Following on the debate about potassium and temperature:
A letter to the Lancet in 1996 (Gibson E (1996) BMJ 313:943) noted that the
reference interval for potassium in Barbados was lower than for more
temperate climes (2.8 - 4.1 mmol/L). The author commented on the
possibility of environmental or dietary factors possibly being involved. A
good place for further study perhaps. A pity Tim Hull is already in Riyadh.
We have studied the effect of temperature on plasma potassium with
different times between collection and centrifugation. At 25 degrees the
maximum fall in plasma potassium (compared to immediate centrifugation) was
0.6 mmol/L at 12 hours after collection, at 37 degrees the maximum fall was
0.7 mmol/L at 2 hours. (Warning, n only = 4). Obviously if centrifugation
was further delayed this was followed by a rapid rise as potassium left the
red cells. At lower temperatures there was no fall seen in plasma potassium.
It would seem possible that the apparent hypokalaemia seen at higher
temperatures may also be dependent on the time between collection and
centrifugation and can occur within usually acceptable centrifugation times.
I would also like to comment on Tim Hull's point about the sum of all the
possible errors in potassium measurement:
Firstly to add a further 0.3 mmol/L for haemolysis (our cuttoff for
allowable effect);
Secondly to say that this does not include unmeasurable effects such as
excessive fist clenching with the tourniquet in place;
Thirdly that while the chance of all of these co-existing in a sample is
very low, we do so many potassium measurements it surely will happen;
and fourthly (as well all teach our students) that an unexplained high
potassium should be repeated, urgently if required (ECG as well) as there
are so many causes of an artefactual hyperkalaemia and so few for an
artefactual hypokalaemia (my list was wrong sample, drip arm, and I will
include delayed separation at higher temperatures).
Graham
At 12:36 PM 7/14/00 +0100, Everitt, Tony wrote:
>Just to add to the potassium reference range debate, there was an
>interesting article regarding an increase in the number of cases of
>hypokalaemia amongst GP patients during the hot summer of 1995. (Masters PW,
>Lawson N, Marenah CB and Maile LJ. High ambient temperature: a spurious
>cause of hypokalaemia). It was postulated that high ambient temperatures
>during sample delivery to the laboratory were responsible for the increase
>in hypokalaemia by stimulating cellular uptake of potassium.
>
>The study was conducted using plasma samples and I looked at our laboratory
>data for serum samples. There was a fall in mean potassium levels in GP
>samples of approx. 0.3 mmol/L during the summer months of 1995, which was
>not mirrored for in-patient samples.
>
>Tony Everitt
>Basildon Hospital
>
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Graham Jones
Staff Specialist in Chemical Pathology
St Vincent's Hospital, Sydney, Australia
Ph: (02) 9361-2170 Fax (02) 9361-2489
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