What levels of C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR) make septic arthritis less likely?

From the Guidelines

Low levels of C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR) make septic arthritis less likely, with a CRP less than 20 mg/L and an ESR below 30 mm/hr suggesting a lower probability of septic arthritis. These inflammatory markers are typically elevated in infectious processes, and their absence or minimal elevation provides reassurance against septic arthritis. However, these values should be interpreted alongside clinical presentation and other diagnostic findings. Normal or near-normal CRP and ESR levels are particularly useful negative predictors when combined with absence of fever, normal joint fluid appearance, and less than 50,000 white blood cells per mm³ in synovial fluid analysis. The reason these markers are valuable is that bacterial infection in a joint space typically triggers a significant inflammatory response that elevates these acute phase reactants. While these markers aren't perfectly sensitive or specific, their low values can help clinicians rule out septic arthritis when considering differential diagnoses for joint pain and swelling. According to a recent study 1, CRP has a sensitivity of 73% to 91% and a specificity of 81% to 86% for the diagnosis of prosthetic knee infection when a cutoff of 13.5 mg/L is used. Another study 1 suggests that elevated C-reactive protein >2.0 mg/dL has been described as an accurate predictor of septic arthritis.

Some key points to consider when interpreting CRP and ESR levels in the context of septic arthritis include:

• The American Academy of Orthopaedic Surgeons (AAOS) guidelines strongly recommend the use of ESR, CRP, and serum interleukin-6 testing for patients being assessed for peri-prosthetic joint infection 1.
• A large multicenter study found CRP and joint aspiration to be the most useful tools to diagnose infection 1.
• The combination of synovial fluid alpha-defensin and CRP tests demonstrated a sensitivity of 97% and a specificity of 100% for the diagnosis of periprosthetic joint infection 1.
• The Kocher criteria, which include fever >101.3 F, erythrocyte sedimentation rate of at least 40 mm/hour, white blood cell (WBC) count of at least 12,000 cells/mm3, and an inability to bear weight on the affected side, can be used to distinguish septic arthritis from transient synovitis 1.

Overall, while CRP and ESR levels can provide valuable information when evaluating for septic arthritis, they should be considered in conjunction with clinical presentation, other diagnostic findings, and guidelines from reputable organizations.

From the Research

Levels of CRP and ESR in Septic Arthritis

• The level of C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR) can be used to determine the likelihood of septic arthritis.
• A study published in 2019 2 found that patients with CRP < 90 mg/L were unlikely to have septic arthritis, as no patients with CRP < 90 mg/L had septic arthritis.
• Another study from 2011 3 found that the sensitivity of ESR was 98% using a cutoff of ≥ 10 mm/h and 94% using a cutoff of ≥ 15 mm/h, while the sensitivity of CRP was 92% using a cutoff of ≥ 20 mg/L.
• A 2014 study 4 determined that a CRP value of 41 mg/L and an ESR value of 32 mm/h were the optimal thresholds to predict an infection, with sensitivity values of 94.1% and 91.2%, respectively.

Threshold Values for CRP and ESR

• A CRP value < 90 mg/L makes septic arthritis less likely, as found in the 2019 study 2.
• An ESR value < 10 mm/h may indicate a lower likelihood of septic arthritis, although the sensitivity of ESR at this cutoff is not well established 3.
• A CRP value < 20 mg/L may also indicate a lower likelihood of septic arthritis, as found in the 2011 study 5.
• The optimal threshold values of 41 mg/L for CRP and 32 mm/h for ESR, as found in the 2014 study 4, can be used to predict an infection, but may not be directly applicable to determining the likelihood of septic arthritis.

Clinical Application

• The use of CRP and ESR in clinical practice can help differentiate between septic arthritis and other conditions, such as transient synovitis or inflammatory arthritis 5, 6.
• A combination of clinical evaluation and laboratory tests, including CRP and ESR, can be used to determine the likelihood of septic arthritis and guide further management 2, 3, 4, 5, 6.