You could try storage in liquid nitrogen for the controls [alternatively it may be better to put the FDA official in there as this may make his jobsworth hat shatter]. If that doesn't work you will have to try a bose-einstein condensate system.
TIM
**********************************************
Prof. Tim Reynolds,
Clinical Chemistry Department,
Queens Hospital,
Belvedere Rd.,
Burton-on-Trent,
STAFFORDSHIRE,
DE13 0RB,
UK.
tel: 01283 511511 ext. 4035
fax: 01283 593064
email: tim.reynolds@queens,burtonh-tr.wmids.nhs.uk
alternative email for the all too frequent occasions when the NHS email connection doesn't work:
[log in to unmask]
-----Original Message-----
From: p=NHS NATIONAL
INT;a=NHS;c=GB;dda:RFC-822=ACB-CLIN-CHEM-GEN(a)JISCMAIL.AC.UK;
Sent: 07 February 2002 21:39
To: p=NHS NATIONAL
INT;a=NHS;c=GB;dda:RFC-822=ACB-CLIN-CHEM-GEN(a)JISCMAIL.AC.UK;
Subject: Long-term frozen analyte stability - a hard problem
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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------ACB discussion List Information--------
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community working in clinical biochemistry.
Please note, archived messages are public and can be viewed
via the internet. Views expressed are those of the individual and
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