Page 144 - Canine Lameness

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difficult and, therefore, direct smears have been recommended to be assessed using broader cate-
gories such as normal, mild‐moderately increased, or markedly increased (Dusick et al. 2014).
Nevertheless, if an automated analyzer is not available, it is important to obtain or estimate a
TNCC as this can be used to determine response to treatment.
Normal canine joints typically have a TNCC of less than 3000 cells/μl. TNCCs in pathologic
joints will vary depending on the underlying pathological cause. Suppurative arthropathies often
yield the highest cell counts, which have been reported to range from >3 000 to >100 000 cells/μl
(MacWilliams and Friedrichs 2003). Nonsuppurative arthropathies typically do not exceed
10 000 cells/μl and most often range from 3 000 to 5 000 cells/μl.

9.3.4 Cytological Analysis

Cytologic analysis of joint fluid is typically performed by evaluating direct smears. If smears prepared
by the clinician at the time of collection are adequate, these will be utilized for interpretation. Many
laboratories will often also prepare their own direct smears and cytospin preparations using sample
from the EDTA tubes; the latter slides can be helpful for distinguishing cellular morphology or charac-
teristics not obvious on direct smears. Slides are generally evaluated microscopically using 10× and
100× objectives to confirm automated TNCCs, provide a differential cell count, and assess cell morphol-
ogy and/or inclusions. Direct smear preparations of synovial fluid in healthy patients typically have a
dense eosinophilic stippled background (Figure 9.3). Cells may align in a linear fashion, commonly
referred to as “windrowing,” due to the viscosity of joint fluid (Figure 9.4A, B). High protein content can
also result in dense preparations, which make it challenging to determine cellular morphology, cellular-
ity, and the presence of infectious agents. The thinnest portions of the slide and/or cytospin slides are
best for cellular evaluation. The number of red blood cells and the presence of platelets/platelet clumps
should be noted as this can indicate increases of WBC proportions due to blood contamination.
Cells frequently seen on cytologic evaluation of normal joints include large and small mononu-
clear cells, synovial lining cells (i.e. synoviocytes), and rare‐to‐occasional neutrophils (Figure 9.3).
Large mononuclear cells are the predominate cell population identified in healthy joints and com-
pose approximately 90% of cells identified. On cytological evaluation, it can be difficult to differenti-
ate synovial macrophages from synoviocytes, which is why the term “large mononuclear cells” is
used. Large mononuclear cells are typically identified by their amount of blue grey to deeply baso-
philic cytoplasm, vacuolization, cytoplasmic inclusions, and the size of their nucleus (approximately
1.0×–1.5× the size of a neutrophil). Synoviocytes can have a similar morphology as macrophages and
have a round to spindle shape, with round‐to‐ovoid nuclei, and a moderate amount of basophilic
cytoplasm. Small mononuclear cells are usually representative of the lymphocyte population pre-
sent. They are identified by a thin rim of cytoplasm, an absence of cytoplasmic vacuolization, and a
nucleus that is round and approximately 0.5–1.0× the size of a neutrophil (Figure 9.3B). Nondegenerate
neutrophils are readily identified by their segmented nucleus and clear, granular cytoplasm
(Figure 9.5E). Of note, in the absence of blood contamination, neutrophil numbers will be low; how-
ever, if blood contamination is present, the number will increase. As mentioned above, it can be
challenging to differentiate cell types in dense cytological preparations, so in some cases, cytospin
preparations or dilution of the sample might be required to fully assess cellular morphology.

9.3.5 Mucin Clot Test
The mucin clot test is a semiquantitative assessment of the hyaluronic acid content in synovial
fluid. The test estimates the ability of the synovial fluid to form a clot after the addition of acetic