Different types of bacteria can cause pneumonia. A blood culture is a test that lets the health care provider know which bacteria may have caused your pneumonia, and which antibiotic should be prescribed. It is best to do the blood culture within 24 hours of arrival at the hospital and before antibiotics are started. It is also important to start antibiotics as soon as possible. A blood culture lets your health care provider know how to best treat you and if any precautions are necessary to prevent the spread of your illness.