Platelets prevent excessive bleeding by helping blood to clot at the site of an injury. An abnormally low platelet count (thrombocytopenia) may result in small vessel bleeding that can cause small red dots on the body (petechiae), or in excessive bleeding from wounds in mucous membranes, skin, or other tissues (ecchymoses and hematomas).

Platelet counts in CLL patients can become compromised by excessive lymphocytes, by treatment, and by related conditions such as Immune Thrombocytopenic Purpura (ITP).

Normal platelet counts are between 150,000 and 350,000 (150 - 350 x 10³µL or 150 - 350 x 10⁹L) per microliter of blood. There are no clear-cut numbers for when platelet counts become a concern. As oncologists gain more experience with decreased platelet counts, the threshold of concern has dropped. It used to be that anyone with a platelet count of lower than 75 was a candidate for platelet transfusion; now it is down into the 10's and 20's - lower yet if there is no sign of bleeding.

Platelet counts below 20 are of serious concern, but anyone is at risk of bleeding episodes if the platelets are decreased below 75.