The operating principle of a photo-optic platelet aggregometer is best described as?

• A. Aggregation on a foreign surface: Platelet aggregation is directly proportional to the difference in platelet counts performed before and after platelet-rich plasma is passed through a column of glass beads.
• B. Change in optical density: As platelets aggregate, the optical density of the platelet-rich plasma decreases.
• C. Electrical impedance: Platelet aggregates are counted as they pass through an aperture, temporarily interrupting the flow of current between two electrodes.
• D. Pulse editing: Editing electronically generated pulses can differentiate the number of free platelets versus platelet aggregates.

Answer: (B)

In a photo-optic platelet aggregometer, the test tracks how light passes through platelet-rich plasma as aggregation occurs. When platelets aggregate, they form larger clumps, so the suspension becomes less turbid and more light can get through. That change is reflected as a decrease in optical density over time. The instrument basically monitors this shift in light transmission to quantify the extent and kinetics of aggregation. The other descriptions refer to different technologies (counting by electrical impedance, bead-column methods, or pulse editing) and don’t describe the photo-optic principle.