Diagnosing a Joint Effusion

Abnormal Accumulation of Fluid in or Around a Joint

Joint effusion of elbow
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Joint effusion is the abnormal accumulation of fluid in or around a joint. You may have heard the terms "water on the knee" or "fluid on the knee," both of which describe an effusion of the knee joint.

Aseptic joint effusion means that the cause is not related to a bacterial infection. In this instance, a viral infection, physical injury, or arthritis may be to blame. Septic joint effusion means just the opposite and is related directly to an infection (including sepsis).

When a joint is affected by inflammatory arthritis, the buildup of fluid is not uncommon. When this happens, the joint will appear swollen and will usually be accompanied by pain, redness, warmth, and a decreased range of motion.

Physical Examination

If a joint is abnormally swollen, the doctor will perform a physical exam to determine whether the swelling of tissues is secondary to the effusion or if the tissues themselves are causing the effusion. It is a fine distinction but one that can help differentiate between, say, an infection and an inflammatory disorder in which the tissues are only collaterally involved.

The doctor will then palpate (examine by touch) the affected joint. This can tell us several things:

  • If the swelling is related to arthritis, the lubricating tissue between the joints, called the synovium, will feel boggy. Moreover, the swelling will tend will be gradual (with the exception of gout can strike suddenly and mainly affect the big toe).
  • An infection is often characterized by swelling, pain, fever, and the inability to move the joint.
  • Acute swelling accompanied by the inability to bear weight (with or without bruising) may suggest a ligament tear or fracture.

Ultrasonography is often used to aid in the diagnosis. In the end, the more that the doctor is able to characterize the condition, the more likely the appropriate treatment will be prescribed.

Joint Effusion on X-ray

While joint effusion is not easily recognized on X-ray, there are characteristics that can assist in the diagnosis. Among them:

  • With a knee joint effusion, the effusion will appear as a rounded, homogeneous mass of soft tissue situated between the front of the thigh bone (femur) and the quadriceps muscles of the thigh.
  • With an elbow effusion, the mass of swollen tissue will have displaced the fat pad surrounding the lower part of the forearm bone (humerus) and appear in a characteristic triangular shape referred to as a "sail sign."
  • Hip effusions, unlike the other joint effusions, are nearly impossible to view on X-ray and would typically require an ultrasound, computed tomography (CT) scan, or magnetic resonance imaging (MRI) scan.

Joint Fluid Analysis

Your doctor will likely want to aspirate (drain) fluid from the affected joint to relieve pressure. If an infection is suspected, the doctor may inject a small amount of cortisone into the joint to reduce inflammation and prevent fluid from building up again.

The aspiration of fluid can also aid in determining the cause of the effusion. Normal synovial fluid will usually be clear with the viscosity of an egg white. Any changes in its appearance or texture can provide clues as to the cause of the effusion.

For example:

  • Cloudy fluid may suggest rheumatoid arthritis. The lab analysis will often reveal elevated levels of defensive white blood cells (more than 2,000 per cubic millimeter).
  • Yellow-green or cloudy fluid may suggest an infection or sepsis. The white blood cells will also be elevated (greater than 20,000 per cubic millimeter). Traces of pus may also be seen.
  • Golden fluid is commonly seen with gout. Microscopic examination will usually reveal needle-like crystals of monosodium urate.
  • Bloody or pink fluid will often be seen with a joint injury. Lab tests will usually reveal an equal number of red and white blood cells.
  • Clear fluid is typically seen with osteoarthritis since it doesn't involve any inflammation. The white blood cell count will usually be below 2,000 cells per cubic millimeter.


Once the joint fluid has been drained, nonsteroidal anti-inflammatory drugs (NSAIDs) and immobilization will be prescribed as the standard treatment of an aseptic effusion. In the event of a septic effusion, antibiotics may need to be prescribed.

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