Roger,
 
Thanks. Unfortunately we are seeing a difference in the opposite direction, i.e. the measured total CO2 is lower than the calculated actual bicarbonate by up to 5 mmol/L in the children on whom we are measuring it. The difference appears to correlate reasonably closely to the estimated carbonic acid/dissolved CO2 concentration. This may suggest that during the centrifugation and separation stage (with exposure to air) that the carbonic acid fraction is being lost as CO2 to the atmosphere. Alternatively the calculated bicarbonate is inaccurate due to changes in pK in ill patients.
 
Thanks
 
Paul Griffiths
Birmingham Children's Hospital
-----Original Message-----
From: Bertholf, Roger [mailto:[log in to unmask]]
Sent: 26 April 2005 14:36
To: [log in to unmask]
Subject: Re: Blood gases query

Unless the Olympus CO2 method is unique, and I don't think it is, it does not measure the bicarbonate concentration. It measures all forms of CO2: bicarbonate, dissolved CO2 gas, carbonic acid, and carbonate. Methods that measure bicarbonate consume the anion and therefore shift the equilibrium in that direction. Methods that measure CO2 do likewise, in the opposite direction. The result is that all forms of CO2 are measured. Therefore, a measured CO2 on a chemistry analyzer will always exceed the calculated bicarbonate, assuming pH and temp are correct in the calculation. The difference is small, since dissolved CO2 accounts for only about 2 mmol/L of total CO2. But there *is* a difference between total CO2 measured on a chemistry analyzer and the bicarbonate concentration calculated by the Henderson-Hasselbalch equation.
 
Roger Bertholf
University of Florida Health Science Center/Jacksonville
-----Original Message-----
From: Clinical biochemistry discussion list [mailto:[log in to unmask]] On Behalf Of Griffiths Paul (RQ3) BCH
Sent: Tuesday, April 26, 2005 8:20 AM
To: [log in to unmask]
Subject: Re: Blood gases query

But which is the correct answer?
And why should they be different?
 
-----Original Message-----
From: Mainwaring-Burton Richard (RGZ) [mailto:[log in to unmask]]
Sent: 26 April 2005 13:11
To: 'Griffiths Paul (RQ3) BCH'
Subject: RE: Blood gases query

Why not simply put these few serum samples through a blood gas analyser ?

 

with best wishes
Richard

Richard Mainwaring-Burton
Consultant Biochemist
Queen Mary's Hospital
Sidcup, Kent
DA14 6LT
020-8308-3084

-----Original Message-----
From: Griffiths Paul (RQ3) BCH [mailto:[log in to unmask]]
Sent: 26 April 2005 13:14
To: 'Mainwaring-Burton Richard (RGZ)'
Cc: ACB mailbase (E-mail)
Subject: RE: Blood gases query

 

Richard,

 

We only do it on a few samples where it is specifically requested, i.e when looking for metabolic acidosis. The suggestion from Adedoyin et al was that using measured serum bicarbonate resulted in an overestimate of the number of patients with possible renal tubular acidosis. Further investigation of these patients showed that the vast majority did NOT have RTA.

 

Paul

 

-----Original Message-----
From: Mainwaring-Burton Richard (RGZ) [mailto:[log in to unmask]]
Sent: 26 April 2005 13:02
To: [log in to unmask]
Subject: Re: Blood gases query

Why not drop bicarb on the main analyser ? (see previous discussion)

That way you save a bundle of money and also the aggravation of spotting the differences !

 

with best wishes
Richard

Richard Mainwaring-Burton
Consultant Biochemist
Queen Mary's Hospital
Sidcup, Kent
DA14 6LT
020-8308-3084

-----Original Message-----
From: Griffiths Paul (RQ3) BCH [mailto:[log in to unmask]]
Sent: 26 April 2005 10:29
To: [log in to unmask]
Subject: Re: Blood gases query

 

Mohammed,

 

We have also recently been looking at this problem. We are finding a significant difference (up to 5mmol/L) between actual bicarbonate calculated on a Bayer 865 blood gas analyser (using whole blood) and measured bicarbonate on an Olympus AU640 (using serum/plasma separated and measured as soon as possible). Other groups have reported this difference (Adedoyin et al, Pediatrics Vol 112, No.6, p463, Dec 2003 suggested that serum bicarbonate measurements were incorrect due to shifts in the equilibrium as a result of the centrifugation/separation process), whereas other groups (Rosan et al, Clin Chem 29/1 69-73, 1983) have suggested that the pK value is not constant in ill patients and that the calculated blood gas bicarbonate is unreliable.

 

We would welcome any comments or suggestions to help explain this difference.

 

Paul Griffiths & Daniel Herrera

Birmingham Children's Hospital

 

-----Original Message-----
From: Mohammad Al-Jubouri [mailto:[log in to unmask]]
Sent: 26 April 2005 09:43
To: [log in to unmask]
Subject: Blood gases query

Dear All

 

Blood gas analysers measure pH and PCO2 using specific electrodes and calculate actual bicarbonate using various algorithms (based on Henderson Hasselbalch equation). It is unclear to me how the calculation of actual bicarbonate varies during acute vs chronic vs mixed acid base disturbances, if it is based on the Henderson equation only? Also can manufacturer's have built in software to provide an interpretation guide for a given set of blood gas readings, or is this considered too risky by them?

 

thanks

 

Mohammad

 

 



Dr. M A Al-Jubouri
Consultant Chemical Pathologist

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