Abbott i-STAT 1 System Manual page 560

Conductivity Methods
Systems using the conductivity method, such as the i-STAT System, measure the electrical conductance of
a whole blood sample. Plasma conducts electrical current, and blood cells act as insulators. A sample with
a relatively high hematocrit has, by definition, a larger proportion of its volume filled by the non-conductive
red blood cells. The overall conductance of the sample will thus be relatively low. In the i-STAT System,
before the measured sample conductance is converted into a hematocrit value, corrections are applied for
the temperature of the sample, the size of the fluid segment being measured, and the relative conductivity
of the plasma component. The first two corrections are determined from the measured value of the calibrant
conductance; the last correction is determined from the measured concentrations of sodium and potassium
in the sample.
i-STAT provides two customization settings for reporting hematocrit results: The "K EDTA" customization
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reports hematocrit results traceable to MH-K EDTA. The "K EDTA" customization reports hematocrit results
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traceable to MH-K EDTA.
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For best agreement of i-STAT and hematology analyzer hematocrit results, the i-STAT customization
setting is selected according to the calibration of the comparative hematology analyzer (MH-K EDTA or
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MH-K EDTA).
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When this Technical Bulletin was first released, the manufacturers of the Advia , Cell-Dyn , Coulter , and
® ® ®
Sysmex , hematology analyzers used samples collected in K EDTA for the purpose of calibrating their
®
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systems to reference methods. The manufacturers of Advia, Cell-Dyn and Sysmex now use samples
collected in K EDTA for the purpose of calibrating their systems to reference methods. Therefore, for best
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agreement between these analyzers and the i-STAT System, the K EDTA customization setting should be
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selected. (Note: The default setting on the i-STAT System is K EDTA.)
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Automated Hematology Analyzer Methods
Automated hematology analyzers do not directly measure hematocrit, but rather calculate hematocrit from
measurements of individual cell sizes and counts. The sample is diluted with an isotonic buffered aqueous
solution and passed through a measuring orifice. Individual cells are counted and sized by one of two basic
mechanisms. Either fluctuation in electrical conductivity or the scatter of collimated light, is measured as
cells pass through the orifice. The hematocrit of the original sample is calculated from the number of cells,
the sizes of the cells and the volume of diluted solution passing through the orifice.
Co-oximetry Methods
Like hematology analyzers, co-oximeters do not directly measure hematocrit. Hematocrit values are
calculated from hemoglobin values that are measured by spectrophotometry. A fixed relationship between
hemoglobin and hematocrit is used to calculate hematocrit.
METHOD COMPARISONS
The discrepancy between the hematocrit values determined by two different systems on an individual
sample has three components: a random component, a systematic component and a method-dependent /
sample-specific component.
Random Component
The random component results from the combination of each system's imprecision. The size of the
component is different on each determination. The imprecision of the i-STAT System is typically about
0.6 %PCV to 0.7 %PCV (1 standard deviation).
Systematic Component
The systematic component results from differences in the systems' calibration, and is constant from sample
to sample.
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Art: 714261-01D Rev. Date: 10-Jan-11