Transcript MLAB 2401: Clinical Chemistry Keri Brophy

MLAB 2401: CLINICAL
CHEMISTRY
KERI BROPHY-MARTINEZ
Assessment of Acid-Base Balance
BLOOD GASES
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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
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.
performed at 37o C, to emulate
body temperature
 Testing
PREANALYTICAL CONSIDERATIONS
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Air bubbles
Causes increases in pO2, pH
 Causes decreased in pCO2
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Clots
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Can not run clotted whole blood on instrumentation
Glycolysis
Cell respiration causes a decrease in pH, pO2
 pCO2 increases
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Temperature
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pH is temperature dependent. For every 1 degree rise in
temperature, the pH decreases about 0.015 units
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
> 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
INSTRUMENTATION

Electrochemistry
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Ion Selective Electrodes
Hemoglobin Concentration
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Spectrophotometry
DETERMINATION
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Three components are directly measured
pH
 pO2
 pCO2
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Values that can be calculated and reported
include:
Total CO2 or bicarbonate ion
 Base excess
 Oxygen saturation
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PH
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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 > 50 mmHg = HYPO ventilation
 pCO2< 30 mmHg= HYPER ventilation

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

SiggaardAnderson
nomogram
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
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Causes of:
Excess bicarbonate
 Deficit of bicarbonate
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O2 SATURATION
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Calculation/Derived
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Requires measured pH and pO2 values
Measured
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Requires a hgb measurement usually obtained by cooximetry
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Co-oximetry: measuring at multiple wavelengths to get
light absorption spectra
REFERENCES
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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 .
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