Introduction: Arthrography and synovial fluid punction are two investigations easily combined as a diagnostic tool for the detection of (a)septic loosening of arthroplasties: diagnostic imaging; X-ray guided injection of iodine-containing contrast agents in (artificial)joints and synovial fluid puncture for microbiological diagnostics. The contrast agent is present in the fluid puncture for culturing. Free iodine is bactericidal, hence the assumption that the iodine causes fault negative culture results.

The older literature (1960–1982) partially supports this assumption and iodine contrast agents have been modernized considerately.

Is the effect the modern contrast agents have on the micro-organisms the cause of the fault negative culture results?

Method: The influence of modern widely used iodine containing contrast agents (Omnipaque¹, Visipaque², UltraVist³, Xenetix⁴) and two 30 year old agents (Hexabrix⁵ en Telebrix⁶) on the growth of the following relevant prosthesis infection causing micro-organisms were examined:

Staphylococcus aureus

Three different techniques were used: a disk diffusion test (classical resistance determination) and time-killing curves tests with a high inoculum (1,5*108 cfe/ml) and a low inoculum (10*3 cfe/ml) at 0, 2 and 24 hours.

Results: Disk diffusion tests: no contrast agent affected the growth of micro-organisms.

The high and low inoculum tests: only the combinations Telebrix with both P. aeruginosa and E. coli showed any growth inhibition but a non-significant (p = 0.07) growth inhibition of log-1. This however, did not impede the detection of these bacteria.

And with all other combinations there was no significant inhibition compared to the saline control and in every combination the percentage surviving number of bacteria was always higher than 30%.

Conclusion: Modern contrast agents do not seem to have a significant inhibitory effect on micro-organisms. They do not seem to be the cause of the fault negative synovial fluid culture results.

Aspiration arthrography using an iodinated contrast medium is a useful tool for the investigation of septic or aseptic loosening of arthroplasties and of septic arthritis. Previously, the contrast media have been thought to cause false negative results in cultures when present in aspirated samples of synovial fluid, probably because free iodine is bactericidal, but reports have been inconclusive.

We examined the influence of the older, high osmolar contrast agents and the low osmolar media used currently on the growth of ten different micro-organisms capable of causing deep infection around a prosthesis. Five media were tested, using a disc diffusion technique and a time-killing curve method in which high and low inocula of micro-organisms were incubated in undiluted media. The only bactericidal effects were found with low inocula of Escherichia coli and Pseudomonas aeruginosa in ioxithalamate, one of the older ionic media.

The low and iso-osmolar iodinated contrast media used currently do not impede culture. Future study must assess other causes of false negative cultures of synovial fluid and new developments in enhancing microbial recovery from aspirated samples.