deparaffinisation

Deparaffinisation, also known as dewaxing, is the laboratory process of removing paraffin wax from formalin-fixed, paraffin-embedded tissue sections in order to allow staining and other aqueous assays to interact with cellular material. The tissue is first prepared by cutting thin sections that are mounted on glass slides and refixed to ensure adherence before staining.

The standard procedure uses organic solvents to dissolve the paraffin. Slides are immersed in xylene or a xylene substitute in one or more baths, typically for several minutes per bath, until the wax is dissolved. This is followed by rehydration through a series of graded alcohols (for example 100%, 95%, 70%) and finally water, so that aqueous stains and reagents can penetrate the tissue. Some laboratories employ alternative dewaxing agents or reduced-toxicity substitutes; the exact solvent sequence may vary with tissue type and downstream protocol. In some cases, dewaxing is combined with heat-induced steps, or performed before immunohistochemical staining.

Safety and environmental considerations are important, as xylene and many solvents are toxic and flammable. Work is conducted in a fume hood with appropriate PPE, and solvent waste is disposed of according to institutional and regulatory guidelines. Deparaffinisation is a prerequisite for most histological stains, immunohistochemistry, and in situ hybridization work on FFPE specimens.