Abbott i-STAT 1 System Manual - page 452
pH - 2
Art:
714181-00L
Rev. Date: 01-Aug-11
The reference range programmed into the analyzer and shown above is intended to be used as a guide
for the interpretation of results. Since reference ranges may vary with demographic factors such as
age, gender and heritage, it is recommended that reference ranges be determined for the population
being tested.
Clinical Significance
pH is an index of the acidity or alkalinity of the blood with an arterial pH of <7.35 indicating an acidemia and
>7.45 alkalemia.
3
Temperature “Correction” Algorithm
pH is a temperature-dependent quantity that is measured on the i-STAT System at 37°C. The pH reading at
a body temperature other than 37°C can be ‘corrected’ by entering the patient’s temperature on the chart
page of the analyzer. See section 12 ‘Procedure for Cartridge Testing’ in the i-STAT 1 System Manual or
section 11 ‘Patient and Control Sample Testing’ in the i-STAT System Manual for details. In this case, blood
gas results will be displayed at both 37°C and the patient’s temperature. The pH at the patient’s temperature
(T
p
) is calculated as follows
4
:
Note: Patient temperature corrected results are only available on cartridges containing pH, PCO
2
, and
PO
2
sensors.
Performance Characteristics
The performance characteristics of the sensors are equivalent in all cartridge configurations.
The typical performance data summarized below was collected in health care facilities by health care
professionals trained in the use of the i-STAT System and comparative methods.
Precision data were collected in multiple sites as follows: Duplicates of each control fluid were tested in the
morning and in the afternoon on five days for a total of 20 replicates. The averaged statistics are presented
below.
Method comparison data were collected using CLSI guideline EP9-A
5
. Venous blood samples were
collected in evacuated tubes and arterial samples were collected in blood gas syringes with lithium heparin
anticoagulant. All sample were analyzed in duplicate on the i-STAT System and on the comparative methods
within 10 minutes of each other. Arterial blood samples were collected from hospital patients in 3 mL 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
6
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”.
4
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.