Surgical tissue is treated with various reagents (eg, alcohols and xylenes) to preserve tissue components and prepare the specimen for paraffin embedding and sectioning. The process is long and involved, and each sequential step is completely dependent on the previous one. The basic processing steps, in order, are:
- Fixation: Formalin stabilizes tissue proteins to prevent further changes, such as decay.
- Dehydration: Increasing concentrations of alcohol (70%, 95%, 100%) remove water from tissue.
- Clearing: Xylene (or xylene substitute such as isopropanol) removes dehydrant (alcohol) from tissue in preparation for paraffin infiltration; the clearant must be miscible (be able to mix) with the paraffin.
- Paraffin infiltration: Wax media displaces the clearing agent in tissue with paraffin in preparation for paraffin embedding.
The chart below is an example of a five hour biopsy processing program on a closed-system processor with vacuum.
| Reagent | Time (minutes) |
| Formalin | 20 |
| Formalin | 20 |
| 70% alcohol | 20 |
| 95% alcohol | 25 |
| 95% alcohol | 25 |
| 100% alcohol | 20 |
| 100% alcohol | 20 |
| Xylene | 20 |
| Xylene | 20 |
| Xylene | 20 |
| Paraffin | 20 |
| Paraffin | 20 |
| Paraffin | 25 |
| Paraffin | 25 |
