Synovial Cell Count Poorly Predicts Septic Arthritis in the Presence of Crystalline Arthropathy

Abstract

Abstract. Introduction: A synovial cell count greater than 50,000/mm3 is the threshold most commonly used to diagnose septic arthritis. This lab value may be nonspecific in the setting of crystalline arthropathy. The purpose of this study was to evaluate the accuracy of diagnosing septic arthritis using a synovial cell count cut-off of 50,000/mm3 in the setting of crystalline arthropathy.Methods: This was a retrospective review of joint aspirations performed between July 1st, 2013 and June 30th, 2016. Synovial fluid samples were evaluated for cell count, crystals, Gram stain, and culture. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the synovial markers were calculated.Results: During the study period, 738 joint aspirations were sent for testing, of which 358 aspirations in 348 patients met inclusion criteria. There were 49 (13.7%) cases of culture-positive septic arthritis, and 47 patients underwent surgical irrigation and debridement. Gout and pseudogout crystals were present in 163 aspirates (45.5%). Three joints (0.8% overall rate) had concomitant crystalline arthropathy and septic arthritis, each of which had a synovial WBC ≥85,000/mm3. Increasing the WBC count cutoff to 85,000/mm3 demonstrated a specificity of 100%, but a PPV of 12.0%.Conclusions: A cut-off of 85,000/mm3 may be more appropriate to diagnose concomitant septic arthritis and crystalline arthropathy. We recommend medical management and observation in patients with crystal-positive joint aspirations unless the synovial cell count is elevated above 85,000/mm3. Prospective studies using this treatment guideline are needed to evaluate its validity and accuracy.

How reliable are the synovial cell count and blood parameters in the diagnosis of septic arthritis?

“…From a different perspective, McGillicuddy et al[ 21 ] showed that a synovial WBC count was less than 50,000/mm3 in 39% of the SA patients in their series. Similarly, Luo et al[ 12 ] found that 38.8% of the patients with SA had a synovial WBC count of <50,000/mm 3 . Li et al[ 22 ] also found that 36% of their patients with SA had a joint WBC count less than 50,000/mm 3 .…”

“…In the present study, culture results were chosen as diagnostic criteria after the exclusion of contaminated cultures, consistent with previous studies. [ 12 , 20 , 33 ] In some studies, the diagnostic criteria were the presence of pus or Newman criteria. [ 12 , 17 ] However, using these criteria, patients who do not actually have SA would be considered as SA and, therefore, the gold standard is the culture result.…”

“…[ 12 , 20 , 33 ] In some studies, the diagnostic criteria were the presence of pus or Newman criteria. [ 12 , 17 ] However, using these criteria, patients who do not actually have SA would be considered as SA and, therefore, the gold standard is the culture result. On the other hand, when the diagnosis is made based on culture results alone, both treatment can be delayed and culture-negative SA can be overlooked.…”

“…[ 16 ] The typical synovial white blood cell (WBC) count threshold has been considered as 50,000/mm3 , as the values higher than 50,000/mm 3 is considered as indicative for SA. [ 12 , 17 ] Also the American Rheumatism Association (ARA) has classified WBC into categories and considered the values as infectious which are higher than 50,000/mm 3 . [ 18 , 19 ] However, it has also been reported that this cell count may not always yield reliable results.…”

“…[ 18 , 19 ] However, it has also been reported that this cell count may not always yield reliable results. [ 11 , 12 ]…”