Low testosterone is found with a blood test. In general, a diagnosis of low testosterone is made if your hormone level is below 300 ng/dL.
Your health care provider will also do a PSA (prostate specific antigen) blood test to check for signs of prostate cancer. Your provider may also do a test to measure the red blood cells in your body (hematocrit). A hematocrit is checked to keep record of your red blood cell count, because it can go up if you take testosterone.