Interpretation of synovial fluid is based on a combination of cytologic features in smears, total protein (by refractometer), nucleated cell count and an estimate of viscosity.
A red blood cell (RBC) count should also be performed on joint fluids because blood contamination will affect nucleated cell counts (false increase), percentage of neutrophils (false increase), total protein (false increase) and viscosity (false decrease). The degree to which these results are impacted depends on the extent of blood contamination and the peripheral leukocyte and neutrophil counts and protein concentration.
In samples of low volume (<0.2 mL) placed in a large EDTA tube (>2 ml), EDTA contributes to the refractive index, which can result in a false increase in total protein.
TP: Total protein, TNCC: Total nucleated cell count, N: Normal, D: Decreased, pmn: Neutrophils, macs: macrophages, ANA: Antinuclear antibody testing, RF: Rheumatoid factor testing.