Septic arthritis is a direct invasion of the joint space by micro-organisms.
A patient presents with a painful right ankle. You perform a tap and send the fluid to the lab. The following result returns(real case):
White Cell Count 95,000 x106/L Polymorphs 87%
Red Cell Count 13000 x 106/L
Gram Stain: No Organisms seen
Crystals: Sodium Urate 2+
Is this Septic Arthritis or Gout?
This is Septic arthritis by definition. We accept that >50,000 white cells point to septic arthritis. If there are more than 75% polymorphs it is septic.
You can have rate and birefringent crystals at the same time.
My rule of thumb is the rule of 50. More than 50,000 x 106/L white cells and > 50% polymorphs EQUALS SEPTIC JOINT