I had a question on HIT testing back in early August that I lost the thread on, but here is a comment on the HIT assay from Dana Haas, St. Luke’s Episcopal Hospital, Houston, TX that I think answers the question. This came, with Dana’s permission, from Pat Letendre’s Medlab list.
Initially when my institution started doing the PF4 assay, we used the GTI polyclonal assay (this is about 10 years ago), but had problems, so we switched to the Stago kit. Our policy is if the test is positive, we send it out for SRA for confirmation. We found a tremendous number of false positives with the Stago kit; also it had a floating positive cut-off as opposed to a set positive cut-off.
Now that GTI has come out with the IgG kit (positive OD >0.4), we have used that for over 3 years, and our false positive rate has dropped tremendously. When I went to ISTH in Boston last year, I found out that Hyphen also has a PF4 kit and their positive cut-off of 0.5 was what all of the speakers were using for their talks, but I have not used that kit.