This document discusses acid-base physiology and pre-analytical errors in arterial blood gas sampling. It defines acid-base balance and the buffers that maintain homeostasis. Respiratory and renal systems regulate pH through mechanisms like hyperventilation and ion transport. Pre-analytical errors in sampling, transport, or storage can affect blood gas results and mislead diagnosis if not addressed. Proper patient identification, rapid analysis, and avoiding air bubbles or clots are important to obtain accurate values.

1. Acid Base Physiology & Laboratory aspects of
ABG
Dr.Rittu Chandel
Junior Clinical Biochemist
Breach Candy Hospital Trust, Mumbai

2. OVERVIEW
• Physiology
• Pre analytical errors

3. ACID BASE DEFINITION
:
Definition Acid Base
Arrhenius increases the concentration of
hydrogen ion (H+) when dissolved
in water
increases the
concentration of hydroxyl
ion (OH−) when dissolved
in water
Bronsted Lowry donates a proton in a reaction accepts a proton in a
reaction.
Lewis accepts a pair of electrons to form
a covalent bond
donates a pair of electrons
for a covalent bond

4. DEFINITIONS
• pH-negative logarithm of hydrogen ion activity
• Strong Acids-acids which ionize completely
• Weak Acids-acids which ionize incompletely
• Buffers-Solutions which resist change in pH

5. EQUATIONS

6. To maintain a constant pH, HCO3- /PCO2 ratio should be constant
When one component of the ratio is altered, the compensatory response
alters the other component to keep the ratio constant

7. REGULATION
• First line of defense : Blood buffers
• Second line of defense : Respiratory regulation
• Third line of defense : Renal regulation

8. BUFFERS OF BODY FLUIDS
Substances which react with an acid or base to minimise change in
the pH…..thus maintaining acid-base homeostasis
“isohydric principle”:
at a given moment, all buffer systems which participate in defence of
acid-base changes are in equilibrium with each other

10. RESPIRATORY REGULATION
• Rate of respiration is controlled by
chemoreceptors in respiratory center which
are sensitive to changes in pH
• In acidosis, respiratory rate is stimulated
resulting in hyperventilation.

11. RENAL REGULATION
• Excretion of H+
• Reabsorption of Bicarbonate
• Excretion of Titrable acid
• Excretion of NH4
+

12. Excretion of H+

13. Reabsorption of Bicarbonate

14. Excretion of Titrable acid

15. Excretion of NH4+

16. PRE ANALYTICAL ERROR
• Patient identification
• Sampling
• Sample transport
• Nature of the sample

17. PATIENT IDENTIFICATION
• Proper labeling is must
• Unique patient identifier bearing name, date
and collection time

18. SAMPLING
• Painful sampling may result in pain,
anxiety and induced hyperventilation
which can result in reduced pCO2
• During sampling from arterial line or
arterial catheter there is risk of dilution
with flush solution
• There are chances of puncturing a vein
and mixed blood can be obtained
• Using too much heparin

19. AIR BUBBLES
• Room air has a pO2, of approximately 150 mm Hg
(sea level) and a pCO2 of essentially zero
• Thus, air bubbles that mix and equilibrate with
arterial blood will shift the paO2 toward 150 mm
Hg and will lower the paCO2
• without air bubbles with air bubbles
• pO2 70 mm Hg pO2 90 mmHg
• pCO2 45.6 mmHg pCO2 45.4 mmHg

20. CLOTTING
• Blood sample should be slowly mixed
between the palms to avoid clotting
• Clotting can block the sample pathway of the
blood gas analyzer
• This can give error, need resampling and delay
the future samples

21. PROLONGED STORAGE
• Cellular metabolism continues after the blood is
collected
• Analyze the sample immediately or due to cellular
metabolism the following picture is seen
Leukocytes and platelets continue to consume oxygen in
the sample after it is drawn and can cause a significant
fall in paO2 over time at room temperature, especially in
the setting of leukocytosis or thrombocytosis.
• Drop in pH, glucose , pO2
• Rise in the values of lactate, ionized calcium and pCO2

22. Lab receives arterial blood sample from emergency department.
Blood gas testing is requested. Sample was transported by
pneumatic tube within 10 minutes from sampling. You notice an
air bubble in the syringe.
What would you do?
a) Sample is acceptable. I would expel the bubble and perform
the analysis.
b) Sample is not perfect, I would expel the bubble and perform
the analysis. I would report the result with a comment.
c) Sample is not acceptable. I would reject the sample and
request repeated sampling.
d) I would call a physician and ask him to decide what to do.

23. 8:00 a.m.
lab receives arterial blood sample, for blood gas testing for an
ICU patient. Sample has been delivered to the lab in a plastic
syringe, on ice.Sampling time was 6:30 a.m.
What would you do?
a) Sample is acceptable. I would thouroughly mix the sample and
perform the analysis.
b) Sample is not perfect, but I would accept it for analysis after
thouroughly mixing it. I would report the result with a comment .
c) Sample is not acceptable. I would reject the sample and
request repeated sampling.
d) I would call a physician and ask him to decide what to do.

24. TAKE HOME MESSAGE
• The ratio HCO3- /PaCO2
• Pre analytical errors create :
• Misdiagnosis
• Missed diagnosis
• Delayed diagnosis

25. THANK YOU
me@drrittuchandel.com
www.drrittuchandel.com