Her-2/neu testing is used as a prognostic marker to help determine how aggressive a breast cancer tumor is likely to be.

It is also used as a predictor of response to therapy, such as hormone therapy and chemotherapy.

The serum Her2/neu test is sometimes used to monitor cancer therapy. If the level is initially elevated then falls, it is likely that treatment is working; if it stays elevated, treatment is not working; and if the level falls then rises, the cancer may be recurring.

Her-2/neu testing is recommended as part of an initial workup of invasive breast cancer and is sometimes done with recurrent breast cancer. It is not diagnostic but helps the doctor determine treatment options and understand more about the tumor’s characteristics.

Serum Her-2/neu is sometimes ordered initially to establish a baseline and then, if elevated, used to monitor cancer treatment. However, this method is not widely used because levels are only elevated when a large amount of cancer is present so early cancers are likely to be negative for serum Her-2/neu.

If an IHC Her-2/neu test is positive, it means that the Her-2/neu gene is over-expressing the Her-2/neu protein. If a FISH test is done, then amplification of the Her-2/neu gene can be detected. If either of these is positive, then the patient is likely to have a tumor that is aggressive, that will respond poorly to hormone treatment, and that will be resistant to chemotherapy. These patients may be considered candidates for Herceptin therapy. [See "Is there anything else I should know?"]

If the IHC is negative but the FISH is positive, the patient still may benefit from Herceptin, but if both are negative the treatment will not be useful.

Her-2/neu-positive tumors are susceptible to Herceptin (tratuzumab), a drug therapy that was created to target Her-2/neu protein. Herceptin attaches itself to the excess protein molecules and inhibits the growth of the cancer. The development of this specialized therapy has increased the use of Her-2/neu testing. Herceptin may be used alone or with some chemotherapy agents but is only useful in those who have Her-2/neu amplification and protein over-expression.

Her-2/neu testing is not available in every laboratory. Both IHC and FISH require experience and special training to perform and interpret. Your doctor will probably send your sample to a reference laboratory and the results may take several weeks to return.

It takes a small amount of cancer tissue to perform the Her-2/neu test. If a sufficient sample is not available, your doctor may try running a serum Her-2/neu test and/or make an assumption that you are Her-2/neu-positive in order to broaden your treatment options.