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Hi, i'm studying for USMLE STEP 1 with Kaplan.
I was testing the Qbank for epidemiology and found this question:
The sensitivity and specificity of a common used fecal immunochemical test (FIT) for detection of colorectal cancer (CRC) are 80% and 90%, respectively. The prevalence of CRC is 1% in individuals aged 45-54. Which of the following is the likelyhood tat a 50 YO individual testing positively on the FIT does not have CRC?
A. 1% B. 7% C. 10% D. 20% E. 93% F. 99%
So, i realized they were asking me for the probability of getting a false positive result with the test and i remembered there was this.... measure, called the "False Positive Rate" which was calculated as 1-Specificity.
So, if specificity for this test is 90%, meaning 0.9, then the False positive rate would be1-0.9 = 0.1, meaning 10%, so i went for answer C.
Turns out the answer was E. 93%, and in the explanation they said that you actually needed to calculate de Positive predictive value and then subtract from 100% that PPV and you would get the answer. With the data given in the question, you can get to a PPV of 7% so the answer was 93% (i'll leave a screenshot of the explanations here so you can see for yourselves).
But, they said that option C. 10% was the false positive rate and that it is "the likelyhood thtat an individual without CRC will test positively on the test"... So this is very confusing to me, because... isn't this exactly what they are asking me in the question???????
I was testing the Qbank for epidemiology and found this question:
The sensitivity and specificity of a common used fecal immunochemical test (FIT) for detection of colorectal cancer (CRC) are 80% and 90%, respectively. The prevalence of CRC is 1% in individuals aged 45-54. Which of the following is the likelyhood tat a 50 YO individual testing positively on the FIT does not have CRC?
A. 1% B. 7% C. 10% D. 20% E. 93% F. 99%
So, i realized they were asking me for the probability of getting a false positive result with the test and i remembered there was this.... measure, called the "False Positive Rate" which was calculated as 1-Specificity.
So, if specificity for this test is 90%, meaning 0.9, then the False positive rate would be1-0.9 = 0.1, meaning 10%, so i went for answer C.
Turns out the answer was E. 93%, and in the explanation they said that you actually needed to calculate de Positive predictive value and then subtract from 100% that PPV and you would get the answer. With the data given in the question, you can get to a PPV of 7% so the answer was 93% (i'll leave a screenshot of the explanations here so you can see for yourselves).
But, they said that option C. 10% was the false positive rate and that it is "the likelyhood thtat an individual without CRC will test positively on the test"... So this is very confusing to me, because... isn't this exactly what they are asking me in the question???????
