title: The Limitations of Gram-stain Microscopy of Synovial Fluid in Concomitant Septic and Crystal Arthritis.
abstract: 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.
  
  
  OBJECTIVE
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Edinburgh Trauma Unit Orthopaedic Surgery, Edinburgh, United Kingdom.
authors.forename: Paul
authors.initials: P
authors.lastname: Stirling
authors.affiliation: North Middlesex University Hospital - Orthopaedics, London, United Kingdom.
authors.forename: Mohammed
authors.initials: M
authors.lastname: Tahir
authors.affiliation: Edinburgh Trauma Unit Orthopaedic Surgery, Edinburgh, United Kingdom.
authors.forename: Henry Dushan
authors.initials: HD
authors.lastname: Atkinson
mesh_terms: D001170:Arthritis, Infectious / Q000175:diagnosis* / Q000453:epidemiology / Q000382:microbiology; D001419:Bacteria / Q000302:isolation & purification*; D001431:Bacteriological Techniques; D015415:Biomarkers / Q000032:analysis; D002131:Calcium Pyrophosphate / Q000032:analysis*; D000070657:Crystal Arthropathies / Q000175:diagnosis* / Q000453:epidemiology / Q000378:metabolism; D003460:Crystallization; D004739:England / Q000453:epidemiology; D005840:Gentian Violet; D006801:Humans; D015994:Incidence; D008853:Microscopy; D010619:Phenazines; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012307:Risk Factors; D013194:Staining and Labeling; D013582:Synovial Fluid / Q000737:chemistry* / Q000382:microbiology*; D014527:Uric Acid / Q000032:analysis*
chemical_list: D015415:Biomarkers; C035863:Gram's stain; D010619:Phenazines; D014527:Uric Acid; D005840:Gentian Violet; D002131:Calcium Pyrophosphate
keywords: Gram-stain; calcium pyrophosphate; crystals; lithium heparin; microscopy; septic arthritis.