Linearity

Here is a question from Kim Kinney at Indiana University Medical Center:

I have a question regarding linearity in coagulation. Since most of our special coag assays are based on standard curves, can you not say that an assay is linear from the low to high standard?  If you want to extrapolate beyond the low to high standard, would you then do a linearity study?  I understand linearity in chemistry where most analytes are a direct measurement but when an assay is based on a standard curve do you still need linearity studies?

Kim, in setting up new assays I first perform the linearity study, set the analytical limits, then choose high and low standards that are 10 to 20% “inboard” from the limits, just to ensure results near these standards are accurate. I’ve also forwarded your question to a couple of method validation experts for further information. Geo.