MLAB 2401: Clinical Chemistry Keri Brophy-Martinez

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MLAB 2401 Clinical ChemistryKeri Brophy-Martinez

• Assessment of Acid-Base Balance

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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

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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

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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 0.015
  units

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Reference Values (ABG)
Component Arterial Blood Mixed Venous Blood
pH 7.35-7.45 7.31-7.41
pO2 80-100 mmHg 35-40 mmHg
O2 Saturation gt 95 70-75
pCO2 35-45 mmHg 41-51 mmHg
HCO3- 22-26 mEq/L 22-26 mEq/L
Total CO2 23-27 mmol/L 23-27 mmol/L
Base excess -2 to 2 -2 to 2

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Instrumentation

• Electrochemistry
• Ion Selective Electrodes
• Hemoglobin Concentration
• Spectrophotometry

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Determination

• Three components are directly measured
• pH
• pO2
• pCO2
• Values that can be calculated and reported
  include
• Total CO2 or bicarbonate ion
• Base excess
• Oxygen saturation

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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.

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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

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pCO2 Measurement

• Partial pressure of carbon dioxide in the blood
• pCO2 measured in mmHg x 0.03 indicates carbonic
  acid (H2CO3)
• pCO2 gt 50 mmHg HYPO ventilation
• pCO2lt 30 mmHg HYPER ventilation

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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

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Siggaard-Anderson nomogram
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Calculated Parameters

• Siggaard-Anderson nomogram
• Base Excess
• Total CO2 and bicarbonate concentration

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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

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O2 Saturation

• Calculation/Derived
• 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

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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
  .