SYNOVIAL FLUID
CHAPTER 11
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Learning Objectives
Upon completing this chapter, the reader will be able to
1. Describe the formation and function of synovial fluid.
2. Relate laboratory test results to the four common classifications of
joint disorders.
3. State the five diagnostic tests most routinely performed on synovial
fluid.
4. Determine the appropriate collection tubes for requested laboratory
tests on synovial fluid.
5. Describe the appearance of synovial fluid in normal and abnormal
states.
6. Discuss the normal and abnormal cellular composition of synovial
fluid.
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Learning Objectives (cont’d)
7. List and describe six crystals found in synovial fluid.
8. Explain the differentiation of monosodium urate and calcium
pyrophosphate crystals using polarized and compensated
polarized light.
9. State the clinical significance of glucose and lactate tests on
synovial fluid.
10. List four genera of bacteria most frequently found in synovial
fluid.
11. Describe the relationship of serologic serum testing to joint
disorders.
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Physiology
• Synovial fluid functions
– Lubrication for the movable joints: diarthroses
– Nutrients for articular cartilage
– Lessens shock of joint compression
• Formation
– Ultrafiltrate of plasma across synovial membrane
– No high weight molecules
– Synoviocytes in synovial membrane secrete hyaluronic
acid that makes the fluid viscous (lubrication)
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Anatomy
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Disorders
• Four classifications of disorders (arthritis)
1. Noninflammatory: degenerative, osteoarthritis
2. Inflammatory: immunologic, lupus erythematosus
(LE), rheumatoid arthritis (RA), Lyme disease
• Crystal-induced, gout and pseudogout
3. Septic: microbial infection
4. Hemorrhagic: trauma, tumors, coagulation
deficiencies
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Classification and Pathologic
Significance of Joint Disorders
Table 11–1 Synovial Fluid Reference Values2
Volume
<3.5 mL
Color
Colorless to pale yellow
Clarity
Viscosity
Leukocyte count
Neutrophils
Crystals
Glucose:plasma difference
Total protein
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Clear
Able to form a string 4 to 6 cm
long
<200 cells/µL
<25% of the differential
None present
<10 mg/dL lower than the blood
glucose level
<3 g/dL
Specimen Collection
& Handling
• Needle aspiration called arthrocentesis
• Normal knee fluid amount 3.5 mL
– >25 mL if inflamed
• Normal fluid does not clot; diseased fluid clots
• Collect in
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–
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–
–
Sterile heparinized or SPS for microbiology
Liquid EDTA (no powdered) for hematology
Heparinized or nonanticoagulated for other tests
Centrifuge nonanticoagulated tube and separate
Sodium fluoride for glucose
• Test ASAP to avoid cellular lysis and changes in crystals
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Appearance
• Normal: clear and pale yellow (egg white)
• Deeper yellow with noninflammatory and
inflammatory; green tinge = infection
• Hemorrhagic or traumatic tap = red
• Traumatic tap: look for decreasing blood in tubes
• Crystal induced: milky
• Turbidity: white blood cells (WBCs) or cellular debris,
fibrin
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Viscosity
• Polymerization of hyaluronic acid
– Essential for joint movement
• Arthritis decreases polymerization
• 4 to 6 cm string from aspirating needle = OK
• Ropes (mucin clot test)
– Add fluid to 2% to 5% acetic acid to form clot
– Rate, good: solid clot; clear fluid, fair: soft formed clot;
low: friable clot; poor: no clot
• Use acetic acid to identify synovial fluid
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Cell Counts
• WBCs most common
• Do not use normal WBC diluting fluid; use normal
saline/methylene blue
• May have to treat viscous fluid with hyaluronidase
or 37°C incubation first
• Perform on a Neubauer counting chamber
– Automated instrument, may need more hyaluronidase
• Normal: <200 WBCs/μL, septic may reach >100,000
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Counting Procedure
•
•
•
•
Line petri dish with moist filter paper.
Place hemocytometer on two small sticks above paper.
Fill both sides of the hemocytometer for compatibility.
For counts less than 200 WBCs/μL, count all nine large
squares.
• For counts greater than 200 WBCs/μL in the above count ,
count the four corner squares.
