Chronic fatigue syndrome (CFS) is hard to diagnose. Fatigue is an extremely common problem, and it can have many other causes. CFS can be diagnosed only by ruling out other conditions.

First, your doctor will take your medical history and do a physical exam. Experts have come up with a specific list of symptoms to decide whether a person has CFS. Doctors use a variety of tests to rule out other conditions. These tests usually include:

• Complete blood count (CBC), which provides important information about the kinds and numbers of cells in your blood.
• Erythrocyte sedimentation rate (ESR), which measures how quickly red blood cells (erythrocytes) settle in a test tube. This tells whether or not certain types of inflammation are present.
• Blood glucose level, used to check for diabetes.
• Thyroid-stimulating hormone test, used to detect problems that affect the thyroid gland.
• Chemistry panel, a blood test that provides information about your general state of health.
• Urinalysis, a urine test that can provide information about your overall health and clues to many conditions.

These are routine lab tests. Other tests may be done if your symptoms, history, and physical exam suggest other possible problems. Additional tests may include:

• ANA (antinuclear antibodies), to check for lupus (systemic lupus erythematosus).
• Rheumatoid factor, to check for rheumatoid arthritis.
• HIV test, to check for human immunodeficiency virus (HIV) infection.
• Tests for Lyme disease, if you may have been exposed to ticks.
• Skin test, for tuberculosis.

Some doctors may order tests that check your immune system. These can be expensive and generally are done only in research settings. In addition, it's hard to know what the findings of these tests mean because so little is known about the immune system's connection to chronic fatigue syndrome.