What laboratory procedure calculates the number of platelets in a known volume of blood?

The procedure that calculates the number of platelets in a known volume of blood is the platelet count. This test specifically quantifies the platelets, which are essential for blood clotting and maintaining hemostasis.

In a platelet count, a blood sample is taken and analyzed, typically using an automated hematology analyzer, which measures the number of platelets present in a given volume, often expressed as platelets per microliter or per liter of blood. The significance of accurately determining platelet levels is crucial for diagnosing conditions related to bleeding disorders, thrombocytopenia (low platelet count), or thrombocytosis (high platelet count).

Hematocrit measures the proportion of red blood cells in the blood but does not provide specific information about platelets. Serum count is not a standard term used in assessing blood components related to platelets, and a complete blood count (CBC) includes a variety of blood components, including red and white blood cells, hematocrit, and also often includes data on platelets; however, the specific procedure for determining platelet levels is explicitly termed the platelet count. Thus, understanding that the platelet count is the dedicated assay for measuring platelets clarifies why it is the correct answer in this context.