Reiter's syndrome can be difficult to diagnose because the symptoms may be mild and occur sporadically. No specific laboratory test can confirm the diagnosis. However, episodes of arthritis of one month with urethritis/cervicitis and conjunctivitis are specific and sensitive for this disease.

Diagnosis begins with a physical examination. A medical history, including the recent symptoms of any infection, and a sexual history may also be taken. A patient's history of any unusual symptoms, fever or pain can help the diagnostic process. The patient may be asked to complete a pain assessment form.

Certain blood tests may be used to rule out other form of arthritis. Some of these tests, which frequently yield positive results for many forms of arthritis, are negative for people with Reiter's syndrome. Many laboratory tests are used to screen for specific kinds of arthritis or other disease. Blood tests that used include:

• Antinuclear antibody (ANA) test and rheumatoid factor test. These usually yield a positive result for some forms of arthritis. However, patients with Reiter's syndrome usually test negative for both tests.
  
  
• C reactive protein test. An elevated level of this protein produced by the liver suggests an inflammatory disease.
  
  
• HLA-B27 test. Presence of this genetic marker in the white blood cells may indicate Reiter's syndrome, or other conditions including ankylosing spondylitis, rheumatoid arthritis or juvenile rheumatoid arthritis.
  
  
• Erythrocyte sedimentation rate (ESR). This blood test measures how long it takes red blood cells (erythrocytes) to separate from plasma in a blood sample. It is nonspecific: An elevated ESR indicates inflammation that could be caused by many conditions.

Other tests that may be performed include:

• Urine tests and stool samples. A physician may test a sample for infections that originated with a gastrointestinal source.
  
  
• Chlamydia test. A swab of cells from the throat and the urethra in men and women and the cervix in women is used to test for this sexually transmitted disease in cases where it is the suspected originating infection. A more advanced urine test can also test for chlamydia. If a patient tests positive for chlamydia, the physician may test for other sexually transmitted infections, including gonorrhea and the human immunodeficiency virus (HIV).
  
  
• Synovial fluid analysis. A needle is used to remove fluid from an inflamed joint. Testing the fluid may indicate several forms of arthritis, such as gout, infectious arthritis or juvenile rheumatoid arthritis.

Most patients with Reiter's syndrome do not show any joint damage on x-rays in the early stages of the disease. The use of x-rays may serve to rule out other forms of arthritis. X-rays that show bony outgrowths or changes in the size of joint spaces may indicate such diseases as osteoarthritis. If Reiter's syndrome is more advanced at diagnosis, the patient may show some joint changes, especially if the joints in the lower back are affected.

The combination of the patient's medical history and test results may enable a physician to diagnose Reiter's syndrome or continue investigating other potential conditions.