This is a theoretical and practical problem that keeps cropping up in our
research laboratory.
Typical issue of the day: "For purposes of FDA use of a particular study, it
is necessary to prove that PTH and 25(OH) Vitamin D are stable for 2 years
at -80C"
Proving stability at -20C is usually possible by comparing paired samples
stored for long periods at -20C and -80C within the same assay. However,
proving stability at -80 is hard to do. In order to show that any minor
instability is not due to assay drift, controls are needed which are
invariably also stored at -80C, rapidly leading to a circular argument. I
have seen use of theoretical arguments based on thermodynamics (ie. if you
know stability at 4C and at 30C you can guess what stability might be
at -80C). Even for assays which can be recalibrated against a gold standard
the problem remains that in the absence of controls, gold-standard
calibration cannot be assumed to result in a stable assay. Does anyone have
any interesting thoughts about this general problem, or have any specific
insight into long term storage of these particular analytes (PTH and Vitamin
D).
Aubrey Blumsohn
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