A comprehensive medical checkup should allow a clinician to differentiate ME/CFS from other conditions. This involves inquiring about the patient's medical history and present disease, as well as inquiring about the symptoms to determine how frequently they occur, how severe they are, and how long they have lasted. A physical examination may reveal signs of infection, injury, or other problems not apparent from just listening to the patient talk about his or her symptoms. Some tests may be needed to rule out other possible causes of illness. These include: blood tests to look for infections, cancer, or other diseases; urine tests to look for evidence of kidney problems or damage caused by alcohol or drugs; and tests of the brain, spinal cord, muscles, joints, and nerves. The tests used to find these problems can be invasive or simply require taking more time than is first available during an office visit. In some cases, an MRI (magnetic resonance imaging) or CT scan (computerized tomography) may need to be done instead. Tests that show whether the body's immune system is working properly are also useful in diagnosing CFS. The two main tests used to detect immune system problems are measuring antibodies against viruses such as Epstein-Barr virus (EBV) or cytomegalovirus (CMV) and looking at certain cells of the immune system under a microscope.