What is the average C-Reactive Protein (CRP) level in patients with septic arthritis?

From the Guidelines

The average C-reactive protein (CRP) level in patients with septic arthritis is typically above 13.8 mg/L, as indicated by a recent study 1. This value is significant because it is associated with a higher risk of adjacent infection, and its elevation is a key factor in the diagnosis of septic arthritis. In the context of septic arthritis, CRP levels are often interpreted alongside other clinical findings, including joint fluid analysis, physical examination findings, and possibly positive joint fluid cultures. Some studies suggest that CRP levels can be much higher, often exceeding 100 mg/L, with most cases showing values between 100-200 mg/L 1. However, it's essential to note that CRP levels alone cannot definitively diagnose a septic joint, as they can be elevated in other inflammatory conditions as well. Serial CRP measurements can be useful for monitoring treatment response, with decreasing levels suggesting effective therapy. Normal CRP is typically less than 10 mg/L, so values significantly above this threshold in the context of joint symptoms should raise suspicion for septic arthritis. Key factors that predict adjacent infection include C-reactive protein above cutoff 13.8 mg/L, absolute neutrophil count above cutoff 8.6 10^3 cells/mL, platelet level below cutoff 314 cells/mL, duration of symptoms >3 days, and age >3.6 years 1. The presence of at least three of these risk factors is classified as high risk for adjacent infection, with a sensitivity of 90% and a positive predictive value of 80% on the initial study, and was validated in a subsequent study with a sensitivity of 86% and a positive predictive value of 91% 1.

Some of the key points to consider when evaluating CRP levels in septic arthritis include:

• CRP is an acute phase reactant produced by the liver in response to inflammation, and its levels rise rapidly within 4-6 hours of an inflammatory stimulus, peaking at 24-48 hours 1.
• In septic arthritis, bacterial infection within the joint space triggers this inflammatory response, leading to elevated CRP levels.
• Joint fluid analysis, showing leukocyte count >50,000 cells/mm³ with >90% neutrophils, is a critical component of the diagnosis of septic arthritis 1.
• Physical examination findings of a hot, swollen, painful joint with limited range of motion, and possibly positive joint fluid cultures, are also essential in diagnosing septic arthritis 1.

Overall, while CRP levels are an important tool in the diagnosis and management of septic arthritis, they should be interpreted in the context of other clinical findings and risk factors.

From the Research

Average CRP for Septic Joint

• The mean CRP in patients with septic arthritis was found to be 308 mg/L in a study published in the Southern medical journal 2.
• Another study published in the same journal found the mean CRP for septic joints to be 13 mg/L 3.
• A study published in The Journal of emergency medicine found that the sensitivity of CRP was 92% using a cutoff of ≥20 mg/L for septic arthritis 4.
• It is worth noting that the values of CRP can vary greatly between different studies, with one study finding that all patients with CRP >200 mg/L had septic arthritis, while patients with CRP 90 to 200 mg/L had a mix of crystal arthropathy and septic arthritis 2.

Comparison with Other Conditions

• The mean CRP in patients with crystal arthropathy was found to be 100 mg/L, which is significantly lower than that of septic arthritis 2.
• The mean CRP in patients with normal or osteo/inflammatory arthritic joints was found to be 25 mg/L, which is also significantly lower than that of septic arthritis 2.
• A study published in the American family physician noted that physical examination findings and serum markers, including erythrocyte sedimentation rate and C-reactive protein, are helpful in the diagnosis of septic arthritis, but are nonspecific 5.

Diagnostic Thresholds

• A study published in The Journal of emergency medicine found that a CRP cutoff of ≥20 mg/L had a sensitivity of 92% for septic arthritis 4.
• Another study published in the Southern medical journal found that a CRP cutoff of 0.8 had a positive predictive value for septic joints, although the exact sensitivity and specificity were not reported 3.
• A study published in the Southern medical journal found that no patients with CRP <90 mg/L had septic arthritis, suggesting that a CRP level below this threshold may be useful in ruling out septic arthritis 2.