Abbott i-STAT 1 System Manual page 561

Method-Dependent / Sample-Specific Component
The method-dependent / sample-specific component results from the specific interferences which affect
different measurement methods. Each sample has specific characteristics, which may cause different
methods to have different types and sizes of measurement error.
The method-dependent / sample-specific component is commonly observed as a random difference
between dissimilar methods. It can, however, also cause an apparent systematic difference component
when a particular sample population has a systematic characteristic. For example, a population of
samples with low mean cell hemoglobin concentration (MCHC) will cause a systematic negative bias on
determinations by some automated hematology analyzers when compared to determinations from both
microhematocrit and conductivity systems.
LIMITATIONS OF THE METHODS
Microhematocrit
Sources of error in the microhematocrit method are discussed in the Procedure for Determining Packed Cell
Volume by the Microhematocrit Method, and include sampling, tube, reading and packing errors.
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Conductivity Methods
Electrolyte Concentration
The conductivity of the whole blood sample is dependent upon the concentration of electrolytes in the
plasma portion. The i-STAT System corrects for the concentration of electrolytes using the measured value
of sodium and potassium. The interference is minimized to an insignificant level.
Other Non-Conducting Elements
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The conductivity method does not distinguish red blood cells from other non-conducting elements such as
proteins, lipids and white blood cells, which occupy volume in the sample. The i-STAT System is calibrated
to read hematocrit accurately when these other elements are at normal levels.
Total Protein
At hematocrit levels less than 40 %PCV, the reading will increase by approximately 1 %PCV for each g/dL
(10 g/L) the protein level is increased outside the normal range of 6.5 g/dL to 8.0 g/dL (65 g/L to 80 g/L).
At hematocrit levels less than 40 %PCV, the reading will decrease by approximately 1 %PCV for each g/dL
(10 g/L) the protein level is decreased outside the normal range of 6.5 g/dL to 8.0 g/dL (65 g/L to 80 g/L). At
hematocrit levels greater than 40 %PCV the interference is about three quarters that size.
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It is important to be aware of the total protein level when using conductivity systems to monitor a
patient on a cardiopulmonary bypass pump. If albumin, or other colloid, is not added to the pump's
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priming solution, the plasma protein will drop by about 3 g/dL to 4 g/dL (30 g/L to 40 g/L). The
conductivity reading will then be systematically low by 3 %PCV to 4 %PCV. For further information
on the use of the coronary bypass mode on the i-STAT System see the Theory section of the i-STAT
or i-STAT 1 System Manual.
It is also important to be aware that the total protein level in premature neonates can be in the range
of 3.6 g/dL to 6.0 g/dL (36 g/L to 60 g/L). Total protein levels may be low in burn patients and in
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patients receiving large volumes of saline-based fluids.
Lipids
Interference from lipids will be about two-thirds the size of the interference from proteins. The protein
interference is larger because it is a charged non-conducting element.
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Art: 714261-01D Rev. Date: 10-Jan-11