From Maria Franco, Orlando Health: Hi, George, when running factor assays (1:10, 1:20, etc), is the agreement between results to r/o inhibitory activity all calculated from the original result (1:10)? Not sure what the most common or recommended practice is. I have heard to find the mean of all results and then try to calculate the deviation. Thanks, Maria.
Hello, Maria, and thank you for your question. The correct method is to compare the result from each dilution with the original (1:10) dilution. If a subsequent dilution, for instance, 1:20, 1:40, or 1:80 produces a result that exceeds the 1:10 dilution result by more than a set percentage, you may suspect the presence of an inhibitor. Many laboratory directors choose 10% as their limit, though I’ve seen presentations where 15% is recommended. This question is also addressed with a little more detail in our audio module, Factor Assays. Once you suspect an inhibitor, the next step is a Bethesda titer. If any of our participants has another approach, please comment below. Geo.
Added September 14: I spoke with Patti Tichenor and Laura Taylor in the University of Alabama at Birmingham Hospital special coagulation lab, and they reminded me that you should also watch for rising values as the dilution increases. For instance, if you see a result of 10% factor VIII in the 1:10 dilution,13% in the 1:20 dilution, 19% in the 1:40, and 24% in the 1:80, that is stronger evidence than say 10%, 13%, 14%, and 15%, even though the change from the 1:10 dilution to the 1:20 jumped by 30%.