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Cell Counting
Flow cytometry can be used in the realm of cell counting where differentiation of multiple populations is necessary, e.g. in blood samples. Flow cytometric cell counting utilizes a sample with a known concentration of fluorescent beads. The sample is run through a flow cytometer and set to stop after a predetermined number of beads are analyzed. Because the concentration of beads is known the volume of sample analyzed can then be easily computed. The number of cellular events can then be easily counted and the concentration determined. Different populations within a sample can be differentiated by scatter properties or fluorescent staining.
 
Beads of a known concentration gated by fluorescence (left) and bacteria of an unknown concentration gated by fluorescence (right) |
Statistics containing the counts of the beads of known concentration (top) and bacteria of unknown concentration (bottom).
The concentration of cells can be found by the following equation:
In the example above the bead concentration was known to be 2.0∙10 beads/mL so the concentration of bacteria can be found by:
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Several companies offer tubes with known quantities of beads or solutions of known bead concentration. Caltag Counting Beads and Molecular Probes CountBrightTM absolute counting beads each contain known concentrations of beads. When using these products add a known volume of product to a known volume of sample, this will allow easy calculation of final bead concentration. BD TruCOUNTTM tubes contain a known quantity of beads. With these tubes add a known volume of sample directly to the tube, then calculate the bead concentration.
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