What lab abnormalities are associated with septic (infectious) arthritis?

From the Guidelines

Septic arthritis typically shows several laboratory abnormalities that help in diagnosis, including elevated white blood cell (WBC) count, increased inflammatory markers such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), and joint fluid analysis with markedly elevated WBC count and predominance of neutrophils. These laboratory findings are essential for diagnosing septic arthritis and differentiating it from other inflammatory arthritides. Some key lab abnormalities associated with septic arthritis include:

• Elevated WBC count, usually above 10,000/mm³
• Increased ESR and CRP
• Joint fluid analysis showing cloudy or purulent fluid with WBC count exceeding 50,000/mm³, and predominance of neutrophils (>75%)
• Gram stain of synovial fluid may reveal bacteria in 50-75% of cases
• Cultures of joint fluid are positive in 60-80% of cases
• Blood cultures are positive in approximately 50% of cases
• Procalcitonin levels may be elevated in systemic infection
• Synovial fluid glucose is typically decreased compared to serum glucose, and protein levels are elevated, as noted in studies such as 1. It's worth noting that while these lab abnormalities are common in septic arthritis, no single test is pathognomonic, and results must be interpreted in clinical context, as highlighted in 1.

From the Research

Lab Abnormalities in Septic Arthritis

The lab abnormalities that would be present in septic arthritis include:

• Elevated erythrocyte sedimentation rate (ESR) 2, 3, 4, 5
• Elevated C-reactive protein (CRP) 3, 4, 5
• Elevated synovial fluid white blood cell counts 4, 6
• Elevated polymorphonuclear cell percentage in synovial fluid 4, 6
• Elevated procalcitonin (PCT) levels 4
• Elevated serum IL-6 and TNF-α levels 4
• Low synovial glucose levels 6
• Elevated synovial lactate dehydrogenase and synovial C-reactive protein levels 6
• Positive culture of the pathogen in synovial fluid or tissue 2, 3, 6

Diagnostic Markers

The diagnostic markers that can be used to distinguish septic arthritis from inflammatory arthritis include:

• Synovial fluid white blood cell counts 4, 6
• Polymorphonuclear cell percentage in synovial fluid 4, 6
• Procalcitonin (PCT) levels 4
• Serum IL-6 and TNF-α levels 4
• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) levels 3, 4, 5

Threshold Values

The threshold values for the diagnostic markers include:

• Synovial WBC count: 28,100 cells/mL for highest accuracy, 40,000 cells/mL for 100% specificity 6
• CRP: 0.8 mg/L as positive cutoff 5
• ESR: 15 mm/h as positive cutoff 5