Blood-Contaminated Joint Fluid Samples Might Still be Useful

Blood-Contaminated Joint Fluid Samples Might Still be Useful

Normal synovial fluid is straw-colored, transparent, and highly viscous. When veterinarians assess this fluid for signs of infection, they are looking for elevated total protein concentrations and white blood cell counts.

Photo: Paula da Silva

To determine whether a horse's joint is infected—which can be a serious side effect of joint injections or lower limb wounds—veterinarians analyze synovial (joint) fluid samples for signs of trouble. Collecting synovial fluid can be a tricky procedure, however, and can result in a blood-contaminated sample. While veterinarians have historically tossed such samples and started over—at the expense of the horse's welfare as well as the owner's wallet—researchers now believe useful information can still be garnered.

James Carmalt, MA, VetMB, MVetSc, FRCVS, Dipl. ABVP, ACVS, professor of equine surgery at the University of Saskatchewan's Western College of Veterinary Medicine, in Saskatoon, described how veterinarians can effectively use and interpret blood-contaminated synovial fluid samples at the 2013 American Association of Equine Practitioners convention, held Dec. 7-11 in Nashville, Tenn.

Normal synovial fluid is straw-colored, transparent, and highly viscous, Carmalt said. When veterinarians assess this fluid for signs of infection, they are looking for elevated total protein (TP) concentrations and white blood cell (WBC) counts.

"Unfortunately, it is not uncommon for an equine practitioner to be unable to provide a definitive diagnosis of septic arthritis for multiple reasons, including a blood-contaminated sample," Carmalt said.

So Carmalt and colleagues conducted a two-part study to produce a mathematical model that would allow veterinarians to approximate TP and WBC from contaminated samples. They first took venous blood (usually from the jugular vein) and synovial fluid samples from 10 horses. The researchers then contaminated these horses' normal synovial fluid with their own blood in 10% increments from 0 to 50%. Using measurements from each sample, Carmalt and his colleagues generated an equation that accounted for the change in TP concentration and WBC counts depending on the level of blood contamination.

In the second part of the study, the team tested their results by using contaminated samples from five additional horses. A blinded researcher then used their formula to predict what the TP and WBC values would have been if the samples had not been contaminated.

In conclusion, Carmalt said, veterinarians can use the method he and his colleagues presented to calculate TP concentration and WBC counts from blood-contaminated synovial fluid samples. In his own practice, Carmalt said he has constructed a Microsoft Excel spreadsheet in which entering the laboratory values will automatically generate blood-free calculated values without having to conduct cumbersome calculations each time.

"However, diagnosing a potential septic arthritis is still challenging when results are above normal but are not above the published cut-off values (indicating a positive result) for septic arthritis," he said. "We believe that in these circumstances, use of other diagnostic tools, including cytology (microscopic examination of cells) as well as the clinical picture and clinician experience, cannot be overrated."

About the Author

Alexandra Beckstett, The Horse Managing Editor

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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