Page 149 - Canine Lameness

9.4 Canine Arthropathies 121

due to the potential for desiccation when submitted in the syringe/needle. It is important to note
that the sensitivity for a true positive result can decrease with small sample volumes (i.e. if less
than 0.5 ml are used in a blood culture flask). On the other hand, use of a culturette swab as the
single method of culture is least likely to produce a positive culture result and therefore should be
avoided (Font‐Vizcarra et al. 2010).
Blood culture flasks are available with and without antibiotic‐binding resin. In general, and par-
ticularly in situations where an animal is receiving antibiotics, use of resin‐containing media is
advised to reduce the activity of antibiotics previously administered to the patient (Lorenzo‐
Figueras et al. 2006). If excessive amounts of fluid are available (such as frequently seen in septic
arthritis), culture from a red top tube in addition to the blood culture flask is ideal since blood
culture flasks have a higher propensity for contamination. As such, matching culture results from
both the red top and blood culture flask cultures provides greater confidence in the diagnosis.
Culturing of the synovial membrane has not been shown to be superior to synovial fluid and,
therefore, likely does not justify the additional cost and potential morbidity associated with the
sample collection procedure (Montgomery et al. 1989).
Infectious agents (bacteria, rickettsial, fungal, protozoal, or viral) typically require additional
specific tests (i.e. culture or genetic isolation) to diagnose. Many agents can infect joints either via
direct penetration or the hematogenous route (Martinez and Santangelo 2017). Some organisms,
such as Borrelia burgdorferi, may not necessarily cause synovial infections, but disease is typically
thought to be due to immune complex deposition, resulting in polyarthritis (Littman et al. 2018).
Additional diagnostics, such as the measurement of lactate and glucose in synovial fluid, as well
as serum C‐reactive protein, have shown promise for including or excluding septic arthritis if cytol-
ogy, culture, and clinical signs are not conclusive (Proot et al. 2015; Hillström et al. 2016). Further
work is warranted to validate use of these tests for diagnosing septic arthritis, particularly given a
lack of knowledge regarding how other musculoskeletal and non‐musculoskeletal conditions may
affect the test results. It should be noted that, prior to adopting these or any other novel assays
clinically, each individual diagnostic laboratory will need to confirm assay sensitivity and specific-
ity for synovial fluid on the designated high‐throughput or point‐of‐care instrument, determine
in‐house reference intervals, and validate their own diagnostic cutoffs.

9.4.2 Nonsuppurative Arthropathies

9.4.2.1 Mononuclear Inflammation
Mild to moderately increased TNCCs with predominantly large mononuclear cells are typically
due to either degenerate joint disease, such as osteoarthritis, or trauma. Both conditions can result
in increased populations of large mononuclear cells, typically both synoviocytes and macrophages,
within the synovial fluid. Investigation into clinical history and additional diagnostic information
is crucial for interpreting synovial fluid that shows increased nuclear cellularity due to large mono-
nuclear cells. If trauma is suspected, evidence of previous or acute hemorrhage and/or other frag-
ments of surrounding structures such as cartilage or bone might also be visualized on cytological
examination.
Foreign body reactions can induce mononuclear‐to‐mixed inflammation, the latter of which
may contain neutrophils, lymphocytes, and/or plasma cells. Pseudogout (deposition of calcium
pyrophosphate crystals in articular cartilage), while very rare in dogs, can cause acute arthritic
clinical signs and is associated with a mixed inflammatory response composed of mononuclear
cells and neutrophilic inflammation. A distinguishing feature of pseudogout is the presence of
variably sized square‐to‐rhomboid‐shaped crystals (Forsyth et al. 2007).