All performance data in this section was generated on OPTI CCA-TS systems with the SRC run daily to check QC. Quality control material was run with each new lot of cassettes.
2 Tietz; Burtis C, et al (Eds.), Textbook of Clinical Chemistry and Molecular Diagnostics, 4th Ed., (Elsevier
8.5.9 Limitations
The performance characteristics are affected by the following sample considerations:
The preferred test liquid is whole human blood for all parameters. It is necessary to tonometer blood to obtain values to evaluate accuracy of PO2 and PCO2 because patient samples must be considered to be unknown. Tonometry of blood introduces potential errors unrelated to the blood gas system being evaluated. Accuracy of the gas values used, temperature control and thermostatting of the tonometer, humidification of the tonometry gases, duration of tonometry and transfer of the sample from the tonometer to the instrument for analysis are examples of potential pre-analytical error.
pH of blood cannot be predicted in tonometry. All tonometered samples analyzed in these studies were analyzed in duplicate on an AVL 995 to establish correlation. Precision of PCO2 and PO2 measurement, as well as pH was evaluated over a 20 day period using two OPTI CCA-TS systems with two replicates per run using a commercially available solution of reduced bovine hemoglobin which has been
demonstrated to be comparable to tonometered whole blood.3
The OPTI CCA-TS system is designed to measure whole blood, serum, or plasma, to be controlled with Standard Reference Cassettes on a daily basis, and with aqueous solutions for each new lot of cassettes. Aqueous controls are portable and quite convenient to use with the OPTI CCA-TS system, however, their low oxygen carrying capacity and temperature sensitivity is well known. Measurements of such materials are more prone to pre-analytic error as well as analyzer-specific errors, compared to similar measurements of whole blood. The OPTI CCA-TS system is no exception to this, and demonstrates somewhat poorer
PO2 precision with aqueous controls than with whole blood, due to the large amount of plastic material comprising its disposable measurement chamber.
The OPTI CCA-TS’s tHb measurement is sensitive to pathologically rapid sedimentation rates of the erythrocytes, often induced by excessive rate and amounts of rouleaux formation4. This is observable as
rapid sedimentation and clarification due to erythrocyte aggregates falling to the bottom of the syringe within minutes of mixing. The OPTI CCA-TS breaks up most of the rouleaux and other aggregates by rapidly aspirating the whole blood sample with high shear rate, however in rare pathologic cases the rouleaux aggregates persist or reform during the aspiration and cause a positive tHb offset of up to 3 g/dL, typically within the range 7-12 g/dL.
Any measurement outside the Measurement Range will be indicated on the display as ‘ LOW ‘ for values lower than the range and ‘ HIGH ‘ for values above the range. However, the printed report will show out- of-range values with reference to the end value of the measurement range; for example, the printed report will show a PCO2 value of 220 mmHg as:
PCO2 > 200 mmHg (Meas.Lim)
For measurement ranges of the individual analytes, see Analyte Section of this Operator’s Manual.
3 Mahoney JJ, Wong RJ, Van Kessel AL, Reduced Bovine Hemoglobin Solution Evaluated for Use as a
Blood Gas Quality Control Material. Clin.Chem.39/5,874-79 (1993)
8.5.10 Interferences
Selected substances endogenous and exogenous to human blood were tested for interference in accord with CLSI EP7-A25
.
These substances were selected on the basis of their optical absorbance orfluorescence properties likely to affect the optical signal measured by the OPTI CCA-TS, or the optical properties of the sensor measured by the analyzer. To cause interference to the optical sensors, the substances must be highly mobile (low molecular weight) and highly colored, in order to penetrate the optode membrane barriers quickly (within the 90 sec. measurement interval), and then strongly absorb light or emit light of the proper color. To cause interference to the tHb and SO2 reflectance measurements, the substances must strongly absorb or scatter red or infrared light, relative to normal whole blood.
The following substances were tested in whole blood at the CLSI-recommended test level or higher, and showed no interference to any measured analyte, including blood gas, electrolytes, and tHb/SO2:
Bile Acids (30 µmol/dL) Bilirubin (40 mg/dL) Beta-Carotene (3.0 mg/dL)
Hemolysis (10%) During hemolysis K+ is released from the blood cells thereby
increasing the measured K+. In the same manner, protein released
from the cells binds ionized Ca++ and decreases the concentration.
While an accurate value is reported, it will reflect the actual changes caused by hemolysis.
Lipemia (equivalent to 3000 mg/dL triglycerides) Elevated white blood cell count (30,000 WBC/µL)
The following substances were tested in plasma at the CLSI-recommended test level or higher, and showed no interference to blood gas and electrolyte analytes:
Coumadin (Warfarin) (12 mg/dL) Dicumarol (Dicoumarin) (11 mg/dL) Procain (Novacaine) (13 mg/dL)
Acetaminophen (Paracetamol) (20 mg/dL)
The OPTI CCA-TS system was evaluated for the interference of sample temperature on measurement (iced samples). No measurable sensitivity to sample temperature was found.
For more detailed information on interferences, see analyte section of this Operator’s Manual.
5 Clinical and Laboratory Standards Institute (CLSI). Interference Testing in Clinical Chemistry;