Say you get one of those annoyingly painful paper cuts, which damages your blood vessel endothelium, causing you to bleed. Normally, the first responders to endothelial damage are platelets, which quickly plug the site of injury - that’s primary hemostasis.
Later, coagulation factors come in to strengthen that platelet plug - that’s secondary hemostasis.
Now, a low number of those first responder platelets is thrombocytopenia, and it’s defined as a platelet count below 150,000 cells per microliter, with the normal range being between 150,000 and 450,000.
Thrombocytopenia is mild if the platelet count is between 100,000 and 150,000, moderate if it’s between 50,000 and 100,000, and severe if it’s less than 50,000.
Generally, the lower the platelet count, the higher the risk of bleeding or bruising.
Spontaneous bleeds starting to happen when the platelet count falls below 30,000, with spontaneous intracranial bleeds developing when the platelet count falls below 10,000.
Low platelets cause mucocutaneous bleeding, which includes petechiae - which are pinpoint superficial skin bleeds, anterior epistaxis - which are usually mild nosebleeds, immediate bleeding after surgical procedures, like tooth extraction, or bleeding from mucocutaneous surfaces, like gingival, gastrointestinal, or vaginal bleeding.
Finally, most surgical procedures can be performed as long as the platelet count is above 50,000.
One quick thing to look for is hemodilution and it happens when the platelet concentration falls as a result of large volume transfusions of platelet-free products, like packed red blood cells or intravenous fluids. This can happen in trauma patients who lose a lot of blood.