CELL-DYN
3000 System Operator’s Manual
3-17
9140240E — May 1995
Chapter 3
Principles of Operation
RBC/PLT Measurement
Overview
An impedance channel is used for the determination of RBC and PLT
data. A 1:12,501 dilution of the sample is made with the Diluent. The
cells are counted and sized using the Impedance method as they pass
through the 60 x 72 µm aperture in the RBC/PLT transducer assembly.
Dynamic thresholding separates the PLTs from the RBCs. The 100 µL
volume of sample dilution that is analyzed is precisely regulated by the
RBC/PLT metering assembly. Data is collected in 256 channels for both
RBCs and PLTs.
Electrical Impedance Measurements
RBCs and PLTs are counted and sized by the aperture impedance
method. This method is based on the measurement of changes in
electrical current which are produced by a particle, suspended in a
conductive liquid, as it passes through an aperture of known dimensions.
An electrode is submerged in the liquid on either side of the aperture in
order to create an electrical pathway through it.
As each particle passes through the aperture, a transitory change in the
resistance between the electrodes is produced. This change produces a
measurable electrical pulse. The number of pulses generated is indicative
of the number of particles that traversed the aperture. The amplitude of
each pulse is essentially proportional to the volume of the particle that
produced it.
Each pulse is amplified and compared to internal reference voltage
channels. These channels are delineated by calibrated size discriminators
to accept only pulses of a certain amplitude. Thus, the pulses are sorted
into various size channels according to their amplitude.
Coincidence Passage Correction
Two or more cells can enter the aperture sensing zone simultaneously
during a measurement cycle. The resistance change created in this
situation generates a single pulse with a high amplitude and increased
pulse area. Thus, it appears that one large cell has passed through the
aperture. Consequently, the cell count is falsely decreased. This count
reduction, referred to as coincidence passage loss, is statistically
predictable because it has a direct relationship to the effective volume of
the aperture and the amount of dilution. Each total cell count for RBCs
and PLTs is automatically corrected for coincidence passage loss.
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