TL:DR: The job of the lymphatic system is to remove excess fluid generated during the course of normal blood circulation from extravascular tissue, and to fight infection. Anything that might increase this burden on the lymph nodes remaining post mastectomy (e.g. venipuncture, IVs, finger sticks, etc.) is discouraged.
As most breast cancers (75%) occur in the upper outer quadrant of the breast, lymphatic drainage - and metastatic cancer cells - goes to the axillary nodes. Some, all, or none of the nodes are removed with the breast tissue, depending on the cancer and the surgeon. All lymph passes through lymph nodes, so when some are removed, the remaining nodes filter more fluid than normally.
The arm on the mastectomy side is permanently off limits to any procedures (technically even blood pressure measurements) if any lymph nodes were removed during the mastectomy (they do not grow back), or if there as been radiation (which damages lymph nodes) to that side. That's because there are a reduced number of lymph nodes in the underarm area to handle the work of the lymphatic system for that arm, and doing anything which might stress the remaining nodes can cause permanent swelling of the arm (lymphedema.)
The goal is to reduce your risk of developing lymphedema. Having a mastectomy alone does not put you at risk for lymphedema. The risk comes when you have lymph nodes removed or damaged. If you had any of the following, you should take precautions to reduce your risk of lymphedema:
-Removal of all lymph nodes in the underarm area, which is called an axillary node dissection
-Removal of a limited number of “key” lymph nodes from the underarm area, which is called sentinel lymph node biopsy
-Having radiation to the chest wall and armpit after surgery
During routine admission to the hospital for something unrelated, if the patient has had a mastectomy or radiation, a brightly colored band is placed on the affected wrist to alert all staff to use the other side if there is a chance you may not be able to voice an opinion (e.g. in the OR or under sedation from pain meds).
Lymphedema can occur any time after mastectomy, often in the first 5 years, but has occurred even up to 20 years later. Because lymphedema is such a problematic condition, anything to reduce the possibility of it developing is recommended, even if the probability is low.
Patients with lymphedema have chronic, progressive swelling, pain, recurrent infections, and significantly decreased quality of life. The swelling can progress to gigantic proportions causing gross disfigurement with severe detrimental effects. In addition, lymphedema is a significant source of biomedical expenditures with one recent study demonstrating a more than $10,000 increase in the annual treatment costs of cancer survivors with lymphedema as compared with those without lymphedema6.
Treatment for lymphedema remains suboptimal and is, in most cases palliative with a goal of preventing disease progression rather than a cure. Medical and surgical treatments have been reported but in general these therapies have been disappointing...
There is no way to know who will develop lymphedema and who will not. The greater the number of axillary lymph nodes removed, the greater the chance of lympedema, but even people with as few as 2 nodes removed have developed it. Therefore, with or without strong supporting evidence, an abundance of caution is advised, as lymphedema is such a terrible post-operative complication.