Art: 714382-00D Rev. Date: 02/20/06 20-5
DETERMINATION OF CELL CONCENTRATION
CPB
Each time a cartridge containing a hematocrit sensor is used, the operator has
the option of selecting, in addition to the sample type, the CPB compensation
algorithm for samples with abnormally low protein levels. The CPB option
is specifically intended for use when samples are collected from patients on
cardiopulmonary bypass. However, the facility may validate its use for other
patient populations known to have protein levels significantly lower than the
normal adult population.
The CPB algorithm infers the total protein level by assuming the pump priming
solution dilutes the hematocrit and total protein equally. Modeling the pre-
pump hematocrit as 43 %PCV and the pre-pump total protein as 7.0 g/dL, the
following graph indicates the inferred total protein and resultant correction.
It is known that direct methods read up to 7% higher than indirect methods
in measuring the concentration of electrolytes. This is because there is an
excluded volume occupied by plasma protein and lipids that is not considered
in indirect measurements. Typically, however, the elevation of results is less
than the full 7% because some of the analyte is bound to protein and other
ions, and is not assayed by direct methods. For each analyte this discrepancy
is characterized, and the result of the direct measurement is adjusted so that
normal ranges are in agreement with indirect reference methods at normal
levels of total protein and lipids.
Hematocrit
In whole blood, plasma conducts electricity while the cellular constituents, red
and white blood cells and platelets, do not. For a sample of a given electrolyte
concentration, as the number of cells per unit volume of plasma increases, the
conductivity of the sample decreases. The total cell concentration in whole
blood can, therefore, be determined from:
1) the known electrolyte concentration of the calibrant,
2) the measured electrolyte concentration of the sample,
3) the measured conductivity of the calibrant and
4) the measured conductivity of the sample.
These measured quantities are determined using a combination of
potentiometric and conductometric sensors.
Direct measurement of hematocrit by the conductometric technique gives a
result related to the non-conducting excluded volume fraction of the sample
fluid. Red blood cell volume is the predominant component of the non-
conducting 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
patients on cardiopulmonary bypass.
Osmotic imbalance causes a discrepancy between direct (conductometric,
spun) and indirect (Coulter) measurements because of variation in the mean
cell volume.
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