The C-reactive protein (CRP) test is a blood test that measures the level of CRP in the blood. CRP is a substance produced by the liver that increases whenever there is inflammation somewhere in the body. High levels of CRP thus indicate inflammation. This information can be important in diagnosing inflammatory conditions or infections, or evaluating the effectiveness of certain treatments in decreasing inflammation or infection.

Many pain conditions may cause significant inflammation that will result in CRP levels as high as 100 times above normal. These conditions include:

• Arthritis. The most common arthritic condition, osteoarthritis (OA), is usually not associated with inflammation, and these patients do not typically have increased levels of CRP. However, rheumatoid arthritis patients usually have elevated CRP. The CRP test can also be used in diagnosing other forms of arthritis, such as:
  
  
• Gout. One of the most painful forms of arthritis, characterized by the buildup of uric acid crystals in the big toe and other joints.
  
  
• Infectious arthritis. Joint inflammation caused by infection.
  
  
• Juvenile rheumatoid arthritis. The most common form of joint inflammation in children.
  
  
• Ankylosing spondylitis. A chronic condition that primarily affects the spine.
  
  
• Reiter’s syndrome. Reactive arthritis involving inflammation of the joints, urethra and the lining of the eye.
  
  
• Pseudogout. Joint inflammation characterized by calcification of articular cartilage.
  
  
• Psoriatic arthritis. A combination of arthritis and the skin disorder psoriasis.
  
  
• Polymyalgia rheumatica. A rheumatic disease involving pain and stiffness in the neck, shoulders and hips.
  
  
• Encephalitis. Inflammation of the brain, usually caused by infections, resulting in severe headaches.
  
  
• Meningitis. Inflammation of the meninges, membranes that protect the brain and spinal cord.
  
  
• Lyme disease. An inflammatory disease caused by bacteria transmitted from ticks.
  
  
• Systemic lupus erythematosus. An autoimmune disorder in which the body mistakenly attacks its own tissues, resulting in inflammation of the skin, joints and internal organs.

• Trauma (e.g., injuries, fractures or burns).
  
  
• Urinary tract infection.
  
  
• Prostatitis. Acute or chronic inflammation of the prostate gland.
  
  
• Sarcoidosis. An inflammatory disease involving the lymph nodes, lungs, liver, eyes, skin or other tissues.
  
  
• Temporal arteritis. A rheumatic disease that involves inflammation of the blood vessels, most often in the head.
  
  
• Myositis. A group of inflammatory muscles conditions.
  
  
• Sacroiliitis. Inflammation of the sacroiliac joints that connect the base of the spine to the pelvic bone.
  
  
• Behcet’s disease. A rare, painful disorder characterized by inflammation of the mouth, genitals and small blood vessels.
  
  
• Rheumatic fever. An infectious condition caused by bacteria. It is usually associated with painful joints but can also involve the heart.

Other conditions that can cause an increase in CRP levels include high blood pressure (hypertension), diabetes and glucose intolerance, skin conditions such as hives and psoriasis, cancer, and bacterial, viral, fungal or parasitic infections.

CRP tests may also be performed after surgery because high levels of the protein may indicate the presence of infection. Though a sedimentation rate (sed rate) test is also used for this purpose, CRP levels may be better tools for early detection because they will show an elevation more quickly.

Although the CRP test can detect inflammation in the body, it cannot determine its location or cause, so it is a nonspecific test like sed rate. Additional diagnostic tests are typically required.

Physicians have hoped that screening for C-reactive protein could predict heart attacks. However, some recent research suggests the CRP test might not reveal additional information about a patient’s risk of heart disease.