Saint Thomas Hospital and Vanderbilt University
Correspondence to: Richard W. Light, MD, Pulmonary Medicine, Saint Thomas Hospital, 4220 Harding Rd, Nashville, TN 37202; e-mail: email@example.com
To the Editor:
Use of the Coulter Counter Analyzers for Particle
Characterization is an accepted reference method for particle size
analysis and is widely used in experimental settings for the
measurement of total cell counts in different body
fluids.1–3 We recently found that in pleural fluids
induced by the intrapleural injection of talc, the total leukocyte
count was higher than expected. We hypothesized that residual talc
particles were being counted as leukocytes by the Coulter counter.
To confirm this hypothesis, we analyzed talc solutions of various
concentrations using a Coulter Z1 Particle Analyzer (Coulter
Electronics; Luton, England). Talc powder (Sigma; St. Louis, MO) was
diluted with 0.9% sodium chloride solution into a slurry. The samples
were treated with a diluent (Isoton II; Coulter Corporation; Miami, FL)
and a lytic reagent to lyse erythrocytes (Zapoglobin II; Coulter
Corporation) before analysis. Standard Coulter counter settings for
detection of leukocytes were used, under that particles with a diameter
between 3.6 μm and 7.47 μm (approximately 25 to 218 fL in
volume) were counted. The first reading was discarded, and the mean of
the next three readings was recorded. The results are shown in Table 1
Talc is a heterogeneous compound with the approximate chemical formula
The size of talc particles varies significantly among commercial
preparations.5 The median diameter of the commercial talc
powder we used was 7.8 μm,5 suggesting that
approximately half of the talc particles would fall within the
detection range of the Coulter counter. Our results confirmed that the
presence of talc particles interferes with the measurement of total
leukocyte counts with the Coulter counter.
The Coulter method of counting particles is based on measurable changes
in electrical resistance produced by nonconductive particles suspended
in an electrolyte. As particles pass through the aperture between
electrodes, they displace their own volume of electrolyte. Volume
displaced is measured as a voltage pulse; the height of that is
proportional to the volume of the particle. This method is independent
of particle shape, color, and density. Hence, talc particles of similar
volume as leukocytes will be counted as leukocytes. Likewise, various
microorganisms in blood or cerebrospinal fluids have been misidentified
as leukocytes by the Coulter method.6–7
Once injected intrapleurally, some of the talc particles will remain in
the pleural space for a variable length of time. These residual
particles will frequently be mixed with the induced pleural fluid.
Investigators should beware that in experimental settings involving the
use of talc, total cell counts in body fluids measured with Coulter
counter may be falsely elevated. Other methods, eg, a
hemacytometer, should be used.
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