOB screen were found to have colon cancer at colonoscopy

D.70% of patients with a positive FOB screen were found to have colon cancer at colonoscopy

E.If a patient has a negative FOB screen, their probability of having colon cancer is 14%

#2
08-07-2014
 USMLE Forums Scout Steps History: Not yet Posts: 46 Threads: 15 Thanked 9 Times in 9 Posts Reputation: 19

here you need to understand what sensitivity and specificity really means

specificity-if a test positive -> patient really have a disease (mnemonic from FA SPIN-specificity rules IN)
sensitivity-if a test negative -> patient really don`t have a disease (mnemonic from FA SNOUT - SeNsitivity rules OUT)

that`s enough to solve the question:
we know that specificity is 70% so 70% of patients with positive test really will have a disease
 The above post was thanked by: DrNewB (08-08-2014)
#3
08-07-2014
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,109 Threads: 31 Thanked 521 Times in 394 Posts Reputation: 540

I will take A
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In USMLE, “Any fool can know. The point is to understand.”

#4
08-07-2014
 USMLE Forums Scout Steps History: Not yet Posts: 17 Threads: 1 Thanked 2 Times in 2 Posts Reputation: 12

A is correct. This answer describe sesitivity (tp\tp+fn). Sesitivity says about percentage of the patients who actually have a disease with positive test (left column of square).

B is incorrect. This answer describe specificity - probability of having negative result in case where patient actually doesn't have the disease (right column of square).

C and D it's abou PPV and NPV
 The above post was thanked by: DrNewB (08-08-2014)
#5
08-07-2014
 USMLE Forums Scout Steps History: Not yet Posts: 46 Threads: 15 Thanked 9 Times in 9 Posts Reputation: 19

well yes, it looks like i described PPV and NPV, so my bad
#6
08-07-2014
 USMLE Forums Addict Steps History: Step 1 Only Posts: 115 Threads: 16 Thanked 54 Times in 32 Posts Reputation: 64

Sensitivity can be thought of as the percentage of people with the disease that will test positive.

In this case, the sensitivity is reported as 86%, therefore the probability of testing positive if a patient has colon cancer is 86%.

B is not correct.

This answer choice specifies the false-negative rate.

C is not correct.

This statement describes the positive predictive value of the test, or the probability of actually having the disease given a positive test result.

D is not correct.

This statement again describes the positive predictive value of the test. This is not equal to the sensitivity or specificity.

E is not correct.

Good job everyone

Last edited by DrNewB; 08-07-2014 at 03:09 PM.
#7
08-07-2014
 USMLE Forums Addict Steps History: Step 1 Only Posts: 115 Threads: 16 Thanked 54 Times in 32 Posts Reputation: 64

Quote:
 Originally Posted by VVDieiev A is correct. This answer describe sesitivity (tp\tp+fn). Sesitivity says about percentage of the patients who actually have a disease with positive test (left column of square). B is incorrect. This answer describe specificity - probability of having negative result in case where patient actually doesn't have the disease (right column of square). C and D it's abou PPV and NPV
Even I thought that B describes specificity and D is NPV. I read it again and again to understand that point
#8
08-07-2014
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,109 Threads: 31 Thanked 521 Times in 394 Posts Reputation: 540

Please where did u get this question. It looks very potent.
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In USMLE, “Any fool can know. The point is to understand.”
#9
08-08-2014
 USMLE Forums Addict Steps History: Step 1 Only Posts: 115 Threads: 16 Thanked 54 Times in 32 Posts Reputation: 64

Quote:
 Originally Posted by nsesereso Please where did u get this question. It looks very potent.
Hi, Its from UsmleRx. I liked this question so thought to share. I have modified it a little though.
 The above post was thanked by: nsesereso (08-08-2014)

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