Les Culank enquired about evidence-based standards for GP specimen transport,
for example specifying limits of time, temperature, maybe other relevant
factors, to avoid spurious elevation of measured serum potassium?

A few years ago we addressed this question for a range of common analytes.
We made aliquots of heparinised whole blood taken from the usual laboratory
volunteers (n=4) and stored them for varying periods of time at 4,15,25 and
37 degrees. The time periods were 4, 8, 12, 24, 48 and 72 hours. At the end
of these time points the samples were separated and the plasma stored until
analysis (Roche Hitachi analyser). For each analyte the changes were
expressed relative to the concentration at time zero. From this data we
then determined allowable times for samples to remain uncentrifuged at the
various temperatures. For "room temperature" we used the data of 15 and 25
degrees and the other temperatures for greater extremes.

As expected the glucose and potassium measurements were the most unstable.
Potassium was interesting and we clearly saw the phenomenon described
elsewhere where the plasma potassium falls before it rises in whole blood
stored at room temperature. For these two analytes our limits are 2 hours
for glucose and 4 hours for potassium. Between these times and 8 hours we
issue the result but with a comment that the true glucose may be 0.5 to 2
mmol/L higher than the measured value (and recommend Grey top) and the
potassium may be 0.5 mmol/L higher or lower than the measured result.
Beyond 8 hours we do not release results for either of these analytes. We
have set limits for most common analytes which allows us to release some
results on samples uncentrifuged fopr several days if needed.

The setting of limits based on the data is my summation alone. I find this
a very problematical area. Any error introduced because of delay is
obviously added to any other errors already present, however being too
strict and not releasing results annoys doctors and patients and destroys
potentially useful data. Personally I am opposed to comments such as
("delayed separation, potassium may be affected") as the clinician can have
no basis for deciding on the magnitude of any possible effect.

Potassium is especially difficult because of the combined effects of
falling then rising levels which MAY give no change after 24 hours (see 25
degree data). I am still grappling with the best manner to handle this.

I will make our protocol available on our website for a period of time if
people are interested (www.sydpath.stvincents.com.au) look under "NEW" on
the home page. It is a word document so you will need word 97 or newer to
see it. I will put the raw data for potassium there as well (excel
speadsheet). It may take a day or two for this to happen. I would
appreciate any thoughts or feedback.

This data was presented at the Australasian Association of Clinical
Biochemistry meeting in 1997 and the abstract is published in CLin Biochem
Revs for that year.

Best wishes,

Graham
Graham Jones

Staff Specialist in Chemical Pathology
St Vincent's Hospital, Sydney
Victoria St, Darlinghurst, 2010
NSW, Australia
Ph: (02) 8382-2170  Fax (02) 8382-2489
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