Fluid Analysis:- Part 8 – Synovial Fluid Analysis, Procedure for Arthrocentesis
Sample of the Synovial fluid
- Synovial fluid is the aspirated fluid from the synovial spaces called Arthrocentesis.
- Collect specimens in three tubes:
- Tube 1 for culture.
- Tube 2 for microscopy, add heparin, and not use EDTA powder.
- You can use liquid EDTA.
- Tube 3 for chemistry.
- For glucose, the patient should have 6 hours fast.
- Synovial fluid can be aspirated from joints of:
Precautions for synovial fluid
- Powered anticoagulants should not be used because this may interfere with crystal analysis.
- The non-anticoagulant test tube should be centrifuged to prevent the cellular elements from interfering with the chemical and serologic analysis.
- All testing should be done as soon as possible to prevent cellular and possible changes in the crystal.
Purpose of the test (Indications)
- To diagnose the cause of synovial fluid formation.
- To differentiate inflammatory to non-inflammatory causes.
- Arthritis due to crystals like gout and pseudogout.
- Malignant tumor involving the joint.
- To inject anti-inflammatory medicines like corticosteroids.
- To monitor chronic arthritic diseases.
Definition of synovial fluid:
- This is often referred to as joint fluid, the viscous fluid found in the cavities of the moveable joints.
- Normally synovial fluid is transudate with <200 WBCs and <25% neutrophils.
Composition of synovial fluid:
- Synovial fluid is formed as an ultrafiltrate of plasma across the synovial membrane.
- This filtration is nonselective except for the exclusion of high molecular weight proteins.
- The contents of the synovial fluid are similar to the plasma values.
- This synovial fluid does not typically clot. But the inflammation due to increased fibrinogen may clot.
- The synovial cells lining the synovium secrete a mucopolysaccharide containing the hyaluronic acid and a small amount of the protein.
- This hyaluronic acid causes noticeable viscosity of the synovial fluid.
- Synovial fluid is present normally in a very small amount. The amount in the large knee joint is less than 3.5 mL.
- This fluid collection can increase inflammation and maybe around 25 mL.
- Synovial fluid contains mucopolysaccharides called hyaluronic acid, which are responsible for the viscosity of the synovial fluid and lubricates the joints.
- An increase in a synovial fluid enough to aspirate is due to some disease.
Functions of the synovial fluid:
- The smooth articular cartilage and synovial fluid reduce friction between the bone during joint movements.
- Synovial fluid supplies nutrients to the cartilage and acts as a lubricant to the joint, bursa, and tendons sheaths.
TYpes of arthritis:
- Damage to the articular membrane produces pain and stiffness in the joints, referred to as arthritis.
- There are the following types of arthritis:
- This arthritis is seen in osteoarthritis, trauma, neuropathic osteo arthroscopy, pigmented villonodular synovitis, and osteoarthritis dissecans.
- Inflammatory or crystal-induced arthritis is due to rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, and pseudogout (chondrocalcinosis).
- In inflammatory or crystal-induced arthritis, there are predominantly polys.
- Septic arthritis:
- It may be due to bacteria, tuberculosis, viral, and fungal.
- Hemorrhagic arthritis:
- It is due to trauma, synovioma, and pigmented villonodular synovitis.
Classification of arthritis:
|Etiological classification||Cause of the arthritis|
|Non-inflammatory||Degenerative joint disease (Osteoarthritis) seen in old age|
|Inflammatory||An immunologic disorder like Rheumatoid arthritis and SLE|
|Septic||Due to microbial infection|
|Hemorrhagic||Coagulation factor deficiency and traumatic injury|
|Due to crystals||Uric acid in gout|
Gout (arthritis) showing:
- The synovial fluid will show characteristic crystals.
- There is increased fluid uric acid.
- There are monosodium urate crystals.
- There are increased monocytes.
- Calcium pyrophosphate is present in chondrocalcinosis (pseudogout).
- Cholesterol is common in chronic arthritis. These are birefringent, square plate-like crystals with notched corners.
Procedure for arthrocentesis to get synovial fluid:
- Sterile the area with an antiseptic solution.
- Can give local anesthesia to decrease the pain.
- Lie the patient on their back with the joint fully exposed.
- The joint may be wrapped with an elastic bandage to compress free fluid within a particular area to get maximum fluid during aspiration.
- Insert a sterile needle into the joint space and get the synovial fluid for analysis.
- If corticoid steroids need to be injected, in that case, connect the already filled syringe with the aspiration needle.
- Keep one mL in a sterile tube for culture.
- Another one mL in a heparinized tube for cell count.
- Also, collect the blood for the comparison of the chemicals.
- Aftercare: Note the severity of the pain.
- Apply ice packs to decrease the pain and swelling.
- Keep the pressure dressing to decrease the collection of fluid or hematoma formation.
- Avoid strenuous exercise for the next several days.
- Avoid if there is skin or wound infection in the aspiration site because of the risk of sepsis.
- There is a risk of joint infection and hemorrhage in the joint area.
