PlateletsThrombocytes · PLT
Cell fragments that plug damaged blood vessels and start clotting.
Platelets are not whole cells but fragments shed from much larger cells in the bone marrow. When a blood vessel is damaged they stick to the injury and to each other, forming the initial plug that stops bleeding before the clotting proteins finish the job.
It matters because both directions carry meaning: a very low count raises bleeding risk, and a raised count can follow inflammation, infection, iron deficiency or, less often, a marrow disorder. Platelets also rise temporarily after hard exercise.
In practice the count is read with mean platelet volume, which describes their average size, and a single unexpected result is repeated, since platelets can clump in the sample tube and read falsely low.