Biostat question

[stronghold]

---

Members don't see this ad.

β-Thalassemia major results from a homozygous genotype that leads to complete absence of both of the β-globin chains. A study subjected 100,000 participants to an intrauterine screening test; 87 tested positive for β-thalassemia major, and the remaining 99,913 tested negative. In 7 of those 87 cases the results were shown to be false positive. Ultimately, 100 of those originally screened were found to actually have the disease.

Question ID: 1909 USMLE Rx. I did not understand the way they calculated the PPV😕

Also, will I have the time to draw 2X2 for this tricky question? I guess it takes more than 2 minutes!

 

[CassieBagley]

PPV = TP/TP + FP
TP=87-7 = 80. It detected 80 TP out of the total 100 actual cases.
FP=7. It detected 7 FP, the remaining were false negative (100- 80 = 20 false negatives)
PPV= 80/(80 + 7).

 

[stronghold]

Thanks. Oh sorry. I wanted to write "specificity". I did not understand how they calculated it!

 

[sharklasers]

Specificity = TN/ (TN + FP)

The total number of negatives are represented by the denominator. They tell you this is 99900.

So all you need to do is fine TN.

TN = 99900 - FP = 99900 - 7 = 99893

Sp = 99893 / (99893 + 7)

Did you really mean they asked you to calculate specificity, this seems a bit ridiculous for the math they normally make you do lol

 

[stronghold]

Thank you very much. What is your strategy? You just draw 2X2 or search through TN, TP...etc? I am using the former and suffer! Don't you think such calculations are time consuming as they need to be written down?

 

[stronghold]

Thank you very much. What is your strategy? You just draw 2X2 or search through TN, TP...etc? I am using the former and suffer! Don't you think such calculations are time consuming as they need to be written down?