Background: Rapid diagnosis of septic arthritis from Gram-stain microscopy is limited
by an inherent false-negative rate of 25-78%. The presence of concomitant crystal arthritis in 5% of
cases represents a particular diagnostic challenge.
Objectives: This study aims to investigate the effects that a concomitant crystal arthropathy has on
the ability of Gram-stain microscopy of synovial fluid to diagnose a septic arthritis.
Methods: This is a 22-year retrospective cohort study. Inclusion criteria were a positive synovial
fluid culture result with a positive clinical diagnosis of septic arthritis. Results were correlated with
the presence or absence of urate and calcium pyrophosphate crystals, and Gram-stain result. During
this time our collection and analysis methods remained unchanged. All samples were collected in
Lithium Heparin containers. Chi-squared test with a p value < 0.05 was considered significant.
Results: 602 synovial fluid samples were included. 162 cases of concomitant crystal arthritis were
identified (27%). Of these, 16 (10%) had an initial negative Gram-stain. Out of the 440 samples
with no crystals detected, 18 (4%) had an initial negative Gram-stain microscopy result (p < 0.05).
Conclusion: The incidence of concurrent septic and crystal arthritis may be higher than previously
thought. Synovial fluid samples in concomitant septic and crystal arthritis are significantly less
likely to have a positive Gram-stain at microscopy than in cases of an isolated septic arthritis. We
would advise the clinician to maintain a high index of suspicion for septic arthritis in these patients.