Impression cytology is a non-invasive technique for the diagnosis of external eye diseases. It has not been extensively used despite its diagnostic potential because of technical inconvenience in the use of conventional membranes and its non-availability in many settings.
We have described here a highly economical, simple and rapid technique for the collection of impression cytology for the diagnosis of HSK, by using an ordinary glass slide. Our preliminary data shows that this technique is comparable to collection of corneal scrapings, especially for the detection of viral antigen (11/15 [73.3%] versus 12/15 [80%], P = 1.00). The result was not statistically significant.
Qualitatively, interpretation of antigen detection in impression cytology smears was very easy, since the background staining was virtually negligible. Interpretation of smears stained by the immunofluorescence assay was easier than smears stained by the immunoperoxidase aassay, since the background staining was somewhat lesser in the former. Additional advantage of this technique is that the glass slides are totally devoid of background fluorescence in comparison to cellulose acetate and biopore membranes. The specificity and sensitivity of this technique was 100% and 73.3% respectively and was comparable to that of corneal scrapings (specificity: 100% and sensitivity: 80%), for the detection of viral antigen. Earlier reports using membranes have shown better sensitivity [6
]. This could be attributed to the inherent property of the membrane devices to which the cells adhere effectively resulting in the collection of a large number of cells[6
]. In contrast, most of the smears made from corneal scrapings which were positive for viral antigen (9/12) showed a troublesome background staining, though the antigen positive cells (virus infected cells) could be identified from uninfected cells.
Quantitatively, adequate number of cells (10-20 epithelial cells/HPF) could be collected in most of the cases. However, the disadvantage of impression cytology was that in a small proportion of cases (2/15 [13.3%]), the material obtained was inadequate. This may contribute to antigen negativity. Similar findings have been reported by Yagmur et al. These authors have shown that the cell number was inadequate for diagnosis in 21% of the samples observed by impression cytology [7
]. The numbers of cells, which can be collected using a glass slide, are probably small when compared to a membrane. Nevertheless, the results of our preliminary study are encouraging. This may be attributed to the method we have adopted for cleaning the slides, which improves cell adhesion.
Considering the ease of performance and cost per patient, this technique was very easy to perform and proved to be very economical. The advantage of this technique is that the cells are collected directly onto the slide. Handling the slide for various staining procedures is easy and permanent mounts can be stored conveniently when immunoperoxidase assays are used. It is worthwhile to note that the cost (figures are based on the actual cost of procuring these utilities in a developing country like India) of collecting an impression cytology on a glass slide is approximately $0.01 whereas the cost of a millipore membrane is $2.00.
The impression cytology smears can be used for other staining procedures like PAP and Giemsa stain. Characteristic features of a viral infection, for example, the presence of multinucleated giant cell (Fig. ), koilocytic changes in epithelial cells (Fig. ) and inflammatory cells (Fig. ), all can be well-delineated in corneal scrapings and impression cytology smears. We have collected impression cytology for such staining procedures in three patients. Further studies are warranted to assess these procedures. Further, impression cytology from other sites like conjunctiva and skin vesicles can be collected by this method, especially for direct staining techniques. This needs further evaluation.
Based on our preliminary results, the technique we have described may be as useful as collection of impression cytology using Biopore membranes and cellulose acetate papers, as described by other authors[6
]. We believe that this procedure is as safe as collection of corneal scraping which is routinely obtained using blade no. 15 on a Bard Parker handle. We did not encounter any difficulty nor untoward incidents during the collection process using the method we have described. It is worthwhile to note that we have used special glass slides with polished edges. Nevertheless, the glass slides can further be modified wherein the square edges can be rounded off. Alternatively, Perspex cover slips can be used for the collection of cells [12
]. However, handling a Perspex cover slip may not be convenient when compared to a glass slide, especially for the collection of corneal impression cytology. This technique can be considered as a suitable alternative procedure where facilities for such devices (Biopore membranes, cellulose acetate paper) are not available. However, our results should be considered with caution, since the sample size is small.