Arthrocentesis with synovial fluid analysis
cell count with differential
glucose
protein
bacterial culture and sensitivity (not 100% sn)
polarized light microscopy for crystals
CBC
ESR - Sn 94% (with 15mm/h cut-off)
CRP - Sn 92% (with 20mg/L cut-off)
Blood Culture
Gonorrhea culture (urethral/cervical/pharyngeal/rectal)
Imaging (may be helpful for excluding other diagnoses - e.g. trauma, osteo, etc)
Immunocompromised
Consider mycobacterial or fungal<unk> 
Leukemia history: predisposed to Aeromonas infections
Periprosthetic infection
Non-emergent: acute microbiological diagnosis is more important than rapid antibiotics
Diagnose with two synovial fluid cultures (avoid collection from a draining sinus)
CRP >100mg/L during first 6 weeks post-op warrants aspiration and may be used to differentiate from superficial skin infection
cell count with differential
glucose
protein
bacterial culture and sensitivity (not 100% sn)
polarized light microscopy for crystals
Consider mycobacterial or fungal<unk> 
Leukemia history: predisposed to Aeromonas infections
Non-emergent: acute microbiological diagnosis is more important than rapid antibiotics
Diagnose with two synovial fluid cultures (avoid collection from a draining sinus)
CRP >100mg/L during first 6 weeks post-op warrants aspiration and may be used to differentiate from superficial skin infection
>1,100 (prosthetic joint)
>25,000; LR=2.9
>50,000; LR=7.7
>100,000; LR=28
>64% (prosthetic joint)
>90%
Viscosity of synovial fluid may actually be decreased in inflammatory or infectious etiologies, as hyaluronic acid concentrations decrease
The presence of crystals does not rule out<unk> ; however, the diagnosis is highly unlikely with synovial WBC < 50,000
Septic arthritis
Transient 
Monoarticular arthritis
Abscess
Reactive arthritis
Cellulitis
Septic bursitis
Primary rheumatologic disorder 