A 62 year old man present to clinic with a two day history of a painful swollen, warm left knee. You perform a diagnostic arthrocentesis and remove 20cc of cloudy fluid. The WBC count from the aspirate is 42,000/mm3.
What 3 characteristics of the synovial fluid will help you assess whether it is inflammatory at the bedside?
Examining the clarity, colour and viscosity of the joint aspirate can give you clues regarding the degree of inflammation present in the joint.
Normal synovial fluid is clear enough that one can see the markings on the syringe through it distinctly, while inflammatory fluid may be hazy or opaque. Normal fluid is also straw-colored and viscous. This means that if a small amount of fluid is ejected from the syringe, it should form a long strand rather than falling in discrete drops. The strand that forms should be at least 10 cm in length. This is also called the “string sign.”
What is your differential diagnosis for a WBC count of 42,000/mm3?
The cell count in normal joint fluid is >200/mm3. Joint fluid is considered clearly inflammatory if the white count is over 2000/mm3. Above values of 80,000/mm3, the fluid is very suggestive of infection. However, it is important to note that infection can also be present in joints with cell counts well under 80,000/mm3, particularly in cases of partial treatment or immunosuppression.
The differential diagnosis for a WBC count of 42,000/mm3 would include infection as well as crystal arthritis, reactive arthritis or a chronic inflammatory arthritis such as psoriatic arthritis or rheumatoid arthritis.