Synovial Fluid Analysis

Synovial Fluid

Contents:

plasma dialysate (water) – enters joint space from synovial capillary network

hyaluronic acid (from B cells) – glycosaminoglycan synthesized and secreted by B cells of the synovial membrane; composed of long chains of repeating disaccharides; steric configuration of these coils gives synovial fluid its high viscosity

small molecules (glucose, electroyles) – diffuse freely in and out of join space

secreted proteins (also from B cells in the synovial membrane) – intermediate-sized molecules which are moderately retarded in diffusion

Functions of synovial fluid: lubricate joint and provide nutrition to the cartilage.

Examination of Synovial Fluid

can yield specific diagnosis (ex: gout), and can help limit the differential diagnosis (ex: <2000 WBC/mm³ = non-inflammatory), can reflect changes in the synovial membrane and supporting structures.

Tests to perform on SF:

(1) Appearance (color and clarity)

Normal: clear to pale straw colored.

Pathologic: varies with degree of inflammation, usually yellow or green tinged with cloudiness depending on WBC count

Bloody: Hemarthrosis (hemophilia, joint trauma, etc.)

Note: bloody fluid with a fatty foam could result from a Tibial Plateau fracture.

(2) Viscosity (String sign is a quick measure of viscosity) – stretch a drop between thumb and forefinger

Decreased when PMN enzymes are released into joint fluid and degrade HA.

(3) Cell counts

normal – less than 200
grade I (non-inflammatory; acute traumatic arthritis, osteoarthritis) – 200-2000
grade II (inflammatory; RA, gout, SLE) – 2000-75,000
grade III (septic; infection, bacterial, mycobacterial, fungal) – 75,000

(4) Chemical tests: glucose, protein

protein: <2.5 gm/dl = non-inflammatory; >2.5 gm/dl = inflammatory
glucose: normally within 20% of simultaneous blood glucose; with increasing inflammation, glucose decreases
a "normal" glucose level does not rule out bacterial infections particularly in a patient with diabetes mellitus

(5) Wet Prep – cells, crystals, fat droplets, debris

Crystals:

Sodium urate (strongly negative birefringence (light test))
Calcium pyrophosphate (weakly positive birefringence)

(6) Gram Stain + Culture – should be done on all inflammatory fluids, even when crystals are noted!

Synovial fluid analysis diagnostic in:

gout and pseudogout (crystals on wet mount)
bacterial infection (gram stain and/or culture)

An acute, hot, painful and swollen joint is a medical emergency. Synovial fluid examination should be performed

Blood in post-traumatic joint effusion (hemarthrosis) may indicate:

Intra-articular fracture
Torn intra-articular structure (e.g., cruciate ligament of the knee)
Tear through synovial membrane into underlying capillary structures.