Don't pass that joint

Risk stratifying hot joints is where we can probably save the most lives in the waiting room. Septic joints have a reported 10-30% mortality and need to be separated from rheumatic, gouty, and traumatic joints.

An ED complaint of a monoarticular swollen joint, even in a pt with a previous diagnosis of gout or rheumatoid disease, usually means the joint is atypical for them. The rest of the history (like fever and decreased ROM) is nonspecific. Risk factors like any pre-existing joint disease (that includes RA, gout, hardware), loss of skin integrity (injections, dialysis, ulcers, IVDA), and immunocompromised status (steroids, DM, HIV) can be more useful. Make sure they dont have a gonococcal rash, but data shows the physical exam is rarely helpful. Unless it is their typical MCP with a history of gout: they need a workup.

The workuptypically consists of labs and joint fluid analysis. The labs are difficult to interpret; a recent study titled, Sensitivity of erythrocyte sedimentation rate and C- reactive protein for the exclusion of septic arthritis in emergency department patients reported that the sensitivity of CRP was only 92% using a cutoff of ≥20 mg/ L. So unless you have a really weak story, almost no risk factors, and a stone cold normal CRP, you need more workup. 

The gold standardworkup has traditionally been needle joint fluid aspiration, but this is far from perfect. (don’t tap a joint through cellulitis.) Crystals don’t help exclude infection, 25% of septic arthritis have joint WBC <40,000, and <25,000 only gives you a 0.32 negative LR, <90% PMNs only gives you a 0.32 negative LR. The Gram stain is only 50% sensitive and the culture is only 80% sensitive. (Check out the JAMA article below; Does this adult patient have septic arthritis? For more depressing news)
The a swollen joint needs to be treated like chest pain. Story, risk factors, exam, labs, arthrocentesis results, all need to be judged together. No test excludes the disease and no constellation of parameters should give you total confidence. Even if you
re are sure enough to send the patient home with a diagnosis of gout or RA flare, make sure tell them to come back before they die of sepsis.  

J Emerg Med. 2011 Apr;40(4):428-31

JAMA. 2007 Apr 4;297(13):1478-88.