The size threshold range used by electrical impedance methods to count particles as platelets is

• A. 0-10 fL
• B. 2-20 fL
• C. 15-40 fL
• D. 35-90 fL

Answer: (B)

Electrical impedance counting uses a size window to separate particles: as each particle passes through the aperture, it changes the electrical current by an amount proportional to its volume, so the instrument gates events by size to count platelets separately from red cells and debris. Platelets are small, with volumes roughly in the single-digit to a few dozen femtoliters, so a counting window around 2–20 fL best captures the platelet population while excluding much of the debris (smaller than platelets) and red blood cells (much larger, around 80–100 fL).

Choosing a range of 0–10 fL would miss the larger platelets that fall above 10 fL. A range of 15–40 fL would miss many normal platelets that fall below 15 fL and could start capturing larger particles that aren’t platelets. A range of 35–90 fL would include large platelets and would begin to count red cell–sized particles, leading to inaccurate platelet counts. Hence 2–20 fL is the typical window that best isolates platelets.