A platelet count of 20,000 or fewer per microliter is considered dangerously low, according to the University of Pittsburgh Medical Center. Internal bleeding or other complications can result from very low platelet counts, which can be fatal if the bleeding affects the brain or the intestines.
A low platelet count, or thrombocytopenia, can be caused by disorders such as leukemia, states the Mayo Clinic. It can also be a side effect of some medications. Although thrombocytopenia often improves once the primary disorder is treated, surgery and blood transfusions may be necessary. Symptoms of thrombocytopenia include easy bruising, prolonged bleeding from cuts, excessive bleeding from surgery and blood in urine or stool. Women who suffer from thrombocytopenia may have heavy menstrual flows.