Background/Purpose: The analysis of synovial fluid (SF) is an important tool to study joint diseases. SF is classified into non inflammatory, inflammatory and septic according to white blood cell (WBC) count. This classification is essential to start the diagnostic process, and to determine whether a rheumatologist evaluation is required. When SF is removed, the WBC decreases with time; an inflammatory liquid could become a false non-inflammatory one in hours. Reagent strip testing of urine is a validate tool to diagnose urinary tract infection, via the detection of leukocyte esterase activity. It has been used to analyze others biological fluids occasionally. SF test immediately performed after an arthrocentesis using reagent strips could have potential benefits as a screening tool.

Methods: Prospective study. We analyzed SF samples collected successively from patients in a tertiary university Hospital (November 2015 to April 2018). All samples were tested within 1 hour after their collection. We analyzed:Visual appearance, WBC (measured by manual leukocyte method), synovial glucose and total protein. Leukocyte esterase was measured by reagent strip test. It was recorded semiquantitatively as a number of plus signs (negative, 1+, 2+ or 3+) using standard color chart found on the container’s label. The cut-off for the WBC by manual leukocyte counting over than 2000 cells/mm was used to differentiate between inflammatory and non-inflammatory specimens. We decided to classify as inflammatory SF the ones with 1+ or more in the leucocyte esterase pad. We compared the WBC (reference standard diagnostic test) with the presence of leukocyte esterase using the leukocyte esterase reagent.

Results: 303 joint fluid samples were analyzed. According to manual leukocyte counting 147 (48.5%) were non inflammatory and 156 (51.5%) inflammatory. Of the inflammatory fluids: 144 (92.3%) were positive according to leukocyte esterase reagent Of the mechanical fluids: 95 (65.5%) were negative according to leukocyte esterase reagent. The sensitivity and specificity of leukocyte esterase reagent was 92.3% and 64.6% respectively. The PPV was 73.5% and NPV was 88.8%. The 12 false-negative results (negative by leukocyte esterase reagent but more than 2 000 leukocytes/mm ³) showed a predominance of mononuclear cells (62.2%), the median WBC count was 4017.7/mm³ and median polymorphonuclear cells percentage was 33.5%, all cultures were negatives. For inflammatory fluids: semi-quantitative results (negative, 1+, 2+ and 3+) were significantly different regarding the mean WBC, mononuclear cells, polymorphonuclear cells count and glucose. We found 20 hemorrhagic SF; the presence of erythrocytes did not altered the results.

Conclusion: Our results demonstrate that leukocyte esterase reagent strips are a rapid, cheap, and sensitive tool to identify inflammatory SF.Leukocyte esterase reagent strips had an excellent sensitivity but a poor specificity, so they could be use as a screening tool in primary care practice or in rural or poor areas. A positive result indicates an inflammatory process, so the patient should be referred to a rheumatologist.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/synovianalysis-using-leukocyte-esterase-reagent-strips/