• For counts greater than 200 WBCs/μL in the above count,
count the five small squares used for a RBC count.2<<AU:
What does superscript 2 refer to?>>
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Differential Count
• Incubate with hyaluronidase, then cytocentrifuge
• Normal cells: monocytes, macrophages, synovial
tissue cells
• Neutrophils: <25%, increase in sepsis
• Lymphocytes: <15%, noninflammatory higher
• All cells may appear more vacuolated
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Differential Count (cont’d)
• Other cells
– LE cells
– Eosinophils
– Reiter cells/neutrophages: vacuolated macrophages with
ingested neutrophils
– Ragocytes (RA cells): neutrophils with small, dark
granules containing RA factor (IgM)
– Lipid droplets: crush injuries
– Hemosiderin granules: pigmented villonodular synovitis
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Crystal Identification
• Important diagnostic test; frequently performed
• Acute and chronic cases
• Metabolic disorders and decreased renal
function
• Other causes: increased blood levels,
degeneration of bone and cartilage, injection of
corticosteroids
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Types of Crystals
• Primary: monosodium urate (MSU) in gout; calcium
pyrophosphate dihydrate (CPPD) in pseudogout
• MSU: impaired purine metabolism, high purine
foods, leukemia chemotherapy, decreased renal
excretion of uric acid
• CPPD: degenerative arthritis, disorders causing
elevated calcium levels
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Types of Crystals (cont’d)
• Hydroxyapatite: cartilage degeneration, only seen
with electron microscopy
• Cholesterol: systemic autoimmune diseases (LE, RA)
appear similar to urine cholesterol crystals: notched
corners
• Corticosteroids: injections, flat, variable plates
• Calcium oxalate: renal dialysis patients
• Artifacts: starch, powdered anticoagulants, dust,
scratches (clean slides, cover slips)
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Slide Preparation
• Examine ASAP
– Crystal changes, MSU and
CPPD are seen
intracellularly and cells
disintegrate
• Initial examination is wet
preparation unstained
under low and high
power
• Crystals may be seen on
differential
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Slide Examination
• Continued examination is done under polarized and
compensated polarized light of wet preparation
• MSU crystals: needle shaped; seen intra- and
extracellularly; may be seen sticking through
cytoplasm
• CPPD crystals: rhombic, square shaped, or short
rods; often seen in vacuoles of neutrophils; MSU
crystals lyse vacuole membranes
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Crystal Polarization
• Both MSU and CPPD
crystals polarize light
• MSU is highly
birefringent and
appears brighter than
CPPD
• Confirm identification
using compensated
polarized light
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Compensated Polarized Light
• Red compensated polarized light: red compensator
is placed between crystal and analyzer, producing a
red background
• Separates light into slow- and fast-moving vibrations
• Align crystals with slow vibration
• Linear structure of molecules causes different colors
to be produced
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MSU and CCPD Crystals Under
Compensated Polarized Light
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MSU and CPPD Under
Compensated Light
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MSU Under Compensated Light
• MSU molecules run parallel to
the long axis, aligned with
slow vibration; fast light is
impeded, producing a yellow
color (negative birefringence)
• CPPD molecules run
perpendicular to long axis and
impede the slow light
producing a blue color
(positive birefringence)
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Chemistry Tests
• As an ultrafiltrate of plasma, normal values are
similar to those of plasma
• Few are clinically important
• Most frequent is glucose
– Normal: not less than 10 mg/dL of plasma glucose
– Draw sample same time as fluid is collected
– Markedly decreased levels seen in inflammatory and
septic categories
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Chemistry Tests (cont’d)
• Synovial fluid protein
– Normal is less than 3 g/dL
• Increased in inflammatory and hemorrhagic categories
• Synovial fluid lactate and acid phosphatase
• Severity and prognosis of rheumatoid arthritis
(RA)
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Reference Synovial Fluid Values
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•
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•
•
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Volume: <3.5 mL
Color: colorless to pale yellow
Clarity: clear
Viscosity: able to form a string 4 to 6 cm long
Leukocyte count: <200 cells/μL
Neutrophils: <25% of the differential
Crystals: none present
Glucose: <10 mg/dL lower than the blood glucose plasma
difference
• Total protein: <3 g/dL
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Microbiology Tests
• Infections caused by inflammation, trauma, and
systemic infections
• Gram stain and cultures are routinely performed
• Culture must include chocolate agar
– Primary organisms are Staphylococcus, Streptococcus,
Haemophilus influenzae, and Neisseria gonorrhoeae
– Patient history determines fungal and Tuberculosis
(TB) cultures
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Serologic Tests
• Majority of related tests are performed on serum
and fluid may only serve as a confirmation
• RA and LE are the most common autoimmune
causes of arthritis
• Lyme disease: arthritis is frequent complication;
test serum for Borrelia burgdorferi antibodies
• Extent of inflammation: test for C-reactive
protein and fibrinogen
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