To evaluate the performance of combined cytology and microcrystal detection in joint fluid for diagnosing septic arthritis.

Retrospective single-center study of joint fluid samples from patients with manifestations suggesting acute or chronic arthritis. The absolute leukocyte count (/mm³) 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−).

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/mm³), the leukocyte count (cutoff, >50,000/mm³), 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/mm³, and leukocytes >50,000/mm³, respectively.

Combining cytological characteristics of joint fluid with the absence of crystals benefits the diagnosis of septic arthritis.