Rev. Date: 01-Aug-11 Art: 714180-00K PO2 - 3
Method comparison data were collected using CLSI guideline EP9-A
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. Arterial blood samples were collected
from hospital patients in 3cc blood gas syringes and were analyzed in duplicate on the i-STAT System and
the comparative method within 5 minutes of each other.
Deming regression analysis
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was performed on the first replicate of each sample. In the method comparison
table, n is the number of specimens in the data set, Sxx and Syy refer to estimates of imprecision based
on the duplicates of the comparative and the i-STAT methods respectively, Sy.x is the standard error of
the estimate, and r is the correlation coefficient.* Method comparisons will vary from site to site due to
differences in sample handling, comparative method calibration and other site specific variables.
* The usual warning relating to the use of regression analysis is summarized here as a reminder: For any analyte, “if the data is collected over
a narrow range, the estimate of the regression parameters are relatively imprecise and may be biased. Therefore, predictions made from these
estimates may be invalid”.
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The correlation coefficient, r, can be used as a guide to assess the adequacy of the comparative method range in
overcoming this problem. As a guide, the range of data can be considered adequate if r>0.975.
Precision Data (mmHg)
Aqueous Control Mean SD %CV
Level 1 65.1 3.12 4.79
Level 3 146.5 6.00 4.10
Method Comparison (mmHg)
Radiometer
ABL500
Radiometer
ABL700
Bayer 845
n 45 29 30
Sxx 3.70 2.04 3.03
Syy 2.78 2.64 3.28
Slope 1.023 0.962 1.033
Int’t -2.6 1.2 -2.9
Sy.x 2.52 3.53 3.44
Xmin ---- 39 31
Xmax ---- 163 185
r 0.996 0.990 0.996
Factors Affecting Results*
Exposure of the sample to air will cause an increase in PO
2
when values are below 150 mmHg and a
decrease in PO
2
when values are above 150 mmHg (approximate PO
2
of room air).
Standing anaerobically at room temperature will decrease pH at a rate of 0.03 per hour, will increase PCO
2
by approximately 4 mmHg per hour and will decrease PO
2
at a rate of 2–6 mmHg per hour.
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Do not ice samples before testing – PO
2
results may be falsely elevated in cold samples. Do not use a cold
cartridge – PO
2
results may be falsely decreased if the cartridge is cold.
*It is possible that other interfering substances may be encountered. These results are representative and your results may differ somewhat due to
test-to-test variation. The degree of interference at concentrations other than those listed might not be predictable.
Factors Affecting Calculated Results
sO
2
values calculated from a measured PO
2
and an assumed oxyhemoglobin dissociation curve may differ
significantly from the direct measurement.
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