Worried about a septic joint? Follow these steps!
If someone has a swollen, erythematous, warm and painful joint, percuss the joint distal to it (distal percussion test).
For example, if you suspect a septic elbow, have the patient extend their arm, hyperextend the hand, and vigorously tap the hand. Same with the knee, have the patient extend the leg and forcefully tap the bottom of their foot.
If the joint is septic, this will elicit significant pain. If there is no pain, then the chances of the joint being septic are very low.
Have the patient perform full ROM movements of the joint in question. If there is pain out of proportion, then a septic joint is possible. If it is painless, then the chances are very low.
Also, inquire about an injury. It is practically impossible for a joint to become septic without the introduction of a foreign pathogen by a penetrating injury/mechanism.
No penetrating injury + negative pain on percussion + painless ROM = extremely low risk of a septic joint.
If the patient meets these criteria, I will forgo tapping the joint for laboratory analysis and treat it as an inflammatory etiology with NSAIDs and possibly steroids. It has never failed me.