How It’s Tested

By measuring the amount of HER2 protein on the cancer cells or the number of HER2 gene copies, doctors determine which patients could benefit from anti-HER2 drugs.

One of the most widely used FDA-approved HER2 tests is the immunohistochemistry (IHC) test, which measures the amount of HER2 protein on the surface of tumor cells. Pathologists apply a stain to the fresh or frozen tumor cells that darkens the protein. Then they look at the intensity and extent of the stain to determine the protein quantity and categorize the tumor as 0 or 1+ (negative), 2+ (borderline) or 3+ (positive).

But IHC is a subjective test that relies on identifying subtle color changes on the stained cell. “Sometimes it can be very difficult to interpret a 1+ and 2+,” says Sophia K. Apple, MD, chief of breast pathology at the David Geffen School of Medicine at the University of California, Los Angeles. IHC tests also have a wide range of variability from laboratory to laboratory and “even a day-to-day variability in [a particular] lab,” she says, because of differences in processing, heating, staining and even room temperature.

The other widely used test, Fluorescent In Situ Hybridization (FISH), can also help resolve these borderline 2+ results. FISH involves coloring the tumor cells preserved in paraffin with a fluorescent stain; the sample is then viewed in a darkroom under a special microscope to determine the number of HER2 gene copies. Mutated genes appear red; normal ones are green. More red than green indicate a positive result. Errors can occur with this test, too, depending on where FISH is sent for analysis and how skilled the lab staff is at identifying the cancerous cells in a tumor sample.

Because as many as 20 percent of current HER2 test results may be inaccurate, the American Society of Clinical Oncology and the College of American Pathologists (ASCO/CAP) recently collaborated on guidelines for doctors. They cite the criteria for positive and negative HER2 findings and recommend follow-up testing if the first result is inconclusive.

The jury is still out on whether two additional tests will prove more accurate than either IHC or FISH. In July the FDA approved Invitrogen’s SPOT-Light HER2 CISH (Chromogenic In Situ Hybridization) test, which is a variation on FISH. The test, which uses an ordinary light microscope to tally the number of gene copies, is faster and less expensive than FISH. Then there’s the HERMark Breast Cancer Assay, which maker Monogram Biosciences says can detect HER2 protein levels with seven to ten times more sensitivity than IHC.

Because your HER2 status will determine the treatment you get, ASCO recommends talking with your oncologist about your test results. Find out which test was done on your tumor, what the results were and, if the results were borderline, whether a follow-up test was (or will be) done. If you have any concerns, tell your doctor you want your tumor retested. A second opinion could help you avoid a treatment that may not be effective against your cancer.