The Coulter Counter is used to count blood cells by measuring which parameter?

• A. Optical density
• B. Electrical impedance
• C. Magnetic resonance
• D. Fluorescence

Answer: (B)

This relies on the Coulter principle: blood cells suspended in a conductive electrolyte pass one at a time through a very small aperture. Each cell temporarily changes the electrical resistance as it displaces the electrolyte, producing a pulse in current. The number of pulses corresponds to the cell count, and the height of each pulse gives information about the cell’s size. So the parameter being measured is electrical impedance (the change in electrical resistance/current as cells pass through the aperture).

Optical density would involve light absorbance, which isn’t how a Coulter counter works. Magnetic resonance and fluorescence rely on magnetic properties or fluorescent labeling, respectively, and are used in different systems.