Ancillary Histochemical, Immunohistochemical, and Molecular Tests
Core principle
Ancillary tests should answer a specific question. Order tests to distinguish the leading differential, not as a reflex.
Concepts to master
- Histochemical stains: PAS for fungi and basement membrane, Grocott methenamine silver, mucicarmine, iron, and elastic stains.
- IHC panels: melanocytic (SOX10, Melan-A, HMB-45), keratin, lymphoid (CD3, CD20, CD30), vascular (CD31, ERG), and p16 for HPV-related lesions.
- Molecular tests: T-cell receptor gene rearrangement, B-cell clonality, and translocations (e.g., t(17;22) in dermatofibrosarcoma protuberans).
- Interpretation pitfalls: loss of antigen expression, aberrant staining, and false positives in inflamed tissue.
Traps
- Using a single IHC marker to define a lineage.
- Overdiagnosing lymphoma on clonality alone without histology.
- Assuming a negative stain excludes disease (e.g., patchy melanocytic markers in desmoplastic melanoma).
- Ignoring fixation artifacts and edge effects.
Worked rule of thumb
When a spindle-cell tumor is ambiguous, use a broad panel: SOX10 (melanoma), CD34 (dermatofibrosarcoma protuberans), factor XIIIa (dermatofibroma), and smooth muscle actin (leiomyoma). A single marker rarely settles the diagnosis.
Key points
- Test to answer a specific differential.
- Panel approach beats single-marker reliance.
- Clonality supports, but does not replace, histology.
- Desmoplastic melanoma can be marker-negative.
- Fixation and artifact affect staining.