Tests and diagnosis

By Mayo Clinic Staff

Water on the knee is a sign of an underlying knee problem, usually arthritis, an infection or an injury. Your doctor may order some of the following tests to determine the underlying cause.

Imaging tests

These studies show the inside of your knee joint. They can help determine whether there is fluid within the joint or in a bursa and may provide information about what's causing fluid to accumulate.

  • X-ray. Your doctor may request an X-ray to make sure you haven't broken or dislocated any bones or to determine if you have arthritis.
  • Ultrasound. This noninvasive imaging test is widely used in Europe and gaining popularity in the U.S. to evaluate joint pain and swelling. An ultrasound exam is less expensive than an MRI. The test can be used to diagnose arthritis or tendon/ligament disorders.
  • Magnetic resonance imaging (MRI). If your history, examination and X-rays don't identify the cause of your swollen knee, you may need an MRI. This test can detect tendon and ligament injuries that aren't visible on X-rays.

Blood tests

A sample of blood taken from your arm can be tested for evidence of:

  • Infection
  • Inflammation
  • Rheumatoid arthritis
  • Lyme disease
  • Bleeding disorders
  • Gout

Joint aspiration (arthrocentesis)

During this procedure, your doctor withdraws fluid from inside your knee to check for the presence of:

  • Blood, which may stem from injuries or bleeding disorders
  • Bacteria, which may be causing an infection
  • The crystals common to gout or to pseudogout


An orthopedic surgeon inserts a small, lighted tube with a magnifying lens (arthroscope) through a small incision in your knee and examines the inside of your knee joint.

June 16, 2012

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