Synovial fluid analysis with leukocyte esterase reagent strip test

To determine whether leukocyte esterase reagent strip test (LERST) analysis could help distinguish inflammatory arthritis from mechanical joint effusion. We analyzed synovial fluid (SF) from consecutive patients with a non-traumatic joint effusion during a 6-month period. Inflammatory SF was defined by white blood cell (WBC) count ≥ 2000/mm³. The LERST was performed with both semi-quantitative visual analysis (VA) and automated colorimetric reader (ACR) analysis. Leukocytes ≥ 1+ was considered a positive LERST result and WBC count was the reference. We obtained 100 SF samples (87 knees, 7 ankles, 5 hips, and 1 elbow) from 100 patients (mean ± SD age 61 ± 17 years, 59% men). The laboratory analyzed 88 SF samples (37 mechanical and 51 inflammatory). The remaining 12 SF samples were 10 hemarthrosis not allowing LERST analysis and 2 samples with coagulum not allowing WBC count. As compared with the laboratory analysis, the LERST had sensitivity and specificity 55% and 89% with VA and 47% and 92% with ACR analysis. The positive and negative predictive values were 87.5% and 59% with VA and 89% and 55% with ACR analysis. We found almost perfect agreement between VA and ACR results (kappa 0.70 [95% CI 0.50–0.90]). The WBC count increased with number of + observed after VA. Our results confirm that the LERST is able to detect inflammation in SF of native joints, thereby representing a useful and cheap tool in primary care. Its low sensitivity limits its use for ruling out inflammatory disorders.

Key Points ? Reagent strip tests can detect inflammation in synovial fluid. ? In primary care practice, this method is cheap and easy to do.