An aliquot of specimen is added to a drop of 20% KOH on a glass slide. This technique is used to determine fungi in thick mucoid specimens or in specimens that contain keratinous material such as skin, hair and nails. The protein components of the host cells are partially digested by the KOH but the fungal cell wall remains intact (Fig. 2.6). Note: Neither India ink or Gram stain can be used after a KOH preparation.

Figure 2.6 Corneal scraping mounted in 10% KOH showing numerous hyphal elements and conidia characteristic of Pseudallescheria boydii. © Kaminski’s digital image library of medical mycology. (Courtesy Dr J.W. Rippon, University of Chicago, U.S.A.) 

Table 1.6 Advantages and disadvantages of KOH