Acute Monoarticular Arthritis (Approach)
Main categories in the DDx:
1. Traumatic – usually fairly easy to sort out by history, but also consider less obvious causes such as hemophilia, coagulopathies that can predispose to hemarthroses with minimal or no trauma. On tap, expect to see bloody fluid.
2. Infectious – main distinction is Gonococcal, vs Nongonococcal. This distinction is important because gram stain and cultures are much less sensitive for Gonococcus vs typical bacterial pathogens.
3. Crystal Deposition – Gout vs Pseudogout. Keep in mind that clinically, they are virtually indistinguishable except by tap and crystal analysis.
4. Inflammatory – includes seronegative spondyloarthropathies (Reactive arthritis, psoriatic, IBD-associated), and Rheumatoid Arthritis. Caveat is that these typically present with polyarticular arthritis and more chronic, but can certainly present with acute monoarticular arthritis.
In general, you should have a fairly low threshold for performing joint arthrocentesis and sending the fluid for analysis.
Things to look for in the fluid:
1. Appearance – purulent, bloody, clear
2. Cell count:
- Normal - < 200 WBC
- Osteoarthritis – 200 – 2000 WBC
- Inflammatory (including Gout, Pseudogout) – 2000 – 100,000
- Infectious- > 50,000-100,000
** Important to note that inflammatory and infectious arthritis have considerable overlap in the WBC count – i.e. with a WBC of 50 – 100k, it can be either inflammatory or infectious **
3. Gram stain and culture – note that the sensitivity of gram stain in infectious arthritis is only 50-70% at best (and much lower for gonococcal arthritis). Cultures are much more sensitive, although they take longer and still are insensitive for gonococcal arthritis.
4. Crystal Analysis – looking for negatively birefringent needle-shaped crystals for gout, and positively birefringent rhomboid shaped crystals for pseudogout.
** Remember that gout/pseudogout and infection can co-exist, so ALWAYS ALWAYS send for gram stain and culture, and have low threshold to treat with Abxs even if crystals are present, until you are sure there is no infection!! **
Along those lines, according to the American College of Rheumatology committee, synovial fluid analysis should be performed in the febrile patient with an acute flare of established arthritis (eg, rheumatoid arthritis, osteoarthritis) to rule out superimposed septic arthritis. Repeated aspiration and synovial fluid analysis may be used to monitor the response of septic arthritis to treatment and may also be valuable for diagnosis of some cases of gout in which the initial aspirate does not have detectable crystals.
(Chanu Rhee MD, 1/27/11)