Normal constituents of the synovial fluid:
|Volume||<1.5 mL (1.0 to 3.5 mL)|
|pH||It is like a serum|
|RBC count||<2000/cmm (0 to 2000/cmm)|
|White cell count||<200 /cmm (0 to 200/cmm)|
|Polys||<20% of the differential (<25%)|
|Lymphocytes||<15% of the differential|
|Monocytes||65% of the differential|
|Macrophages||variable in number|
|Glucose||<10 mg/dL lower than the blood level|
|Total protein||<3 G/dL (1 to 3 G/dL)|
|Uric acid||This is equal to the blood level (male = 2 to 8 mg/dL and female = 2 to 6 mg/dL)|
Findings in different types of arthritis:
|Color||yellow, clear||yellow to white (Turbid yellow)||yellow to green||red to brown (blood or xanthochromic)|
|TLC cmm||<5000||10,000 to 100,000||10,000 to 200,000||50 to 10,000|
|Etiology (causative agent)||Degenerative joint disorder||Immunologic, Rheumatoid, lupus||Staph. Aureus, H.influenzae,
Strep, pneumococci, Neisseria
|Lactate mg/dL||normal||normal to high||>250 positive||normal|
|glucose serum /fluid mg/dL||<10||>25||>25||<10|
Laboratory tests advised are:
The synovial fluid analysis includes:
- WBC count.
- Generally WBCs <200/cmm and RBCs <2000/µL.
- There is an overlap between infectious and noninfectious arthritis.
- In bacterial arthritis, neutrophils >75%.
- Neutrophils >90% are diagnostic of bacterial arthritis.
- Protein and lactate level:
- Estimate on synovial fluid protein, uric acid, and lactate.
- Increased uric acid level indicates gout.
- Increased protein and lactate level suggests a bacterial infection.
- Mucin clot (Ropes test):
- A few drops of the synovial fluid add 10 to 20 mL of 5% acetic acid in a test tube.
- After one minute, shake the test tube.
- Normal findings = Firm clot with surrounding fluid having a clear appearance.
- Well-formed clot = In non-inflammatory arthritis (good clot).
- Clot good to poor = Non-infectious arthritis.
- The clot is poor = Bacterial arthritis.
- Clinical significance of mucin clot:
- It helps to diagnose inflammatory joint diseases.
- But this test lake specificity is 49% and has a low predictive value of 52%.
- This is not the test to diagnose joint diseases.
- Allow the drip of the fluid from the needle.
- Note the string formation.
- Normal and non-inflammatory synovial fluid = String is 3 cms long.
- Acute inflammatory conditions = Fluids drips and very little string or no string formation.
- Synovial glucose level:
- It is 10 mg/dL less than the blood glucose normally and is always within the 20 mg/dL range.
- It is better to get blood glucose and the synovial fluid sample simultaneously for comparison.
- The patient should have fasted for 6 to 8 hours.
- In degenerative arthritis, synovial fluid glucose is normal.
- Synovial fluid glucose level falls with increasing severity of the inflammatory process.
- This is the lowest in septic arthritis. This may be 50% less in comparison to serum glucose levels (<40 mg/dL). It is seen in only 50% of the cases.
- 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.
- In bacterial arthritis, there is an increased level of protein and lactate.
- Rheumatoid arthritis shows more lymphocytes (lymphocytosis).
- Gram stain
- It is done for the diagnosis of:
- It is positive in 40% to 75% of the cases.
- It can diagnose Gonorrhea.
- It is done for the diagnosis of:
- AFB stain
- It is done to rule out tubercle bacilli.
- Complement level is done which is low in:
- Systemic lupus erythematosus.
- Rheumatoid arthritis.
- Other immunologic arthritis.
- Total complement may be increased in Reiter’s syndrome.
- Complement level is done which is low in:
- RA factor:
- This may be positive before the blood test is positive for Rheumatoid arthritis on the synovial fluid.
- LE cell phenomenon may be positive on synovial fluid.
- Cholesterol crystals are found in rheumatoid arthritis.
- Culture and sensitivity
- This is the best tool for diagnostic purposes.
- This is advised for bacteria and fungi.
- If the patient has taken antibiotics, then it will decrease the positivity.
- Polarized microscopy
- It is done in the presence of crystals.
- The calcium pyrophosphate dihydrate crystals are birefringent in pseudogout (blue on red background).
- Cholesterol crystals are seen in rheumatoid arthritis.
Summary of the lab finding of synovial fluid analysis:
- Noninflammatory disease:
- Clear and straw color.
- WBCs count = <2000 WBCs, and Neutrophils are 25%.
- Inflammatory diseases:
- The fluid is turbid yellow.
- <75000 WBCs, and Neutrophils >50%.
- Septic arthritis:
- Fluid is opaque (Turbid yellow).
- >7500 WBCs, and Neutrophils >75%.
- Culture positive (Gonorrheal disease, TB, or other organisms).
Synovial fluids findings in different causes of arthritis:
|Diseases||Viscosity||Mucin clot||Glucose||Total cell count||Neutrophils||Special tests|
|Normal synovial fluid||Normal or increased||Good clot||After fasting same level||0 to 200/cmm||0 to 25%|
||Normal to 30 mg/dL, below blood level||2000 to 50,000/cmm||40% to 90%||Urates crystals|
||Normal||2000 to 50,000/cmm||35% to 85%||calcium pyrophosphate crystals|
||Normal to 30 mg/dL, below blood level||2000 to 50,000/cmm||70% to 80%||RA factor is positive|
|Systemic lupus erythematosus (SLE)||Normal or low||Good to fair||Normal to 20 mg below blood level||2000 to 5000/cmm||10% to 30%||LE cell preparation|
|Trauma with hemorrhage||Normal||Good||>5000/cmm||25 to 50%||many RBCs|
|Acute bacterial arthritis||Low||Poor||>40 mg below the blood level||>50,000/cmm||75% to 95%||
|Acute rheumatic fever||Low||Good to poor||Small decrease||2,000 to 15,000/cmm||50% to 60%||Negative|
Question 1: What is the glucose level of synovial fluid in gout,
It may be normal or 30 mg/dL less than the serum.
Question 2: What is the normal volume of the synovial fluid.
It is normally 1.0 to 3.5 mL.