Art: 714382-00D Rev. Date: 02/20/06 20-13
QUALITY CONTROL AND THE i-STAT COAGULATION TESTS
Operating Principles
of the Coagulation
Cartridge–Overview
The i-STAT coagulation cartridges measure the time required for complete
activation of the coagulation cascade once initiated by the activator.
Coagulation instruments determine this time by sensing a characteristic
change in a measured property of the sample. In the i-STAT System the
measured property is the concentration of an electroactive marker . The time
to clot is indicated by a relative increase in the concentration as measured by
an amperometric sensor.
i-STAT dries the activator and a precursor of the electrochemical marker (a
substrate to the thrombin enzyme produced by the coagulation cascade) onto
the wall of the reaction chamber during the manufacturing process. At the
beginning of the test the system agitates the blood back and forth across the
chamber wall to mix these reagents into the blood sample.
Quality System for
Coagulation Cartridge
The critical performance feature of the coagulation cartridge centers on the
repeatability of the reagent mixing process. The accuracy to which the reagent
is mixed into the blood sample directly impacts the accuracy of the result.
The system quantitatively confirms the accuracy of the mixing step by
monitoring the key parameters of mix uniformity, magnitude and timing.
These quality tests are performed on each coagulation cartridge.
i-STAT’s microfabrication production processes are inherently capable of
creating sensors with highly reproducible characteristics. For the measurement
of blood gases, electrolytes and chemistries, this means that the i-STAT System
requires only a one-point calibration, using a calibrant solution packaged in
the cartridge, to meet the demanding requirements for clinical accuracy. As
described in the Quality Control section of the i-STAT System Manual, the
calibrant solution is also used to verify the integrity of the sensors as a key
component of the quality system.
For the measurement of ACT and PT, the required accuracy for the
amperometric sensor to detect the relative increase in concentration of
the electroactive marker is more modest. A calibrant solution is required
neither for a one-point calibration nor to verify the wetup characteristics of
the sensor. Instead, the magnitude and rate of change of current is assessed
quantitatively throughout the test in order to verify the quality of the mix,
and the integirity of both the sensor and the reagent coating.
Regulatory Aspects
of the Quality System
for Coagulation
Alternatives to traditional quality systems have been developed that are suitable
for ensuring the performance of unit-use in-vitro diagnostic systems. These
alternative systems rely upon a variety of internal self-tests and electronic/
optical checks. As unit-use devices have become more widespread in clinical
practice, regulations and guidance documents have adapted to recognize the
effectiveness of these alternative quality systems, albeit with some variation.
For example, some state regulations require that the alternative quality
system include an on-board “wet” control. The i-STAT Quality System for
the coagulation test is able to address this requirement even though the
cartridge does not contain an on-board wet calibration fluid. The quantitative
confirmation that the activator and the marker are accurately mixed into the
blood sample is a “wet” test that acts as a control of the most critical aspect of
the coagulation test.
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