How are test results interpreted by a board-certified pathologist?
• A: Interpretation of IHC relies on a qualitative scoring system on a scale of 0 to 3+. A patient may score a 0 (negative), 1+ (negative), 2+ (borderline), or 3+ (positive) on an IHC test based on the reviewer’s interpretation of staining intensity and completeness of membrane staining.9 With FISH testing, the results are quantitative instead of qualitative; tumors are interpreted as HER2 “negative” or “positive” by enumerating the HER2/neu gene copy number.4 Limitations in assay precision (particularly for the IHC method) and in the direct linkage between assay result and overexpression of the Herceptin target (for the FISH method) make it inadvisable to rely on a single method to rule out potential Herceptin benefit.
