Total and differential cell counts should be performed promptly upon the receipt of the SF sample (i.e. within one hour from the collection) using automated methods (analysers with a suitable mode for body fluid analysis). Alternatively, total and differential SF cell counts might be determined manually (by means of light microscopy). The normal SF total white blood cell (WBC) count is < 200 x106/L. Manual differential WBC count should be performed in a stained cytocentrifuged preparation which allows the identification of cell types with diagnostic implications. The predominant cells found in SF are lymphocytes, monocytes, and macrophages, with a few neutrophils and synovial lining cells. The normal value of SF polymorphonuclear leukocytes (%PMN) is < 10%.

Although total WBC count and differentials have limited value in identifying specific joint conditions due to considerable intra-individual variations, it is accepted in clinical practice that the WBC count and %PMN moderately correlate with the degree of joint inflammation (3, 5, 8, 10, 11, 18).