Synovial Fluid

Sample of the Synovial fluid 

1. Synovial fluid is the aspirated fluid from the synovial spaces called Arthrocentesis.
2. Collect specimens in three tubes:
   1. Tube 1 for culture.
   2. Tube 2 for microscopy, add heparin and do not use EDTA powder.
      1. You can use liquid EDTA.
   3. Tube 3 for chemistry.

Synovial fluid examination

3. For glucose, the patient should have 6 hours of fasting.
4. Synovial fluid can be aspirated from joints of:
   1. Knee.
   2. Shoulder.
   3. Elbow.
   4. Wrist.
   5. Ankle.
   6. Hip.

Precautions for synovial fluid

1. Powered anticoagulants should not be used because this may interfere with crystal analysis.
2. The non-anticoagulant test tube should be centrifuged to prevent the cellular elements from interfering with the chemical and serologic analysis.
3. All testing should be done as soon as possible to prevent cellular and possible changes in the crystal.

Contraindication for arthrocentesis

1. Avoid in case of skin infection or wound in the area of aspiration.

Purpose of the test (Indications)

1. To diagnose the cause of synovial fluid formation.
2. To differentiate inflammatory from non-inflammatory causes.
3. Arthritis due to crystals like gout and pseudogout.
4. Malignant tumor involving the joint.
5. To inject anti-inflammatory medicines like corticosteroids.
6. To monitor chronic arthritic diseases.

Definition of synovial fluid:

1. This is often referred to as joint fluid, the viscous fluid found in the cavities of the moveable joints.
2. It is clear and straw-colored with few WBC, and no crystals are found.
3. There is good mucin clot formation.
4. Normally synovial fluid is transudate with <200 WBCs and <25% neutrophils.

Composition of synovial fluid:

1. Synovial fluid is formed as an ultrafiltrate of plasma across the synovial membrane.
2. This filtration is nonselective except for the exclusion of high molecular weight proteins.
3. The contents of the synovial fluid are similar to the plasma values.
   1. This synovial fluid does not typically clot. But the inflammation due to increased fibrinogen may clot.
4. The synovial cells lining the synovium secrete a mucopolysaccharide containing hyaluronic acid and a small amount of the protein.
   1. This hyaluronic acid causes noticeable viscosity of the synovial fluid.

Synovial fluid and cavity for aspiration

5. Synovial fluid is present normally in a very small amount. The amount in the large knee joint is less than 3.5 mL.
   1. This fluid collection can increase inflammation and maybe around 25 mL.
6. Synovial fluid contains mucopolysaccharides called hyaluronic acid, which are responsible for the viscosity of the synovial fluid and lubricates the joints.
7. An increase in a synovial fluid enough to aspirate is due to some disease.

Functions of the synovial fluid:

1. The smooth articular cartilage and synovial fluid reduce friction between the bone during joint movements.
2. Synovial fluid supplies nutrients to the cartilage and acts as a lubricant to the joint, bursa, and tendons sheaths.

Synovial fluid functions

Types of arthritis:

1. Damage to the articular membrane produces pain and stiffness in the joints, referred to as arthritis.
2. There are the following types of arthritis:
3. Noninflammatory:
   1. This arthritis is seen in osteoarthritis, trauma, neuropathic osteo arthroscopy, pigmented villonodular synovitis, and osteoarthritis dissecans.
4. Inflammatory:
   1. Inflammatory or crystal-induced arthritis is due to rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, and pseudogout (chondrocalcinosis).
   2. In inflammatory or crystal-induced arthritis, there are predominantly polys.
5. Septic arthritis:
   1. It may be due to bacteria, tuberculosis, viral, and fungi.
6. Hemorrhagic arthritis:
   1. It is due to trauma, synovioma, and pigmented villonodular synovitis.

Classification of arthritis:

Gout (arthritis) showing:

1. The synovial fluid will show characteristic crystals.
   1. There is increased fluid uric acid.
   2. There are monosodium urate crystals.
   3. There are increased monocytes.
2. Calcium pyrophosphate is present in chondrocalcinosis (pseudogout).
3. Cholesterol is common in chronic arthritis. These are birefringent, square plate-like crystals with notched corners.

Procedure for arthrocentesis to get synovial fluid:

1. Sterile the area with an antiseptic solution.
2. Can give local anesthesia to decrease the pain.
3. Lie the patient on their back with the joint fully exposed.
4. The joint may be wrapped with an elastic bandage to compress free fluid within a particular area to get maximum fluid during aspiration.
5. Insert a sterile needle into the joint space and get the synovial fluid for analysis.
6. If corticoid steroids need to be injected, in that case, connect the already-filled syringe to the aspiration needle.
   1. Keep one mL in a sterile tube for culture.
   2. Another one mL in a heparinized tube for cell count.
7. Also, collect the blood for the comparison of the chemicals.
8. Aftercare: Note the severity of the pain.
   1. Apply ice packs to decrease the pain and swelling.
   2. Keep the pressure dressing to decrease the collection of fluid or hematoma formation.
   3. Avoid strenuous exercise for the next several days.
9. Contraindication:
   1. Avoid if there is skin or wound infection in the aspiration site because of the risk of sepsis.
10. Complication:
    1. There is a risk of joint infection and hemorrhage in the joint area.
    2. There may be a hemorrhage in the joint.

