Which expression correctly describes a platelet count in SI units?

Which expression correctly describes a platelet count in SI units?

• A. 178 * 109/L
• B. 178 * 103/L
• C. 356 * 109/cu. mm
• D. 712,000 * 109/L

Answer: (A)

In SI units, platelet count is expressed as the number of platelets per liter, using a 10^9 factor to reflect the typical large numbers. So a count written as 178 × 10^9/L means 178 billion platelets per liter, which fits the normal clinical range of roughly 150–450 × 10^9/L. The other options would undervalue or misstate the quantity: using 10^3 per liter would give only 178,000 platelets per liter, which is far too low; expressing per cubic millimeter (which is the same as per microliter) with a 10^9 multiplier would yield an impossibly large count; and an even larger multiplier times per liter would be unrealistic. Therefore, 178 × 10^9/L is the correct SI expression.

In SI units, platelet count is expressed as the number of platelets per liter, using a 10^9 factor to reflect the typical large numbers. So a count written as 178 × 10^9/L means 178 billion platelets per liter, which fits the normal clinical range of roughly 150–450 × 10^9/L. The other options would undervalue or misstate the quantity: using 10^3 per liter would give only 178,000 platelets per liter, which is far too low; expressing per cubic millimeter (which is the same as per microliter) with a 10^9 multiplier would yield an impossibly large count; and an even larger multiplier times per liter would be unrealistic. Therefore, 178 × 10^9/L is the correct SI expression.