ITC IRMA TruPoint Blood Analysis System - Manual (Page 146)

C.2
Potentiometric measurements
The IRMA TruPoint potentiometric sensors generate a voltage which is
related to ion concentration according to the Nernst equation:
E = E° + S log (C1/C2)
› E is the voltage generated
› E° is a constant for the sensor
› S is the sensitivity of the sensor
› C1 and C2 are the ion activities outside and inside the sensor
membrane.
The IRMA TruPoint system uses direct (undiluted) measurement methods.
Differences are known to exist between direct and indirect (diluted) methods
due to the measurement of ion activity rather than concentration1. Direct
measurements read up to 7% higher than indirect methods for electrolytes
because of the excluded volume occupied by plasma proteins and lipids. The
IRMA TruPoint system measures electrolyte ion activity, and has been
calibrated to agree with standard laboratory reference methods that are
performed on plasma or serum
2 .
Amperometric measurements
The IRMA TruPoint amperometric sensors generate a current which is related
to analyte concentration according to the relationship:
i = S (C) + B
› i is the current generated
› C is the analyte concentration or partial pressure of the test sample
› B is the current generated in the absence of the analyte
Whole blood glucose measurements are approximately 12% to 15% lower
than plasma concentrations
15 . The IRMA TruPoint system measures glucose
concentrations in whole blood, and has been calibrated to agree with standard
laboratory reference methods that are performed on plasma or serum.
Conductimetric measurements
The IRMA TruPoint analyzer determines hematocrit based on electrical
conductivity, which is related to hematocrit in whole blood:
% hematocrit = ƒ (1 / )
› = sample resistance
› conductivity = 1 /