Estimating the likelihood of cancer? - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
Home
USMLE Articles
USMLE News
USMLE Polls
USMLE Books
USMLE Apps
Go Back   USMLE Forums > USMLE Step 2 CK Forum

USMLE Step 2 CK Forum USMLE Step 2 CK Discussion Forum: Let's talk about anything related to USMLE Step 2 CK exam


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 07-06-2012
USMLE Forums Addict
 
Steps History: 1 + CS
Posts: 159
Threads: 52
Thanked 71 Times in 32 Posts
Reputation: 81
Stats Estimating the likelihood of cancer?

This question was in Kaplan CK Qbank and I cannot figure out the explanation, if any one can explain it to me plesae

55 year old man visit his PCP with a complaint of urinary frequency. Examination shows a 1 cm nodule on his prostate. The physician orders a PSA. By common standards, a PSA level greater than 4 is considered abnormal. Using this standard, the test has a sensitivity of 80% and specificity of 90%. A recently published article found that in a cross sectional study, 10% of men of this age have prostate cancer. The result on the patients PSA is 7. What is your best estimate of likelihood that this man has cancer.?

a. 13%
b. 25%
c. 36%
d. 47%
e. 58%
f. 69%
g. 72%
h. 81%
Reply With Quote Quick reply to this message
The above post was thanked by:
svmle (07-07-2012)



  #2  
Old 07-06-2012
USMLE Forums Scout
 
Steps History: Not yet
Posts: 61
Threads: 7
Thanked 82 Times in 32 Posts
Reputation: 92
Default

72%
????
Reply With Quote Quick reply to this message
  #3  
Old 07-06-2012
USMLE Forums Addict
 
Steps History: 1 + CS
Posts: 159
Threads: 52
Thanked 71 Times in 32 Posts
Reputation: 81
Default

nah the answer is D but i want to know how to get to it
Reply With Quote Quick reply to this message
 
  #4  
Old 07-06-2012
USMLE Forums Addict
 
Steps History: Step 1 Only
Posts: 127
Threads: 3
Thanked 113 Times in 62 Posts
Reputation: 123
Default

The question is asking you to calculate the positive predictive value of the test. I think that the best way to calculate PPV is to use a 2x2 square, but some people prefer to memorize the formula.

Let's say you have 100 patients and 10 of them have prostate cancer, so 90 of them are healthy. If you work backwards from sensitivity and specificity, you can fill out this 2x2 chart:


.......................Disease present.......................Disease absent

Positive test ............A..................................... ......B

Negative test............C..................................... ......D

Total.......................10......................... ..................90



Sensitivity = % of patients with the disease who have a positive test
You have 10 patients with the disease, so if the sensitivity is 80%, then you have 8 patients in Box A.

Specificity = % of patients without the disease who have a negative test
You have 90 patients without the disease, so if the specificity is 90%, then you have 81 patients in Box D.

So:
.......................Disease present.......................Disease absent

Positive test ............8..................................... ......B

Negative test............C..................................... ......81

Total.......................10......................... ..................90




And then you can work out the remaining boxes B and C with simple subtraction:
(10 patients with disease) - (8 patients with disease and positive test) = 2 patients with disease and negative test
(90 patients with no disease) - (81 patients with no disease and negative test) = 9 patients with no disease and positive test



.......................Disease present.......................Disease absent

Positive test ............8..................................... ......9

Negative test............2..................................... ......81

Total.......................10......................... ..................90








So, back to the question, which asks you to calculate the percentage chance of your patient having the disease if he has a positive test. Based on our table, 17 patients have a positive test and 8 of them have disease.

Therefore, the PPV of the test is 8/17 = 47%.
Reply With Quote Quick reply to this message
The above post was thanked by:
datson (08-16-2016), greentrees (11-19-2016), Krazy (10-21-2013), mbbs2010 (07-06-2012), svmle (07-07-2012)
  #5  
Old 07-06-2012
mayursn39's Avatar
USMLE Forums Addict
 
Steps History: 1+CK+CS
Posts: 189
Threads: 20
Thanked 506 Times in 99 Posts
Reputation: 516
Smile

i can give it a try.
What they seem to be asking is 'what is the probability that the positive test is actually positive" in other words the PPV
Now we have to construct the 2 X 2 table..
Putting in the values from the question
The table is as
------------------DISEASED----------------- NON DISEASED
POSITIVE TEST..........8................................... ..9
(>4 PSA)

NEGATIVE TEST..........2................................... .81
(4< PSA)

TOTAL.....................10...................... .............90 TOTAL 100


PPV = 8/8+9 = 8/17 = 0.47

so the probability that the test is actually positive and the chance the patient actually has prostate ca is 47 %..
Correct me if i am wrong anywhere...
__________________
Don`t Let Others Tell You What You Cannot Do !!!!
Reply With Quote Quick reply to this message
The above post was thanked by:
Krazy (10-21-2013), svmle (07-07-2012)
  #6  
Old 03-04-2017
USMLE Forums Newbie
 
Steps History: CK Only
Posts: 1
Threads: 0
Thanked 0 Times in 0 Posts
Reputation: 10
Default

wow! i was looking for that for so long!!
please quick question - Does that quotation work only if you select 90 and 10 for total 100 patients? because i tried different number and its not 47 at the end....(
Reply With Quote Quick reply to this message
  #7  
Old 4 Weeks Ago
USMLE Forums Scout
 
Steps History: Not yet
Posts: 40
Threads: 0
Thanked 3 Times in 3 Posts
Reputation: 13
Default

The answer is D
Reply With Quote Quick reply to this message
  #8  
Old 4 Weeks Ago
USMLE Forums Scout
 
Steps History: Not yet
Posts: 34
Threads: 0
Thanked 1 Time in 1 Post
Reputation: 11
Default

47% is the answer
Reply With Quote Quick reply to this message



Reply

Tags
Biostatistics-Epidemiology, Step-2-Questions

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
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.

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.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Likelihood Ratio kalpana USMLE Step 1 Forum 2 02-17-2016 10:24 AM
The likelihood that the difference is due to chance nativedoc2 USMLE Step 1 Forum 11 08-30-2011 07:07 PM
The likelihood that he has the disease? thunderbolt USMLE Step 2 CK Forum 15 05-17-2011 09:28 AM
The likelihood that the patient is having cancer achistikbenny USMLE Step 1 Forum 4 06-29-2010 12:13 AM
Step 3 and Residency Matching Likelihood Steve13 USMLE Step 3 Forum 2 04-19-2010 11:08 AM

RSS Feed
Find Us on Facebook
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)