Synovial Fluid Analysis: Common Questions

Synovial fluid is the viscous fluid found in the synovial joints. It is also called as joint fluid. The word synovium is derived from Greek which means “with Egg” as the synovial fluid resembles that of egg white.

The synovial fluid is produced by the cells of the syovial membrane. Synovial fluid is an ultra filtrate of the plasma. It contains proteins derived from blood as well as from the secretions from synoviocytes.

There are two types of synoviocytes
a. Type A synoviocyte/Macrophage like: which are derived from monocytes/macrophages, they are primarily phagocytic in function. They remove the debris from the synovial fluid.
b. Type B cells/Fibroblast like: These are actually the proper synoviocytes which produces specialized matrix constituents including hyaluronan, collagens and fibronectin.

The difference is in the content of Hyaluronic acid which is very high in the synovial fluid. The viscosity of the synovial fluid is due to the high content of hyaluronic acid.

The functions of synovial fluid are
a. Lubrication of joints
b. Nutrition to the articular tissue
c. As shock absorbers: which reduces the shock of compression of joints during walking or jogging.

Synovial fluid is collected by a procedure called arthrocentesis. This is a needle aspiration of the affected joint.

Diagnostic	Therapeutic
Suspected arthritis	Relief of pain due to effusions of fluid or blood
Joint effusions	Drainage of collections in septic arthritis
Crystal arthropathy	Injecting medications
Monoarticular arthritis

The different specimen containers are
a. Heparinized tube: For cell counts
b. Plain tube : For chemical examination and immunological studies
c. Sterile containers: For microbiological examination.

The components of synovial fluid examination are
a. Physical examination: Volume, Color , Clarity, Viscosity, Clotting and Mucin clot.
b. Chemical examination: Proteins, Glucose, Lactic acid, Lactate dehydrogenase and Uric acid.
c. Microscopic examination : Cell count, Cell type, Crystals and Microbiologic examination.

Normally, the synovial fluid is colorless to pale yellow and clear. The following color changes are found in different conditions

Deep yellow, clear	Non inflammatory effusions
Deep yellow and turbid	Inflammatory effusions
Milky/white	In the presence of crystals
Red, brown or xanthochromic	Presence of hemorrhage

The synovial fluid is expressed into the test tube from the tip of the syringe, one drop at a time. Normally the synovial fluid will form a string, approximately around 4-6 cm before it breaks.
Normally the synovial fluid is highly viscous. In noninflammatory conditions the viscosity will be reduced .In inflammatory, septic or crystal induced diseases the viscosity is absent.

Mucin clot test is also known as Rope test. This is performed to estimate the integrity of mucin ie, hyaluronic acid-protein complex. One part of synovial fluid is mixed with four parts of 2% acetic acid. Normal synovial fluid forms a ropy clot which indicates good integrity of hyaluronate. If the ropy clot is very fragile and breaks up easily then it means that the integrity of mucin is poor.

The mucin clot test is interpreted as
Good (solid clot),
Fair (soft clot),
Low (friable clot), and
Poor (no clot).

Presence of fibrinogen results in the formation of clot. It is caused due to damage to synovial membrane or because of traumatic aspiration. The clot interferes with the microscopic examination.

The normal synovial fluid parameters are as follows

Physical examination
Color and clarity	Colorless to pale yellow and clear
Viscosity	High
Mucin clot test	Good
Chemical examination
Protein	1–3 g/dL.
Glucose	less than 10 mg/dL lower than serum levels
Uric acid	6 to 8 mg/dL.
Lactic acid.	less than 25 mg/dL
Microscopic examination
Cell count	Leukocytes 150 -200 cells/micro L
Cell type	Neutrophils 25% of the differential
Crystals	Absent
Microbiological examination
Culture	Negative

These are the neutrophils which have phagocytosed immune complexes like IgG, IgM, compliment and rheumatoid factor. The cytoplasmic granules appear dark. Ragocytes are found in Rheumatoid arthritis.

There are actually the mononuclear cells/macrophages which have phagocytosed the neutrophils. These are found in Reiter syndrome.

Physical examination	Non inflammatory	Inflammatory	Septic	Hemorrhagic	Crystal induced effusion.
Color and clarity	Dark yellow, slightly cloudy	Dark yellow/whitish, cloudy to turbid	White/yellow or green Turbid	Red/brown or cloudy	Milky/cloudy/turbid
Viscosity	Reduced	Absent	Absent	Absent	Absent
Mucin clot test	Fair	Poor	Poor	Poor	Poor
Microscopic examination
Cell count	200- 2 000 WBCs	2,000 to 50,000	50,000–200,000 WBCs	50–10,000	500–200,000
Cell type	Less than 30% neutrophils	Greater than 50% neutrophils	Greater than 17%% neutrophils	Less than 50% neutrophils Plenty of red blood cells.	Less than 17%% neutrophils
Crystals	Negative	Negative	Negative	Negative	Plenty Gout: needle shaped Pseudogout : rhomboid.
Microscopic examination
Culture	Negative	Negative	positive	Negative	Negative cc

The following parameters should be considered when one is looking for crystals in the Synovial fluid
a. Shape
b. Birefringence
c. Location (intra/extracellular)
d. Quantity (sparse/plenty

	Condition	Shape	Birefringence	Location	Quantity
Monosodium urate crystals (MSU)	Gout	needle- shaped	Negative	Can be seen inside or outside the cells.	Plenty
Calcium pyrophosphate dehydrate (CPPD)	Pseudo gout	Short sticks , rectangles or small squares	Weak positive	Intra and extracellular	Plenty
Calcium hydroxyapatite	Symptomatic inflammation	Irregular or Chinese coin shape	Negative	extracellular	Few
Cholesterol	Rheumatoid . rarely osteoarthritis and gout	Large square with notch at one of the corner	Negative/positive	Extracellular	Few to plenty
Lipid	acute monoarthritis, chronic polyarthritis, and pigmented villonodular synovitis	Maltese cross	Positive	Extracellular	Few to plenty
Calcium oxalate	Renal damage and oxalosis	Square, pyramidal, short stick or oval	Strong positive	Extracellular	Plenty