An old problem that continues to cause us confusion in Lewisham, SE london.
I should appreciate experiences of others.
Since changing from vacutainer lithium heparin (plasma) to clotted blood
(serum) collections for electrolytes approx 2 yrs ago (K+ reference range
increased by +0.3 mmol/L) I am frequently finding problems with spurious
"high" potassium results, > 5.1mmol/L. This results in patient
inconvenience having one or more repeat tests and considerable staff time
spent phoning results. In turn fewer cases of hypokalaemia are seen. The
usual problems of GPs putting samples in fridges, cold transport boxes,
haematological abnormalities, etc have been eliminated. It even occurs when
the departments phlebotomists take the samples, just a few moments from
specimen reception.
Although a strong advocate for li heparin plasma I was encouraged to change
from lithium heparin vacutainers for 2 main reasons.
1 Spurious high plasma total protein on the Kodak 950 compared to serum -
far more than could be accounted for by fibrinogen were causing problems.
2. All but 2 of the other 40 laboratories in the UK South Thames region
used clotted blood.
We continue to use lithium heparin for A&E, ITU, paediatric and "urgent"
requests. We have used two brands of tubes with no apparent difference in
the frequency of hyperkalaemia. Our reference ranges for serum and plasma
were established by analysing paired samples from 200+ blood donors, 100+
each sex. The samples were conventional polypropylene blood tubes, not
evacuated. Samples were separated within 2hrs and each analysed on two
instruments (Kodak 700s) at Lewisham and Guy's. Very few differences of >
0.5 mmol/L were found.
There have been several publications over the past 25 years highlighting
the problems of spurious potassiums on clotted blood but most of us use
them for convenience.
I should appreciate a response on the lines of:
1. What sample type
2. What type/brand of blood collection tubes
3. Approx. incidence of unexpected hyperkalaemia - not related to
haemolysis, delayed sample receipt, clinical conditions in which
hyperkalaemia would be expected.
I shall produce a summary of respnses.
Thanks
Paul Eldridge Phone 44 181 333 3255
University hospital Lewisham Fax 44 181 690 8891
London SE13 6LH UK
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