Contaminant

In pathology, a contaminant refers to tissue or cells that mistakenly become included within a patient’s tissue sample during the laboratory process. Unlike a floater, which accidentally appears only on a microscope slide, a contaminant is embedded directly into the patient’s paraffin tissue block. As a result, contaminants repeatedly appear each time additional slides (recuts) are prepared from the same block. Contaminants can come from another patient’s specimen or a different part of the same patient’s specimen.

How do pathologists identify a contaminant?

Pathologists first notice potential contaminants when examining a slide under the microscope and seeing tissue that does not seem to belong with the rest of the sample. To confirm if the suspicious tissue is a contaminant, pathologists typically request additional slides from the same paraffin tissue block. If the unexpected tissue consistently reappears on these additional slides, pathologists identify it as a contaminant.

Once confirmed, pathologists document the contaminant on the affected slides, typically circling the unexpected tissue and labeling it as a “contaminant” for clear identification. Additional investigations may be performed to determine the source, especially if the contaminant is concerning or abnormal.

What is the significance of a contaminant in a pathology report?

Identifying and clearly documenting contaminants is important because their presence can potentially affect patient diagnosis and treatment. A contaminant that appears abnormal or malignant (cancerous) might lead to confusion or incorrect medical decisions if not properly recognized and addressed.

For this reason, pathologists carefully evaluate each contaminant to determine if further action is necessary. If a contaminant appears benign (noncancerous) and unrelated to the patient’s actual condition, no further investigation beyond clear documentation is typically needed. However, if the contaminant is suspicious or potentially malignant, a more detailed investigation may be required to determine its origin and clinical significance. Accurate documentation helps ensure patient safety, prevents diagnostic errors, and maintains laboratory quality.

Types of contaminants

Pathologists categorize contaminants into three types based on their source and potential impact:

• Type 1 – Same case, wrong part: Tissue from the same patient but from a different area or specimen. For example, tissue from a patient’s stomach biopsy was accidentally embedded in their colon biopsy sample.

• Type 2 – Unrelated tissue from another case: Tissue from a completely different patient’s specimen is mistakenly included in the sample. This can happen due to handling errors in the laboratory.

• Type 3 – Atypical or malignant tissue of uncertain origin: Abnormal or cancerous tissue whose exact source is unclear. Type 3 contaminants typically require extensive investigation—including additional tissue studies or specialized tests—to determine their origin and potential clinical impact.

What is the difference between a contaminant and a floater?

Although similar, a contaminant is different from a floater. A contaminant is tissue mistakenly embedded directly in the patient’s original paraffin tissue block, causing it to consistently appear on all slides made from that block. In contrast, a floater refers only to tissue or cells that accidentally appear on a microscope slide but are not actually present in the patient’s tissue block itself. Floaters do not reappear on additional cuts from the original block.

Correctly distinguishing between contaminants and floaters helps pathologists maintain accuracy in diagnoses, reduce potential errors, and ensure high-quality patient care​