A month ago my sister had a mastectomy. Today she is going back into surgery because the surgeon has to “put the drains back in.” Can someone explain that?
In general, surgeons place drains to remove tissue fluids that weep into the wound after surgery, to prevent fluid collections (which may potentially become infected or cause other issues).
Many drains are removed within a few days of surgery. They are there to prevent postoperative complications, like infection, but they are themselves an avenue for infection.
In some patients, fluid collections may re-accumulate after the drains have been removed, for various reasons. Sometimes these go away on their own. Sometimes they respond to a simple needle aspiration or two. If they are large or thought to be infected, placement of a new drain may be required. This is often done percutaneously (through the skin) with imaging guidance under local anesthesia.
Obrigado.
I had a lumpectomy last March (2015), which is a procedure that, in my case, did not require drains when I left the hospital. But about three weeks after, the space where the tissue was removed spontaneously filled with fluid (called a “seroma”) and the fluid burst through the already-healed incision. I woke up on a Sunday morning and the front of my nightgown was soaked. My surgeon said the filling with fluid is a natural reaction to the tissue being “injured” by the surgery. (Think of a fluid-filled blister, for instance.) To make a long story short, I wound up having to push sterile gauze strips up into the now-ruptured incision twice a day for two months to absorb the fluid that was continually being produced. The hole had to heal from the inside out. Anyhoo, eventually it got better, but it was quite a mess there for a while.
My best to your sister. 
What part are you unclear about? Are you asking “what is a drain?” or “why were the drains not in place the whole time?” or “why do they need to be back in after not being needed for awhile?” or “why does she have to go into surgery for that instead of the doctor just doing it in the office?” or “why do you need a freaking surgeon just to put a couple drains back in?” or something else entirely?
• drain might have been pulled loose by accident and need to be reinserted
• doctor might have INACCURATELY thought they were no longer necessary and it turns out doctor was wrong about that and needs to put them back in
•doctor might have ACCURATELY thought their presence would interfere with an intermediary phase of healing but need to be present for a later phase of healing.
• drains might have become dysfunctional — clogged up or the insertion point no longer corresponds to where the fluid buildup is concentrated, for example — and “put back in” is a vague and underinformative translation of “surgeon needs to reinsert the drains”