septic arthritis vs. transient synovitis in kids

HOW CAN WE TELL THE DIFFERENCE?
--lots of overlap in signs/symptoms
--one article came up with a decent decision rule


J Bone Joint Surg Am. 1999 Dec;81(12):1662-70.
--looked at 282 cases, excluded 114 atypical patients (e.g. immunocompromised), leaving 168
--38 had "true" septic arthritis
--septic arthritis defined by positive culture or joint WBC >= 50,000 cells/mm3

--four things they decided on that might differentiate:

• history of fever
• non-weight-bearing
• erythrocyte sedimentation rate (ESR) >= 40 mm/hr
• serum WBC > 12,000 cells/mm3

--predicted probability of septic arthritis using these four predictors:

• < 0.2 percent for zero predictors
• 3.0 percent for one predictor
• 40.0 percent for two predictors
• 93.1 percent for three predictors
• 99.6 percent for four predictors

BOTTOM LINE:
--septic arthritis is bad, transient synovitis not so bad
--hard to tell sometimes, signs/symptoms are often similar
--useful: history of fever, non-weight bearing, ESR>40, WBC>12k
--if none of the above: unlikely septic arthritis
--more of the above: worry a bit more


Submitted by S. Lee.


Reference(s): kocher article, picture

What is the Synovial WBC cutoff in Septic Arthritis?

GOOD QUESTION:
--so you're worried about a septic joint
--you've tapped it, now the results are back...


UPTODATE.COM:
--suggests WBC 15,000-100,000/mm3, PMNs >75% for likely septic arthritis


REVIEW ARTICLE 1:
KEY POINTS:
--recent joint surgery or cellulitis over a prosthetic joint were the only findings on H&P that significantly alter the probability of nongonococcal septic arthritis


--positive likelihood ratios with synovial white blood cell (sWBC) counts of...

• 0 to 25,000 WBC per mm3 (or uL): +LR = 0.33
• 25,000 to 50,000: 1.06
• 50,000 to 100,000: 3.59
• >100,000: infinity 

--their conclusion: synovial WBC (>50,000/mm3--if I've done the conversion correctly) can increase, but not decrease, the probability of septic arthritis


REVIEW ARTICLE 2:
KEY POINTS:
--most common associated symptoms (found in >50% of patients): joint pain, joint swelling, fever


--positive likelihood ratios with synovial white blood cell (sWBC) counts of...

• <25,000/microL: LR 0.32
• >25,000/microL: LR, 2.9
• >50,000/microL: LR, 7.7
• >100,000/microL: LR, 28.0 

--synovial WBC differential mattered too: >90% PMNs, LR 3.4; <90% PMNs, LR 0.34

BOTTOM LINE:
--synovial WBC >25k/mm3 is suspicious
--synovial WBC >50k/mm3 very suspicious
--synovial WBC >100k/mm3 don't think too hard
--lots of PMNs (75%+) on the diff, don't think too hard either

Reference(s): review article 1, review article 2, uptodate.com