American Society for Clinical Pathology (ASCP) Histotechnologist Practice Test

Which agent is most commonly used for clearing in histology?

1. Formaldehyde

2. Xylene

3. Acetic acid

4. Ethanol

The correct answer is: Xylene

Xylene is the most commonly used clearing agent in histology due to its effectiveness in removing excess alcohol from tissue samples after the dehydration process. During tissue preparation, tissues are typically dehydrated through a series of graded alcohol solutions, and once this step is complete, xylene is used to clear the tissue. The reason xylene is preferred is that it effectively penetrates tissues and replaces the alcohol, preparing the samples for infiltration with paraffin wax. This is essential for achieving clear and well-defined sections when the tissue is subsequently cut for microscopic examination. Other agents like formaldehyde are used for fixation, acetic acid may be utilized in certain staining procedures or for specific fixation of nucleic acids, and ethanol serves mainly in the dehydration process rather than clearing. Thus, while these agents play important roles in histological preparation, xylene stands out as the standard clearing agent for its specific purpose and compatibility with embedding media.