Can Immunohistochemistry (IHC) be negative in malignancy?

Can Immunohistochemistry (IHC) Be Negative in Malignancy?

Yes, immunohistochemistry (IHC) can be negative in malignancy, as demonstrated by cases where tumors show negative IHC staining for specific markers but are confirmed malignant through other diagnostic methods. 1, 2

Understanding IHC Negativity in Malignancy

• IHC negativity in malignancy can occur in several scenarios, particularly evident in breast cancer HER2 testing where discordant results between IHC and fluorescence in situ hybridization (FISH) have been documented 2
• Approximately 1.5% of invasive breast cancers show HER2 IHC-negative but FISH-amplified (IHC-/FISH+) results, representing cases where malignancy exists despite negative IHC staining 2
• In hematologic malignancies, IHC and flow cytometry show substantial but not perfect agreement (κ=0.69), indicating that some malignancies may be missed if relying solely on IHC 3

Clinical Examples of IHC Negativity in Malignant Conditions

Breast Cancer HER2 Testing

• When concurrent IHC results are negative (0 or 1+) but ISH shows HER2/CEP17 ratio ≥2.0, these cases are still considered malignant despite negative IHC 1
• In cases with average HER2 signals/cell ≥6.0 but negative IHC (0 or 1+), guidelines recommend considering these specimens HER2 negative but they remain malignant breast cancers 1
• A meta-analysis showed that 4% of breast cancers with IHC scores of 0/1+ (negative) were actually FISH positive, indicating malignancy despite negative IHC 4

Mesothelioma and Other Malignancies

• In malignant pleural mesothelioma (MPM), the sensitivity of IHC markers ranges from 45% to 100%, meaning some cases may show negative staining for certain markers while still being malignant 1
• Sarcomatoid mesothelioma frequently does not express typical 'mesothelial' markers, demonstrating that IHC negativity can occur in this malignant subtype 1
• In some cases, expression of keratins may be the only demonstrable feature in malignant mesothelioma when other IHC markers are negative 1

Diagnostic Implications

• Relying solely on IHC for diagnosis can lead to false negatives in malignancy detection, potentially resulting in missed treatment opportunities 2
• In breast cancer, patients with HER2 IHC-negative/FISH-positive results would lose the opportunity for potentially beneficial anti-HER2-targeted therapy if only IHC testing had been used 2
• For lymphomas, IHC has the advantage of combining markers with morphology and tissue architecture, but may still be negative in some malignant cases 3

Complementary Testing Approaches

• Guidelines recommend using a combination of diagnostic methods rather than relying solely on IHC, especially in cases with suspicious clinical or morphological features 1
• For HER2 testing in breast cancer, both IHC and ISH methods are recommended to ensure accurate diagnosis and appropriate treatment decisions 1
• In hematologic malignancies, flow cytometry can complement IHC, especially in cases where IHC might be negative 3
• In colorectal cancer, combining H&E with IHC and quantitative RT-PCR has shown improved detection of micrometastases that would be missed by IHC alone 5

Common Pitfalls and Caveats

• Pre-analytic tissue processing issues can affect IHC results, potentially leading to false-negative staining in malignant tissues 1
• Lack of antibodies that function consistently in variably processed formalin-fixed paraffin-embedded tissue can result in negative IHC despite malignancy 1
• Subjective interpretation of IHC staining can lead to misclassification of malignant cases as negative 1
• For certain malignancies like sarcomatoid mesothelioma, diagnostic accuracy of IHC markers is significantly reduced, requiring additional testing methods 1

In conclusion, while IHC is a valuable diagnostic tool, negative IHC results do not definitively rule out malignancy. Multiple testing methods should be employed when clinical suspicion remains high despite negative IHC findings.