how do I differentiate and treat immune-mediated polyarthritis versus septic arthritis in dogs?

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Definitive differentiation requires synovial fluid analysis: non-septic neutrophilic inflammation in multiple joints confirms immune-mediated polyarthritis (IMPA), while septic arthritis requires the combination of history, physical examination, and synovial fluid cytology plus culture for definitive diagnosis. 710 --- Differentiating IMPA from Septic Arthritis Distribution pattern is the first clinical clue. IMPA is a polyarthropathy typically affecting the carpus and tarsus, whereas septic arthritis is typically a monoarthropathy of a larger, more proximal appendicular joint such as the elbow, hip, stifle, or hock. 7 Preexisting osteoarthritis increases the risk of septic arthritis in any joint. 7 Radiographic findings help characterize but do not confirm either diagnosis. Septic arthritis produces secondary changes including increased soft tissue opacity within and surrounding the joint, osteophytosis, osteolysis, or mixed lytic and proliferative changes to the juxta-articular bone — critically, these bony changes affect all bones contributing to the joint, distinguishing septic arthritis from primary osteomyelitis or bony neoplasia. 7 Erosive IMPA (e.g., rheumatoid arthritis, periosteal proliferative arthritis) is identified by subchondral bone lysis on radiographs; the non-erosive form shows no such changes. 4 Synovial fluid C-reactive protein (CRP) does not reliably separate the two conditions. Synovial CRP is significantly lower in osteoarthritis versus both IMPA and bacterial infective arthritis, making it useful for distinguishing inflammatory from non-inflammatory arthropathy — but synovial CRP does not differ significantly between IMPA and bacterial infective arthritis. 3 In IMPA cases, serum CRP is positively associated with synovial CRP, and synovial neutrophil count is positively associated with synovial CRP in both IMPA and bacterial infective arthritis groups. 3 Serum CRP alone has only fair discriminatory potential between IMPA and steroid-responsiv