Dear Epi 204,
During the course review session yesterday, I said "Double Gold Standard Bias" and then skipped over Double Gold Standard Bias and launched into an accurate description of Imperfect Gold Standard Bias. Double Gold Standard Bias is also called Differential Verification Bias. This is when patients with a positive index test get the immediate, invasive gold standard and patients with a negative index test get clinical follow up. Bias occurs if there are some patients on whom the invasive gold standard would give a different answer than would clinical follow-up. For example, in some patients, the invasive gold standard would be positive but clinical follow-up would be negative. On these patients, the index test can't be wrong. If the index test is positive, the patient gets the invasive gold standard, which is also positive. If the index test is negative, the patient gets clinical follow-up, which is also negative. In this scenario, Double Gold Standard Bias falsely increases both sensitivity and specificity relative to a study that uses a single gold standard (either the invasive standard in everybody or clinical follow-up in everybody).
Imperfect Gold Standard Bias is also called Copper Standard Bias. This is when the study's single reference standard for categorizing D+ vs. D- doesn't necessarily give the "true" disease status. Yesterday morning, my description of this bias was correct, but again, I initially mislabelled it. I hope you won't confuse bias due to the use of a single copper standard with bias due to the use of two different gold standards depending on the results of the index test.
Both Double Gold Standard Bias and Copper Standard Bias are covered in EBD-2 Chapter 4 and the Kohn/Carpenter/Newman paper, both of which are posted on the course website. Tom's lecture, which I just finished re-watching at 1.6X, is also very clear, as always. I got a laugh when, near the end of the lecture, he was talking about a study of diagnosing pertussis and said, "It is not until you look at the results that you can tell this study is complete garbage."
Best,
MAK