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9-12 Operator’s Manual – OPTI CCA-TS2
9 OPERATING PRINCIPLES
9.5.9 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 P O
2
and P CO
2
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 P CO
2
and P O
2
measurement,
as well as pH was evaluated over a 20 day period using two OPTI CCA-TS2 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-TS2 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-TS2 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-TS2 system is no exception to this, and demonstrates
somewhat poorer P O
2
precision with aqueous controls than with whole blood, due to the large amount of
plastic material comprising its disposable measurement chamber.
The OPTI CCA-TS2’s tHb measurement is sensitive to pathologically rapid sedimentation rates of the
erythrocytes, often induced by excessive rate and amounts of rouleaux formation
4
. 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-TS2 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 P CO
2
value of 220 mmHg as:
P CO
2
> 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)
4
J.B.Henry, Clinical Diagnosis and Management by Laboratory Methods, 19th Ed., 1996, p.590,777