Concerning serum and plasma reference intervals for potassium:
We use a similar collection tube protocol as David Alter with heparin
plasma for inpatients and serum for outpatients, largely for the reasons he
has mentioned.
We use the same reference interval for both.
A large part of my justification is the variability involved in circulating
potassium, its collection and measurement.
Dr Westgard's site gives a within-person biological variation of 4.6%,
which gives a 95% confidence limit of +/- 0.45 mmol/L at a level of 5 mmol/L.
There are obviously many collection artefacts such as fist clenching which
we do not control for.
The effect of delay between collection and centrifugation/separation may
introduce at up to 0.4 mmol/L up or down depending on the temperature and
the delay.
The median CV for potassium in the RCPA-AACB EQA program in Australia
(including within- and between-run variation) was 2.0%.
Given these possible, and largely uncontrollable variations, it would seem
a poor return for effort to give different ranges for the different fluids.
Best wishes,
Graham
Graham Jones
Staff Specialist in Chemical Pathology
St Vincent's Hospital, Sydney
Victoria St, Darlinghurst, 2010
NSW, Australia
Ph: (02) 9361-2170 Fax (02) 9361-2489
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