Obtaining Joint Fluid for Analysis:
Joint fluid, also referred to as synovial fluid, can be aspirated from a joint
using a needle and syringe. The procedure can be performed in a doctor's office and the fluid may be sent to a laboratory for analysis. Joint fluid analysis is also commonly referred to by other names:
- synovial fluid analysis
- joint tap
Removal of joint fluid has a dual purpose - diagnostic and therapeutic. Finding the cause of joint swelling is the goal. The removal of fluid may also help relieve pain and pressure on the joint.
Sterile technique must be followed to obtain fluid for a joint fluid analysis. Precautions are taken so that bacteria is not introduced into the joint. A sterile needle is used to draw joint fluid from a joint, but first the skin is sterilized using betadine or povidine. A local anesthetic is also used.
After the fluid is withdrawn, the doctor can inject medicine (usually a corticosteroid) into the joint using the same injection site. A bandage is applied after the needle has been removed.
Once the joint fluid arrives at the laboratory for analysis, it is observed by human eye for color and clarity. Normal joint fluid is viscous and appears clear to light yellow. Cloudy joint fluid is abnormal and suggestive of inflammation
or an infection. Bloody joint fluid is also abnormal and may be caused by trauma to the joint.
Joint fluid is examined under a microscope for the presence of blood cells, crystals, and bacteria. Normal joint fluid has none or few blood cells. Large numbers of red blood cells indicate bleeding in the joint. Large numbers of white blood cells can occur with infection, inflammatory arthritis, gout
, or pseudogout
Crystals are an abnormal finding. Uric acid crystals indicate gout; CPPD crystals occur with pseudogout. Bacteria is also abnormal. A culture can identify the source of infection.
Joint fluid can also be tested for glucose, protein, and lactic dehydrogenase (LDH). Abnormal joint fluid results which may indicate inflammation or infection are:
- Glucose - less than 40 mg/dl
- Protein - greater than or equal to 3 g/dl
- LDH - great than 333 IU/L
Joint fluid is observed in a plain tube after one hour for formation of a fibrin clot. The quality of clot is graded, but any clot shows a problem with the synovial membrane
. Another test known as the mucin clot test (acetic acid is added to synovial fluid) estimates the production of hyaluronate.
Blood tests or other laboratory tests can also be ordered which will offer more clinical evidence and support evidence from the joint fluid analysis.
Clinical Diagnosis by Laboratory Methods, Todd-Sanford.