1. MLAB 2401: Clinical Chemistry Keri Brophy-Martinez
Assessment of Acid-Base Balance

2. Blood Gases Purpose Represents the acid/base status of entire body
Provides information of lung function
Sample type
Whole Blood
Arterial Sample – ABG
Preferred sample
Sites are radial, femoral or brachial artery
Venous & Capillary Blood
Can be used, but not preferred
Assessment performed STAT

3. Specimen Collection & handling
Collected in heparinized plastic syringe
(no air bubbles & no clots!!!)
Often Collected by Respiratory Therapy
Collected anaerobically and put on ice. Ice serves to slow cell metabolism.
Testing performed at 37o C, to emulate body temperature

4. Preanalytical considerations
Air bubbles
Causes increases in pO2, pH
Causes decreased in pCO2
Clots
Can not run clotted whole blood on instrumentation
Glycolysis
Cell respiration causes a decrease in pH, pO2
pCO2 increases
Temperature
pH is temperature dependent. For every 1 degree rise in temperature, the pH decreases about units

5. Reference Values (ABG)
Component
Arterial Blood
Mixed Venous Blood pH
pO2
mmHg
35-40 mmHg
O2 Saturation
> 95%
70-75%
pCO2
35-45 mmHg
41-51 mmHg
HCO3-
22-26 mEq/L
Total CO2
23-27 mmol/L
Base excess
-2 to +2

6. Instrumentation Electrochemistry Hemoglobin Concentration
Ion Selective Electrodes
Hemoglobin Concentration
Spectrophotometry

7. Determination Three components are directly measuredpH
pO2
pCO2
Values that can be calculated and reported include:
Total CO2 or bicarbonate ion
Base excess
Oxygen saturation

8. pH Measurement
Measure of the hydrogen ion activity based on bicarbonate-carbonic acid buffer system
pH electrode has a thin membrane of glass separating two differing H+ concentrations, a H+ exchange occurs in the outer layers of the glass, causing a potential to develop.
A calomel half-cell or reference electrode is also immersed in the solution.
Both the pH and reference electrode are connected through a pH meter. The meter can measure voltage difference between the two and convert to pH units.

9. pO2 Measurement Partial pressure of oxygen in the blood
Measured by the O2 electrode to determine oxygen content
pO2 electrode or Clark electrode measures the current that flows when a constant voltage is applied to the system
As dissolved O2 diffuses from the blood a change in current occurs which offers a direct pO2 measurement

10. pCO2 Measurement Partial pressure of carbon dioxide in the blood
pCO2 measured in mmHg x 0.03 indicates carbonic acid (H2CO3)
pCO2 > 50 mmHg = HYPO ventilation
pCO2< 30 mmHg= HYPER ventilation

11. pCO2 Measurement
The pCO2 electrode or Severinghaus electrode consists of a pH electrode with a CO2 permeable membrane covering the glass surface. Between the two is a thin layer of dilute bicarbonate buffer.
Once the blood contacts the membrane and the CO2 diffuses into the buffer, the pH of the buffer is lowered
Change in pH is proportional to the concentration of dissolved CO2 in the blood

12. Siggaard-Anderson nomogram.

13. Calculated Parameters
Siggaard-Anderson nomogram
Base Excess
Total CO2 and bicarbonate concentration

14. Base Excess Determination of amount of base in the blood
Determines the source of acid-base disturbance
Base deficit usually indicates metabolic acidosis
Causes of:
Excess bicarbonate
Deficit of bicarbonate

15. O2 Saturation Calculation/Derived Measured
Requires measured pH and pO2 values
Measured
Requires a hgb measurement usually obtained by co-oximetry
Co-oximetry: measuring at multiple wavelengths to get light absorption spectra

16. References
Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins.
Carreiro-Lewandowski, E. (2008). Blood Gas Analysis and Interpretation. Denver, Colorado: Colorado Association for Continuing Medical Laboratory Education, Inc.
Jarreau, P. (2005). Clinical Laboratory Science Review (3rd ed.). New Orleans, LA: LSU Health Science Center.
Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson .