Introduction

Synovial fluid analysis typically includes a quantitative and qualitative interpretation such as volume, color, viscosity, total nucleated cell count and differential nucleated cell count. Since the primary goal of synovial fluid analysis is to classify the fluid as noninflammatory or inflammatory during the microscopic exam, a fast and simple analysis can be performed on a small volume of fluid (i.e., drops), if necessary. This is performed by assessing viscosity as a slide is being prepared for microscopic examination, and assessing mucin quality, nucleated cell counts and differential interpretation during the microscopic examination.

Arthrocentesis is performed aseptically, usually under (at least) light sedation. Selection joints to sample may be dictated by physical exam and radiographic findings. Those joints that are obviously effusive should be sampled. However, in dogs and cats, even if only one joint appears clinically affected, fluid should be collected from multiple joints in order to document a polyarthropathy. The carpi and stifles are most frequently sampled.

Examination

Viscosity: Viscosity is easily assessed by gross evaluation, during the preparation of the microscope slide. Once a drip is placed on the microscope slide, gently lift the tip of the syringe away from the slide. Normal synovial fluid will form at least a 1 inch long string before the strand is broken. Microscopically, normal synovial fluid has a very dense granular eosinophilic background as illustrated in the figure.

Total nucleated cell counts: Once the slide is smeared and stained, the tot al nucleated cell count can be estimated during the microscopic examination. The estimation should be performed in an area of the slide that has a monolayer of cell sand where a clear distinction between the nucleus and the cytoplasm can be made. The formula for estimating cell counts microscopically is nucleated cells/µl = (average # of nucleated cells per field) x (microscope objective power)2.

Normal cell counts in the dog may range from 500 cells/µl up to 2,000 cells/µl. For cats cell counts rarely exceed 500 cells/µl. When examining the fluid on a 100 x oil objective, if the fluid has a normal cell count, in the dog you should typically see less than 1 or at most, occasionally 2 nucleated cells after examination of 10 microscopic fields. In the cat, there should be less than 1 nucleated cell seen in 10 microscopic fields.

Differential cell count: Normal synovial fluid contains >90% mononuclear cells (Figure above) and <5% nondegenerate neutrophils. The mononuclear cells should consist predominantly of macrophages and possibly low numbers of small, well-differentiated lymphocytes. Occasionally, synoviocytes (clasmatocytes) may also be observed. These are round to oval cells with abundant basophilic cytoplasm and an eccentrically placed oval nucleus that has condensed chromatin pattern. The differential cell count may be affected, sometimes dramatically, by the presence of blood contamination. Peripheral blood introduced into the fluid during the collection process will cause the addition of increased number of nucleated cells, mostly neutrophils, sometimes making an interpretation of inflammatory vs. noninflammatory joint disease difficult.

Noninflammatory joint disease: In patients with noninflammatory joint disease, the nucleated cell count is either normal or mildly increased (less than 1 nucleated cell in every 2 or 3 100 x oil fields). Mononuclear cells will predominate, the majority of which will be macrophages (Figure on right). Some of the macrophages will be moderately to markedly reactive with increased cytoplasmic vacuolation. Typically, a few neutrophils will be present (<10%). Though, with hemorrhagic or traumatic lesions, a moderate number of neutrophils (<25%) may be. There are four differentials for noninflammatory disease: Degenerative joint disease, trauma, hemarthrosis, and neoplasia.

Inflammatory joint disease: The cardinal features of most inflammatory arthropathies are increased cell counts most of which will be neutrophils. During microscopic examination there will be typically be 1 to many nucleated cells per 100 x oil field. Neutrophil percentages will be high, often approaching 90% of the cell population, but any time there is a predominance of neutrophils present, a diagnosis of inflammatory joint disease should be made. This differential needs to account for any peripheral blood contamination that may be present and could have increased the number of neutrophils observed. Even if the cell count is normal or mildly increased, the joint fluid will still be classified as inflamed if an increased number of neutrophils are present. Occasionally, an inflammatory arthropathy may be associated with increased lymphocyte percent (e.g., lymphoplasmacytic synovitis, late feline chronic progressive polyarthritis). There may be an association between this condition and anterior cruciate rupture in the dog.

In animals with inflammatory joint disease, an underlying cause may or may not be identified. In dogs and cats, most inflammatory joint disease involves multiple joints and often has an immune-mediated component to the disease. Most infectious related joint disease in the dog involves a single joint. Except ions include polyarthropathies associated with tick-borne illnesses such as Borrelia burgdorferi, Anaplasma phagocytophilum and Ehrlichia ewingii. In the cat, viral arthropathies may occur in which case, infectious agents will not be identified and the inflammation may either be neutrophilic, macrophagic, or lymphocytic.

In summary, analysis of synovial fluid will allow the distinction between patients with inflammatory vs. noninflammatory joint disease. Only occasionally will a definitive diagnosis be obtained by synovial fluid analysis alone. Therefore, once the disease process is classified as inflammatory or noninflammatory, further analysis to determine an underlying cause must be explored.