PROCEDURE MANUAL FOR THE i-STAT SYSTEM
28
REV. DATE: 16-Oct-12 ART: 714446-00L
PRINCIPLES OF MEASUREMENT
Sodium, Potassium, Chloride, Ionized Calcium, pH, and PCO
2
are measured by ion-selective electrode potentiometry. Concentrations are calculated from the measured potential
through the Nernst equation.
Urea
is first hydrolyzed to ammonium ions in a reaction catalyzed by the enzyme urease. The ammonium ions are
measured by an ion-selective electrode and the concentration is calculated from the measured potential through the
Nernst equation.
Glucose
is measured amperometrically. Oxidation of glucose, catalyzed by the enzyme glucose oxidase, produces hydrogen
peroxide. The liberated hydrogen peroxide is oxidized at an electrode to produce an electric current which is
proportional to the glucose concentration
.
Creatinine
is hydrolyzed to creatine in a reaction catalyzed by the enzyme creatinine amidohydrolase. Creatine is then hydrolyzed
to sarcosine in a reaction catalyzed by the enzyme creatine amidinohydrolase. The oxidation of sarcosine, catalyzed by
the enzyme sarcosine oxidase, produces hydrogen peroxide. The liberated hydrogen peroxide is oxidized at the platinum
electrode to produce a current which is proportional to the creatinine concentration
.
Lactate
is measured amperometrically. The enzyme lactate oxidase, immobilized in the lactate biosensor, selectively converts
lactate to pyruvate and hydrogen peroxide. The liberated hydrogen peroxide is oxidized at the platinum electrode to
produce a current which is proportional to the lactate concentration.
PO
2
is measured amperometrically. The oxygen sensor is similar to a conventional Clark electrode. Oxygen permeates
through a gas permeable membrane from the blood sample into an internal electrolyte solution where it is reduced
at the cathode. The oxygen reduction current is proportional to the dissolved oxygen concentration.
Hematocrit
is determined conductometrically. The measured conductivity, after correction for electrolyte concentration, is
inversely related to the hematocrit.
ACT
is determined amperometrically. The conversion of a thrombin substrate is initiated by mixing a whole blood sample
(without anticoagulant) with a particulate clotting activator – either Celite brand diatomaceous earth or kaolin. The
substrate used in the electrogenic assay has an amide linkage that mimics the thrombin-cleaved amide linkage in
fibrinogen. The product of the thrombin-substrate reaction is the electroactive compound that is detected
amperometrically. The time of detection is measured in seconds and the result is reported as a whole blood time (WBT).
PT/INR
is determined amperometrically. The conversion of a thrombin substrate is initiated by mixing a whole blood sample
(without anticoagulant) with tissue thromboplastin. The substrate used in the electrogenic assay has as amide linkage
that mimics the thrombin–cleaved amide linkage in fibrinogen. The product of the thrombin–substrate reaction is the
electroactive compound that is detected amperometrically. The time of detection is measured in seconds and reported as
INR and/or seconds.
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