What's the big deal with septic arthritis? It's not really an emergency is it?
- Within days of onset, septic arthritis destroys cartilage and can leave the joint with permanent joint disability occurring in 25-50% of the cases.
- The mortality rate for in-hospital septic arthritis ranges from 7% to 15%, despite antibiotic use.
Ok ok. So that's pretty bad. How do I make sure I don't miss a septic arthritis?
- Age >80 (+LR 3.5)
- Diabetes (+LR 2.7)
- Rheumatoid arthritis (+LR 3.5)
- Joint surgery within past 3 mo (+LR 6.9)
- Hip/knee prosthesis (+LR 3.1)
- Skin infection (+LR 2.8)
- Skin infection + prosthesis (+LR 15)
List of common myths and dogma regarding septic arthritis:
- Fever Sensitivity 46%. Specifificity 31% (1)
- So you’d only miss 54% of septic joints relying on fever.
"If the serum WBC isn't elevated, it's not a septic joint."
- Sensitivity WBC >10,000 is 90% (1)
- If you're ok with missing 10% of septic joints.
"If the CRP and ESR aren't elevated, it's not a septic joint"
- Sensitivity ESR >30 is 95% (1) - miss 5% of septic joints relying on a very low cutoff for ESR
- Sensitivity CRP >100 is 77% (1) - miss 23% of septic joints relying on CRP
"If the synovial WBC <50,000 it's not a septic joint” (My personal favorite)
- Sensitivity WBC >50,000 is 62% (1)
- So 38% of septic joints are missed relying on a synovial WBC >50,000
"If the neutrophile count is <90% it's not a septic joint"
- Sensitivity PMN >90% is 73% (1)
- Missing 27% of septic joints
"If the gram stain is negative, it's not a septic joint"
- Sensitivity of gram stain 50-80% (2)
- Missing 20-50% of septic joints.
- You have to follow the culture for 48 hours.
- Depending on their risk factors, your clinical suspicion, and synovial WBC, you may or may not start antibiotics until the cultures come back
"If there are crystals, it's not a septic joint"
- 5% of crystal monoarthritis have a concomitant septic arthritis (3)
- Crystals have also been found in asymptomatic patients, so crystals definitely can’t exclude an infection
"Synovial fluid culture is the gold standard rule out test for septic arthritis. If the culture is negative, it's not septic arthritis."
- Synovial fluid culture is the most important test we have to rule out septic arthritis, but it's not perfect. There have been multiple reports of cases of septic arthritis where other diagnostic tests (such as imaging), clinical course, etc resulted in a definitive diagnosis of septic arthritis despite an initial negative culture from synovial fluid. (4, 5)
- The incidence of culture-negative septic arthritis is certainly small, but incredibly difficult to give a number too since there is no true gold-standard test to compare a 48 hour synovial culture to.
- The take home message is that when septic arthritis is suspected, empiric antimicrobial therapy is warranted following arthrocentesis until culture data are available. Even if the gram stain and culture are negative, if the patient seems to be responding to empiric therapy, continuing a full treatment course of antibiotics may be necessary given the lack of any true gold standard diagnostic test.
How do I perform an arthrocentesis?
- Sterile prep
- Consider using ultrasound. (I’m a big fan of ultrasound as an adjunct to look for the effusion… especially for small joints like wrists and ankles.)
- Lidocaine injection
- 18 or 20 gauge needle to aspirate fluid
- Location of needle entry for ankle, elbow, knee, shoulder via very photo from @ClinicalEMed below:
- Margaretten, Mary E., et al. "Does this adult patient have septic arthritis?." Jama 297.13 (2007): 1478-1488.
- Genes, N., and M. Chisolm-Straker. "Monoarticular arthritis update: Current evidence for diagnosis and treatment in the emergency department." Emergency medicine practice 14.5 (2012): 1-19.
- Papanicolas, Lito Electra, Paul Hakendorf, and David Llewellyn Gordon. "Concomitant septic arthritis in crystal monoarthritis." The Journal of rheumatology 39.1 (2012): 157-160.
- Ryan, M. J., et al. "Bacterial joint infections in England and Wales: analysis of bacterial isolates over a four year period." Rheumatology 36.3 (1997): 370-373.
- Sharff, Katie A., Eric P. Richards, and John M. Townes. "Clinical management of septic arthritis." Current rheumatology reports 15.6 (2013): 1-9.
A piece of comic relief.
It is always gout by @