Combining cytology and microcrystal detection in nonpurulent joint fluid benefits the diagnosis of septic arthritis

Objective: To evaluate the performance of combined cytology and microcrystal detection in joint fluid for diagnosing septic arthritis. Methods: Retrospective single-center study of joint fluid samples from patients with manifestations suggesting acute or chronic arthritis. The absolute leukocyte count (/mm(3)) was recorded; as well as the differential counts, particularly of neutrophils (%). Microcrystals were sought and bacteriological cultures performed. Septic arthritis was defined as positive cultures of joint fluid or blood samples. Diagnostic performance was assessed based on sensitivity, specificity, the receiver-operating characteristics (ROC) curve with the area under the curve (AUC), and the positive and negative likelihood ratios (LR+ and LR-). Results: Two hundred and eight joint fluid samples were included. The diagnoses were septic arthritis (n=28), chondrocalcinosis (n=41), gout (n=28), rheumatoid arthritis (n=33), spondyloarthritis (n=31), osteoarthritis (n=18), and undifferentiated arthritis (n=29). Among cytological parameters, those having the best diagnostic performance were the neutrophil count (cutoff, >50,000/mm(3)), the leukocyte count (cutoff, >50,000/mm(3)), and the percentage of neutrophils (cutoff, >95%); corresponding LR+ values were 8.93, 5.76, and 4.55, respectively. Neutrophil percentages lower than 80% had an LR-value of 0.07. Combining these cytological variables with the absence of crystals improved the diagnostic performance, yielding LR+ values of 11.36, 10.94, and 10.82 for neutrophils >95%, neutrophils >50,000/mm(3), and leukocytes >50,000/mm(3), respectively. Conclusion: Combining cytological characteristics of joint fluid with the absence of crystals benefits the diagnosis of septic arthritis. (C) 2016 Published by Elsevier Masson SAS on behalf of Societe francaise de rhumatologie.