Unfortunately there is no simple solution to this problem. An average difference between serum & plasma potassium can easily be calculated, but there are significant inter-patient differences. We carried out a small study to quantify these effects in our lab;
Blood was taken into Plasma Separating Tubes (PST) and Serum Separating Tubes (SST) at the same time from 32 patients where the 'Renal' profile had been requested. PST & SST tubes were transported to Pathology Reception together, and centrifuged at 3000 rpm for 10 minutes. These samples were processed in the same way as all samples delivered from wards; no 'fast track' treatment was given. Samples were analysed on the day of collection.
Serum potassium was on average 0.13 mmol/l higher than plasma potassium, with a range of -0.1 mmol/l to +0.6 mmol/l. This is close to published data which quotes; NEJM 325, 1107 (1991) Average 0.36, range 0.00 - +0.70; Clin. Chem. 37, 967 (1991) Average 0.20 Range -0.20 - +0.60
NEJM also quotes correlation (r = 0.55) with platelet count, with an increase in potassium of 0.1 mmol/l for every 100x109 platelets (Platelet RR 150 - 400 x 109)
We currently use serum for 99% of all our requests. Of course, the 'correct' procedure would be to use plasma for U&E's, and serum where plasma is unsuitable, but the thought of doubling the number of tubes we handle is daunting.
Paul Waller
Clinical Biochemistry
Northwick Park Hospital
-----Original Message-----
From: p=NHS NATIONAL INT;a=NHS;c=GB;dda:RFC-822=acb-clin-chem-gen-request(a)mailbase.ac.uk;
Sent: Wednesday, July 12, 2000 10:52 PM
To: p=NHS NATIONAL INT;a=NHS;c=GB;dda:RFC-822=acb-clin-chem-gen(a)mailbase.ac.uk;
Subject: Reference range for plasma vs serum potassium
We switched to heparin samples from the Emergency Department to speed up the process and avoid problems with clots but note that plasma potassium values are slightly lower ( by 0.3 - 0.5 mmol/L) than in serum. Some hospitals in this region have also gone to plasma but have not changed from the serum range of 3.5 - 5.0 mmol/L. Would anyone kindly advise on what the plasma range should be? I am sure it must be reported in some text.
Many thanks for your input.
Mario D'Costa Ph.D. FCACB
Director of Clinical Chemistry and Core Lab
St. Joseph's Health Centre, Toronto
Assistant Professor, University of Toronto
Tel: (416) 530-6263
Fax: (416) 530-6559
Email: [log in to unmask]
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|