Standardization of Conjunctival Impression Cytology

Lack of standardization limits the potential of conjunctival impression cytology as a clinical and research tool. This may be attributed to the variety of filter paper currently used. MF Millipore membrane filters of pore sizes 8.0, 3.0, 0.45, 0.22, and 0.025 micron were tested. Samples obtained from 30 eyes of rabbits were randomized and scored by four masked observers for cellularity and morphologic preservation. Cellularity was significantly greater with pore sizes 8.0, 3.0, and 0.45 micron versus 0.22 and 0.025 micron (p < or = 0.001), with an 83% correlation among four scorers. In contrast, morphology was better preserved in the smaller pore size papers (0.22 and 0.025 micron) when compared with larger pore sizes (p = 0.048). Using the best two filter papers (0.22 and 0.025 micron) and an ophthalmodynamometer, either 40, 60, or 80 g of pressure was applied for 3 s to each pore size paper to see whether cellularity could be increased. Cellularity was greater with pore size 0.22 than 0.025 micron (42.3 +/- 19.8 versus 8.7 +/- 6.4). Regardless of the pore size of the filter paper, cellularity was significantly improved at 60 g when compared with either 80 or 40 g. The results show that to maximize cell acquisition, a paper with medium pore size (0.22 micron) and a pressure of 60 g may be the best choice.