A decrease in the number of platelets circulating in the blood. Platelets are cells necessary for the clotting of blood. There are many causes of thrombocytopenia including certain medications, alcohol, causing less severe platelet decrease. It can also occur at any stage of HIV and sometimes occurs when HIV antiretroviral treatment induces an immune response. It can be the first and only sign of HIV infection. It can be a serious condition leading to abnormal bleeding and in advanced cases bone marrow problems. Your doctor will usually monitor you platelet count during your standard blood tests.