Biostatistics #19 - Screening tests (part b) - USMLE Forums
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#1
06-10-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269
Biostatistics #19 - Screening tests (part b)

Your screening facility can process 1000 people per week. Assume you are attempting the early detection of a disease with a prevalence of 2% and that your test has a sensitivity of 95% and a specificity of 90%.

1.- How many of the people screened in a week will test positive?

a. 19
b. 98
c. 103
d. 117
e. 200

2. Of these, in how many will the results be truly positive? and in how many will they be falsely positive?
a. 19
b. 98
c. 103
d. 117
e. 200

3. Of these, in how many will they be falsely positive?
a. 19
b. 98
c. 103
d. 117
e. 200

4.- Since a patient tested positive, what is the probability that he has the disease?
a. <0.01%
b. 2%
c. 10%
d. 16%
e. 84%
f. 95%
 The above post was thanked by: aknz (10-08-2011), Amenah (10-08-2011), dinosaur108 (10-08-2011), excellence (05-18-2014), nevillenic (06-10-2011), NGaleas (06-11-2011), praru (06-23-2016), rulz (10-08-2011), struggle (06-10-2011), sumedha (10-08-2011)

#2
06-10-2011
 USMLE Forums Guru Steps History: Not yet Posts: 298 Threads: 45 Thanked 713 Times in 210 Posts Reputation: 723

1. D, 117
2. A, 19
3. B, 98
4. D, 16%

Since prevalence is 2% out of 1000 people per week, 20 people will have disease and 980 will not.

With sensitivity of 95%, 19 people with disease will screen positive (20*.95)
With specificity of 90%, 882 people without the disease will screen negative (980*.90)

This means that there will be 1 false negative (20-19) and 98 false positives (980-882)
 The above post was thanked by: bebix (06-10-2011), dinosaur108 (10-08-2011), dr z (06-14-2011), excellence (05-18-2014), nevillenic (06-10-2011), struggle (06-10-2011), sumedha (10-08-2011)
#3
06-10-2011
 USMLE Forums Guru Steps History: Not yet Posts: 298 Threads: 45 Thanked 713 Times in 210 Posts Reputation: 723

Took me a few minutes to get all the math straight but once I was able to come up with the 2X2 table it made the questions straight forward, assuming I got the table right!
 The above post was thanked by: struggle (06-10-2011)
#4
06-10-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 236 Times in 143 Posts Reputation: 246

I am trying to solve it..its hard though
 The above post was thanked by: bebix (06-11-2011)
#5
06-10-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by struggle I am trying to solve it..its hard though
just give it a try...btw, apx85īs answers are correct
#6
06-10-2011
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 406 Threads: 30 Thanked 140 Times in 108 Posts Reputation: 150

i m trying too
 The above post was thanked by: bebix (06-11-2011)
#7
06-11-2011
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 124 Threads: 5 Thanked 204 Times in 70 Posts Reputation: 214
Ans.

Great question to practice screening tests, using a different approach. Thanks.
1.D.117
2.A.19
3.B.98
4.16%

I agree with apx85, number 4 refers to the PPV, which is calculated with:
PPV= TP/TP+FP
 The above post was thanked by: bebix (06-11-2011)
#8
10-08-2011
 USMLE Forums Master Steps History: Step 1 Only Posts: 566 Threads: 25 Thanked 522 Times in 263 Posts Reputation: 532

Great and i finally get it.. took me a day to review and understand... i hate this type of question... THANKS bebix!

Answers: 1. 98-19 = 117 will be positive
Answer: 2. 19 will be TRUE prositive
Answer 3. 98 will be Falsely Positive
Answer 4. PPV will be 19/19+98 = 16%
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__________________
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 The above post was thanked by: bebix (10-08-2011)
#9
10-08-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by rulz Great and i finally get it.. took me a day to review and understand... i hate this type of question... THANKS bebix! Answers: 1. 98-19 = 117 will be positive Answer: 2. 19 will be TRUE prositive Answer 3. 98 will be Falsely Positive Answer 4. PPV will be 19/19+98 = 16%
Well done!!!!
 The above post was thanked by: dinosaur108 (10-08-2011), rulz (10-08-2011)
#10
10-08-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 623 Threads: 111 Thanked 424 Times in 264 Posts Reputation: 434

Thanks bibex,great work

 Tags Biostatistics-Epidemiology, Step-1-Questions

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