The problem may not be due to diffusion in your laboratory. However, some manufacturers may assign calibrator values based on the practices of labs which do not so assiduously handle their samples. Thus, using such calibrators in your lab would cause factitiously high bicarbonate values.
Kent
Kent C. Dooley PhD FCACB
Clinical Chemist
LifeLabs
3201 - 4464 Markham Rd.
Victoria BC V8Z 7X8
tel: 250 881-3100 ext. 2120
email: [log in to unmask]
web: www.LifeLabs.com
-----Original Message-----
From: Clinical biochemistry discussion list [mailto:[log in to unmask]] On Behalf Of COLLINS MICHAEL (RM1) Norfolk and Norwich University Hospital
Sent: Wednesday, February 23, 2011 1:53 AM
To: [log in to unmask]
Subject: Re: Bicarbonate assay
I disagree that the problem is due to diffusion from open samples. Calibration with a primary standard (in our case sodium bicarbonate eliminates the difference between Abbott Architect and other analysers. When we first introduced the Architect results on patient samples were 3mmol/L lower than the Vitros analysers they replaced (samples analysed on Architect first).
Our samples are capped until they go on the track and are only decapped a few minutes before they reach the analyser spurs. Equilibrium with the air in the tube presumably occurs before the sample has been centrifuged. Old analysers typically sampled from the bottom of the sample cup but the sample cups tended to be uncapped in carousels or racks for some time before analysis. The Abbott low results are due only to standardisation.
Mike Collins
BMS3
Biochemistry Automation
Norfolk & Norwich University Hospital
England
[log in to unmask]
http://www.nnuh.nhs.uk/
-----Original Message-----
From: Clinical biochemistry discussion list [mailto:[log in to unmask]] On Behalf Of Dooley, Kent
Sent: 22 February 2011 17:36
To: [log in to unmask]
Subject: Re: Bicarbonate assay
I suspect that much of the confusion around Bicarbonate standardization (we have had similar problems with Siemens bicarbonate)arises from modern chemistry analysers sampling from the top few millimetres of the specimen. Since CO2 can diffuse rapidly into the air but (H2CO3) diffuses more slowly in aqueous solution, one would expect there to be a layer of depleted bicarbonate at the air-liquid interface.
We have conducted a small experiment on our Advia 1800 where we let specimens sit undisturbed on the analyser and have seen significant(sic) decreases in bicarbonate in 30 minutes. The changes can be "eliminated" by sampling mixing the tube.
In many large volume labs samples can sit on a track for considerably longer than 30 minutes.
I suspect that this phenomenon and manufacturers attempts to meet the demands of their largest customers confers confusion on the process of assigning calibrator values.
Kent C. Dooley PhD FCACB
Clinical Chemist
LifeLabs
3201 - 4464 Markham Rd.
Victoria BC V8Z 7X8
tel: 250 881-3100 ext. 2120
email: [log in to unmask]
web: www.LifeLabs.com
-----Original Message-----
From: Clinical biochemistry discussion list [mailto:[log in to unmask]] On Behalf Of Soha Zouwail
Sent: Tuesday, February 22, 2011 2:15 AM
To: [log in to unmask]
Subject: Bicarbonate assay
Dear all,
I wonder if any of the Abbott Architect users are having a problem with the bicarbonate method which is negatively biased by ~4 mmol/L. Abbott told us that no other users have a similar problem! if this is true, I wonder what have they done to solve that, has anyone used a different standard?
Thank you
Soha
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