To examine the utility of using urea concentrations for determining Synovial Fluid (SF) joint volume in effused and non-effused joints.

Knee joint SF was aspirated from 159 human study participants with symptomatic osteoarthritis of at least one knee either directly (165 knees) or by lavage (110 knees). Serum was obtained immediately prior to SF aspiration. Participants were asked to rate individual knee pain, aching or stiffness. SF and serum urea levels were determined using a specific enzymatic method run on an automated CMA600 analyzer. Cell counts were performed on direct SF aspirates when volume permitted. The formula for calculating SF joint volume was as follows: V_j=C_D(V_I)/(C−C_D) with V_j=volume of SF in entire joint, C_D=concentration of urea in diluted (lavage) SF, V_I=volume of saline injected into joint, and C=concentration of urea in undiluted (neat) SF derived below where C=0.897(C_S) and C_s=concentration of urea in serum.

There was an excellent correlation (r²=0.8588) between SF and serum urea in the direct aspirates with a ratio of 0.897 (SF/serum). Neither urea levels nor the SF/serum ratio showed any correlation with Kellgren Lawrence (KL) grade, or cell count. While urea levels increased with age there was no change in the ratio. Intraarticular SF volumes calculated for the lavaged knees ranged from 0.555 to 71.71ml with a median volume of 3.048ml. There was no correlation of SF volume to KL grade but there was a positive correlation (P=0.001) between SF volume and self-reported individual knee pain.

Our urea results for direct aspirates indicate an equilibrium state between serum and SF with regard to the water fraction. This equilibrium exists regardless of disease status (KL grade), inflammation (cell count), or age, making it possible to calculate intraarticular volume of lavaged joints based upon this urea method. Most of the joint volumes we calculated fell within the previously reported range for normal knees of 0.5–4.0ml. The positive correlation between SF volume and knee symptoms reinforces the clinical utility of this method for quantifying SF volume.