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