This test charts or maps the genetic structure of the cells in the body. With the help of this test specific genes or parts of gene can be isolated and identified. In breast cancer this test is performed on the tumor cells or cancerous tissue withdrawn from the breast mass through biopsy. In breast cancer this FISH test specifically tests the presence of HER2 gene in the mass withdrawn. It the tissue pathologically examined as traits of extra or copies of HER2 gene than it are indicative of the fact that this cancer will respond positively to HER2 receptors. More copies of this gene imply that the tissue has increased number of HER2 receptors which stimulate the growth of these cells. As compared to other method of HER2 testing FISH is not available easily everywhere.
The other method of HER2 testing being IHC or ImmonoHistoChemistry. This test is not very conclusive and on usually this test is conducted by the doctor on the patient first. Then for accuracy if required FISH test is conducted as it is considered to be more accurate.
Also different laboratories and different pathologist have their own measure of categorizing positive and negative HER2 status. Also it is quite possible that tissue from different parts in the same cancerous growth can have diverse HER2 status. With one portion being HER2 positive while other part being HER2 negative.
HER2 testing is very vital to give a definite prognosis and treatment line, but in absence of an accurate diagnosis of the HER2 testing women have chances of receiving less better care as expected. Especially in borderline cases if a HER2 positive growth is classified as the test results as negative, the treating doctor may not prescribe medicines to block this hormone receptor cells and the woman may loose on the benefits of a targeted therapy. Similarly, a HER2 negative cancerous growth may be diagnosed as positive and with the resultant additional hormonal targeted therapy the patient may be exposed to risks.