A surgical drain is a small plastic tube that is sometimes used after an operation. It is put inside you during the surgery by the doctor and will stick out of your body until it is removed, usually a few days later.
Surgeons use small plastic drainage tubes after surgery to help drain away fluid: without them, the fluid might gather up and cause infection. Or, in the case of an operation on the lungs or chest, air might gather up and squash the lungs.
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It connects to a small plastic bag that collects any fluid or air that has drained away from where you had the operation. Not all operations require a drain to be in: your surgeon will advise you if one is necessary.
They are usually special small, flexible plastic tubes that the surgeon places during the operation and then leaves poking out your skin, attached to a small bag.
Without a drain, the fluid might gather up in a large pool and be painful. The pressure of the fluid inside might stop the wound healing properly. So the surgical drain allows extra fluid to drain off harmlessly. A few days later, when the fluid production has faded away, the drain can be removed painlessly.