User's Manual

i15 Blood Gas and Chemistry Analysis System User Manual Theory
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Chapter 11 Theory
The system utilizes the electrochemical method to determine the concentrations of blood gases
and blood chemistries, and utilizes the conductivity method to determine the concentration of Hct.
Many parameters can also be calculated such as cH
+
, HCO
3
-
act, HCO
3
-
std, BE(ecf), BE(B),
BB(B), and so on.
11.1 Measurement Method
Methods
Measurements are performed on undiluted specimens. Undiluted methods are also called direct
methods, whereas methods that dilute the sample are called indirect methods.
For electrolytes, direct methods measure the free ion concentration of analyte (apparent or free
ion activity) per unit volume of plasma water, and indirect methods measure the concentration of
analyte per unit volume of plasma. It is known that the direct method gives the clinically
significant result for electrolytes. When there is disagreement between the methods, such as when
the patient has abnormal total protein or lipid levels, it results from interference on the indirect
method. At normal levels of protein and lipids the systematic offset between methods is often
corrected for in commercial direct measuring instruments so that the normal ranges for all
instruments are in agreement. Sensors have been calibrated in the manufacture so that normal
ranges are in agreement with indirect reference methods at normal levels of total protein and
lipids.
Direct measurement of hematocrit by the conductometric technique gives a result related to the
non-conducting excluded volume fraction of the sample. Red blood cell volume is the
predominant component of the nonconducting volume, but proteins, lipids, and white blood cells
also contribute. Elevated hematocrit readings are expected at abnormally elevated levels of these
components. Decreased hematocrit readings are expected at abnormally low levels of protein,
such as found in hemodiluted samples taken from cardiopulmonary bypass. Osmotic imbalance
causes a discrepancy between direct (conductometric, spun) and indirect (Coulter) measurements
because of variation in the meancell volume.
Sensors
Sensor refers to the electrode embedded in test cartridges. There are three different types of
sensors: