Decision-making after joint aspiration: C-reactive protein and synovial white blood cell count as laboratory indicators for surgical joint lavage in pediatric septic arthritis.

BACKGROUND

Pediatric septic arthritis requires immediate recognition, as delayed diagnosis can cause severe joint damage and long-term dysfunction. In the absence of guideline-based cut-off values for laboratory markers, surgical decisions are often made on a case-by-case basis. Due to the scarcity of evidence specific to the pediatric population, treatment strategies are often based on adult data, highlighting the need for targeted research in this population. To address this gap, we developed a diagnostic algorithm that incorporated reliable predictive factors.

METHODS

Of 443 joint aspirations performed in our clinic (2014-2024), 132 (29.8%) were for suspected septic arthritis. After applying exclusion criteria, 80 cases were included. Clinical (fever, pain with movement, comorbidities), laboratory parameters at the time of joint aspiration (serum CRP, synovial white blood cell count [syWBC], serum white blood cell count [seWBC], synovial neutrophil perventage [syN%] and radiological data (radiographs, CT and MRI if available) were collected. Septic arthritis was defined by detection of pathogens in joint aspirate via culture or PCR.

RESULTS

A pathogen was identified in 25% (20/80) of cases, with Kingella kingae (30%) being the most frequently detected organism, followed by Staphylococcus aureus (25%). Regression analysis revealed CRP (p < 0.01), syWBC (p = 0.04), but not syN% (p = 0.51) as predictors. ROC analysis yielded optimal cutoff values for CRP (69 mg/L; AUC = 0.82; 95% CI 0.71-0.93) and syWBC (65,000 cells/µL; AUC = 0.79; 95% CI 0.66-0.92). A diagnostic algorithm using CRP > 69 mg/L alone, or CRP < 69 mg/L combined with syWBC > 110,000 cells/µL, predicted septic arthritis with a sensitivity of 85% (95% CI 0.62-0.97; p < 0.01) and a specificity of 90% (95% CI 0.79-0.96; p < 0.01).

CONCLUSION

CRP was slightly more accurate than syWBC in predicting septic arthritis. When combined in an recursive partitioning model, these parameters demonstrated strong diagnostic performance. In cases where CRP measurements may be unreliable, an elevated syWBC count represents a CRP-independent alternative, although with reduced specificity.

LEVEL OF EVIDENCE: Level III: a retrospective case-control study.