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The Jackson-Pratt Drain (also called a JP Drain) is a medical device that is commonly used as a post-operative drain for collecting bodily fluids from surgical sites. The device consists of an internal drain connected to a grenade-shaped bulb via plastic tubing. The flexible bulb has a plug that can be opened to pour off collected fluid. Each time fluid is removed, the patient, caregiver or healthcare provider squeezes the air out of the bulb and replaces the plug. The resulting vacuum creates suction in the drainage tubing, which draws fluid from the surgical site.
Another method involves folding the drain in half while it is uncapped, then while folded, recapping the drain. This action causes fluid to be gradually sucked out of the body and into the bulb itself. The bulb may be repeatedly opened to remove the collected fluid and squeezed again to restore suction. It is best to empty drains before they are more than half full to avoid the discomfort of the weight of the drain pulling on the internal tubing.
Patients or caretakers can "strip" the drains by taking a damp towel or piece of cloth and bracing the portion of the tubing closest to the body with their fingers, run the cloth down the length of the tube to the drain bulb. One can also put a little bit of lotion or mineral oil on their fingertips to lubricate the tube to make stripping easier. The portion of the tube closest to the exit point of the drain from the body should be gripped first, and once the length of the drain is stripped, the end closest to the surgical site should then be released. This increases the level of suction and helps to move clots through the drainage tube into the bulb.
JP drains come in flat and round forms, and these are available in varying sizes. The flat drains are measured in millimeters and the round drains are measured in French sizes. 
The purpose of a drain is to prevent fluid (blood or other) build-up in a closed ("dead") space, and causing either disruption of the wound and the healing process, or becoming an infected abscess, with either scenario possibly requiring a formal drainage/repair procedure (and possibly another trip to the operating room). The drain is also used to evacuate an internal abscess before surgery, when an infection already exists.
Common uses 
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