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Curr Rheumatol Rev. 2018;14(3):255-257. doi: 10.2174/1573397113666170329123308.

The Limitations of Gram-stain Microscopy of Synovial Fluid in Concomitant Septic and Crystal Arthritis.

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Edinburgh Trauma Unit Orthopaedic Surgery, Edinburgh, United Kingdom.
North Middlesex University Hospital - Orthopaedics, London, United Kingdom.



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.


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.


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.


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).


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.


Gram-stain; calcium pyrophosphate; crystals; lithium heparin; microscopy; septic arthritis.

[Indexed for MEDLINE]

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