What are the characteristic findings on a Complete Blood Count (CBC) in a patient with septic arthritis?

From the Guidelines

In septic arthritis, the CBC typically shows an elevated white blood cell (WBC) count, often exceeding 12,000 cells/mm³, as indicated by the Kocher criteria 1. This elevation in WBC count is a key indicator of an active infection. Neutrophilia, an increased percentage of neutrophils, is commonly present as these cells respond to bacterial infections. The CBC may also reveal other markers of inflammation, although these are not as directly relevant to the CBC results as the WBC count. It's essential to note that while these CBC changes strongly suggest infection, they aren't specific to septic arthritis alone. Definitive diagnosis requires joint fluid analysis showing WBC counts typically above 50,000 cells/mm³ with predominant neutrophils, along with positive cultures from the synovial fluid. The CBC serves as an important initial screening tool that helps guide further diagnostic steps, as emphasized in guidelines for evaluating infections 1. Key points to consider in the CBC results for septic arthritis include:

• Elevated WBC count, often exceeding 12,000 cells/mm³
• Presence of neutrophilia
• Potential for other markers of inflammation to be elevated, though these are not exclusive to septic arthritis
• The necessity of joint fluid analysis for definitive diagnosis.

From the Research

Septic Arthritis and CBC

• With septic arthritis, the complete blood count (CBC) may show:
  • Elevated white blood cell count (WBC) 2, 3
  • Increased neutrophil-to-lymphocyte ratio (NLR) in serum, which is associated with 90-day treatment failure and mortality 2
  • Elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), although these are nonspecific markers 3
• The synovial fluid analysis is more specific for diagnosing septic arthritis, with a white blood cell count greater than 50,000 cells/mm^3 or a neutrophil percentage greater than 90% being indicative of infection 4, 2
• However, it's important to note that a high white blood cell count in the synovial fluid can also be caused by other conditions, such as gout 4

Diagnostic Markers

• Serum NLR is a novel biomarker that has been shown to be more accurate for diagnosing septic arthritis than traditional markers such as WBC and CRP 2
• Synovial fluid NLR is also a useful marker for diagnosing septic arthritis, with a threshold of 25 being optimal for diagnosis 2
• Elevated serum WBC and CRP are associated with treatment failure, while elevated serum NLR is associated with both treatment failure and mortality 2

Limitations of Current Evidence

• There is limited research on the specific CBC findings in patients with septic arthritis, and more studies are needed to fully understand the relationship between CBC results and septic arthritis diagnosis and prognosis 5, 3