Regarding the use of serum for ionised calcium.
Tietz has some useful comments (p 1403 of 3rd Edition of Tietz Textbook of
Clinical Chemistry). He suggests that a serum separator tube, once
centrifuged and unopened remains suitable for ionised Ca for "hours at 25°C
and days at 4°C".
We have confirmed this for both full and partly filled tubes up to 24 hours
at RT. There was minimal pH variation over this time and correcting for pH
made no significant difference.
I gather there are some NCCLS guidelines which offer similar advice: Use
serum separator tube; fill tube to capacity; centrifuge within 4 hours; do
not open until ready to test (possibly collect a separate tube).
I hope this is helpful.
Graham Jones
Staff Specialist in Chemical Pathology
St Vincent's Hospital Sydney
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