Normal constituents of the synovial fluid:

Source 1

Findings in different types of arthritis:

Synovial fluid laboratory tests are:

The synovial fluid analysis includes:

1. WBC count.
   1. Generally, WBCs <200/cmm and RBCs <2000/µL.
   2. There is an overlap between infectious and noninfectious arthritis.
   3. In bacterial arthritis, neutrophils >75%.
   4. Neutrophils >90% are diagnostic of bacterial arthritis.
2. Protein and lactate level:
   1. Estimate synovial fluid protein, uric acid, and lactate.
   2. Increased uric acid level indicates gout.
   3. Increased protein and lactate level suggests a bacterial infection.
3. Mucin clot (Ropes test):
   1. A few drops of the synovial fluid, add 10  to 20 mL of 5% acetic acid in a test tube.
   2. After one minute, shake the test tube.
   3. Interpretations:
      1. Normal findings = Firm clot with surrounding fluid having a clear appearance.
      2. Well-formed clot = In non-inflammatory arthritis (good clot).
      3. Clot good to poor = Non-infectious arthritis.
      4. The clot is poor = Bacterial arthritis.
   4. Clinical significance of mucin clot:
      1. It helps to diagnose inflammatory joint diseases.
      2. But this test lake specificity is 49% and has a low predictive value of 52%.
      3. This is not the test to diagnose joint diseases.
4. Viscosity:
   1. Allow the drip of the fluid from the needle.
   2. Note the string formation.
      1. Normal and non-inflammatory synovial fluid = String is 3 cms long.
      2. Acute inflammatory conditions = Fluids drips and very little string or no string formation.
5. Synovial glucose level:
   1. It is 10 mg/dL less than normal blood glucose and is always within the 20 mg/dL range.
   2. It is better to get blood glucose and the synovial fluid sample simultaneously for comparison.
      1. The patient should have fasted for 6 to 8 hours.
   3. In degenerative arthritis, synovial fluid glucose is normal.
   4. Synovial fluid glucose level falls with increasing severity of the inflammatory process.
   5. This is the lowest in septic arthritis. This may be 50% less than serum glucose levels (<40 mg/dL). It is seen in only 50% of the cases.
   6. Glucose is also low in rheumatoid arthritis, SLE, and gout. It is a mild to moderate decrease and is usually 40 mg/dL below the serum level.
   7. In bacterial arthritis, there is an increased level of protein and lactate.
   8. Rheumatoid arthritis shows more lymphocytes (lymphocytosis).
6. Gram stain
   1. It is done for the diagnosis of infection/noninfectious causes:
      1. It is positive in 40% to 75% of the cases.
      2. It can diagnose Gonorrhea.
7. AFB stain
   1. It is done to rule out tubercle bacilli.
8. Complement
   1. The complement level is done, which is low in the:
      1. Systemic lupus erythematosus.
      2. Rheumatoid arthritis.
      3. Other immunologic arthritis.
   2. Total complement may be increased in Reiter’s syndrome.
9. RA factor:
   1. This may be positive before the blood test is positive for Rheumatoid arthritis on the synovial fluid.
   2. LE cell phenomenon may be positive on synovial fluid.
10. Cholesterol crystals are found in rheumatoid arthritis.
11. Culture and sensitivity
    1.  This is the best tool for diagnostic purposes.
    2. This is advised for bacteria and fungi.
    3. If the patient has taken antibiotics, then it will decrease the positivity.
12. Polarized microscopy
    1. It is done in the presence of crystals.
    2. The calcium pyrophosphate dihydrate crystals are birefringent in pseudogout (blue on red background).
    3. Cholesterol crystals are seen in rheumatoid arthritis.

Summary of the lab finding of synovial fluid analysis:

1. Noninflammatory disease:
   1. Clear and straw color.
   2. WBCs count = <2000 WBCs, and Neutrophils are 25%.
2. Inflammatory diseases:
   1. The fluid is turbid yellow.
   2. <75000 WBCs, and Neutrophils >50%.
3. Septic arthritis:
   1. Fluid is opaque (Turbid yellow).
   2. >7500 WBCs, and Neutrophils >75%.
   3. Culture positive (Gonorrheal disease, TB, or other organisms).

Synovial fluids findings in different causes of arthritis: