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This article is about clinical dialysis; for the laboratory technique, see Dialysis (biochemistry)

In medicine, dialysis is a method for removing waste such as urea from the blood when the kidneys are incapable of this, i.e. in renal failure.


The decision to dialyse

In acute renal failure, dialysis is generally initiated when the renal function has deteriorated to an extent that it is threatening the body's physiology.

In chronic renal failure (or end-stage renal failure, ESRD), the problem is usually longstanding, and decision is based on the possibility of a renal transplant, complications of the malfunctioning kidney (e.g. hyperkalemia, uremia) and personal factors (such as tiredness due to the uremia).

Types of dialysis

The two types of dialysis are hemodialysis and peritoneal dialysis.

  • In hemodialysis, the patient's blood is passed through a tube into a machine that filters out waste products. The cleansed blood is then returned to the body.
  • In peritoneal dialysis, a special solution is run through a tube into the peritoneal cavity, the abdominal body cavity around the intestine. The fluid is left there for a while to absorb waste products, and then removed through the tube.

Peritoneal dialysis

There are three types of peritoneal dialysis. Continuous ambulatory peritoneal dialysis (CAPD), the most common type, needs no machine and can be done at home. Continuous cyclic peritoneal dialysis (CCPD) uses a machine and is usually performed at night when the person is sleeping. Intermittent peritoneal dialysis (IPD) uses the same type of machine as CCPD, but is usually done in the hospital because treatment takes longer. Prior to any peritoneal dialysis, a catheter is placed in the patient's abdomen, running from the peritoneum out to the surface, near the navel. This is done as a short surgery.

CAPD is typically done in the patient's home and workplace, but can be done almost anywhere; a clean area to work, a way to elevate the bag of dialysis fluid and a method of warming the fluid are all that is needed. The main consideration is the very high potential for infection with an open catheter; peritonitis is a common complication, as are infections of the catheter exit site or "tunnel" (path from the peritoneum to the exit site). Because of this, patients are advised to take a number of precautions against infection.

Step-by-step description of peritoneal dialysis (a CAPD exchange):

  1. The supplies and materials needed for an exchange are gathered in one clean location. Notable amongst these is a bag of dialysis fluid (also called dialysis solution), a solution comprised of a known amount of a glucose dissolved in water. The strength of this solution determines the osmotic gradient, and therefore the amount of water that diffuses out of the bloodstream. Common strengths of glucose are 0.5%, 1.5%, 2.5% and 4.25%. 1.5% is approximately fluid-neutral; it neither adds nor removes fluid and is used for patients who are primarily concerned with waste removal rather than fluid regulation. Higher concentrations lead to greater water removal. Electrolytes are also present in the fluid to maintain proper body levels. Patients weigh themselves, and measure temperature and blood pressure daily to determine whether the body is retaining fluid and, thus, what strength of fluid to use. Dialysis fluid typically comes premixed in a disposable bag-and-tube apparatus; no additional equipment is needed. The apparatus consists of two bags, one empty and one with the fluid, connected via flexible tubing to a Y-shaped fitting. The bag is heated to body temperature, to avoid causing cramping. Dry heat is used; common methods include microwaves, heating pads and solar radiation (often using the dashboard of a car, for instance while travelling).
  2. The patient, who performs the entire procedure themselves, dons a disposable surgical mask, scrubs their hands using antibacterial soap, and tucks a clean towel into the waistband of their pants to protect their clothing. The bag of dialysis fluid is removed from the protective packaging, and is hung from an IV stand or other elevated location, such as a coat hook. The tubing attached to the bag of fluid is uncoiled, and the second (empty) bag is placed on the floor. The Y-shaped connector is attached to the catheter tip; a protective cap must be removed from both of these before the connection is made, and the two portions of the connector are not permitted to touch anything, to avoid possible contamination.
  3. Once connected to the system, the patient clamps the tubing connected to the full bag of dialysis fluid and then releases the twist valve located in the tip of their catheter; this permits fluid to flow into or out of the peritoneal cavity. Because the full bag of fluid is clamped off but the empty bag is not, the effluent (used dialysis fluid) from within the peritoneum can drain out of the catheter and into the lower, waste bag. Emptying the abdomen of fluid takes approximately fifteen minutes, and the patient is free to perform tasks such as reading, watching television and editing a Wikipedia article about dialysis.
  4. When the abdomen has drained, the lower, drain, bag is clamped off. The twist valve in the catheter is also closed. The clamp is then removed from the upper tubing, permitting dialysis fluid to drain out. The clamp to the drain bag is briefly opened and some fluid is drained directly from the upper bag into the lower bag. This clears the line of air and other impurities. The drain line is then clamped off and the twist valve on the catheter end is opened. This permits fluid to enter the peritoneum. Filling the abdomen with fresh fluid takes about fifteen minutes, and the patient enjoys the same freedoms as while draining.
  5. Once the entire bag of fluid (an amount varying primarily based on body size, ranging from 1500 to 3000 mL) has been introduced to the abdomen, the patient then cleans their hands again (typically using an antiseptic alcohol-based cleanser) and puts the surgical mask on. The Y-connector is detached from the catheter tip and a protective cap is placed on the end of the catheter.
  6. The effluent is inspected after a dialysis exchange is complete; a cloudy effluent indicates probable peritoneal infection. The effluent is drained into a toilet, and the various dialysis supplies are discarded with normal garbage.


The principle of hemodialysis (UK: haemodialysis) is somewhat different. It works by having the blood flow along one side of a semipermeable membrane, with the dialysis solution flowing along the other side. Due to the difference in osmolarity between the two liquids, water traverses the membrane in order to dilute the dialysis liquid, carrying along the unwanted blood contents.

The dialysing fluid is used at body temperature, and consists of a solution of glucose, amino acids and mineral ions. The solution is sterilized. Urea diffuses into the dialysing fluid, which does not contain the compound. However, concentrations of glucose, amino acids and minerals are either similar to those of normal plasma to prevent loss, or higher than those of plasma to act as nutrient supplements. If a strong glucose solution is used, osmosis of water from plasma would occur.

Dialysis is conducted in a dedicated facility, either a special room in a hospital or clinic that specializes in hemodialysis, or (rarely) it can be done in the patient's home. Nurses and technicians working in the facility have special training specific to dialysis.

A prescription for dialysis by the renal physician (nephrologist) will specify various parameters for setting up dialysis machines, times and durations of dialysis sessions. In the US and UK, 3-4 hour sessions, 3 times a week are typical, although there are patients who dialyse 2 or 4 or 5 per week. There are also a small number of patients who undergo nocturnal dialysis for 8 hours per night 6 nights per week.

Step-by-step description of hemodialysis:

  1. Before or around the time the patient arrives for his/her scheduled session, a dialysis machine will be prepared. There are many models of dialysis machines, but typically in modern machines there will be a computer, CRT, a pump, and facility for disposable tubing and filters. The filters (the actual artificial kidneys) are cylindrical, clear plastic outside with the filter materiel visible inside (looks like thick paper). They are perhaps 15-18 inches long, and 2-3 inches thick. They have tubing connectors at both ends. The technician or nurse will setup plumbing on the machine in a moderately complex pattern that has been worked out to move blood through the filter, allow for saline drip (or not), allow for various other medications/chemicals to be administered. How the plumbing is setup may vary between models of machine and they types of filters. For some filters, it is necessary to clear sterilizing fluid from the filter before connecting the patient. This is done by altering the plumbing to push saline through the filter, and carefully checked with a type of litmus test.
  2. The pump does not directly contact the blood or fluid in the plumbing - it works by applying pressure to the tubing, then moving that pressure point around. Think of a disk with a protrusion in it. Put this into a close fitting 270 degree enclosure. Put plastic tubing between the enclosure and the disk, entering and exiting in the 90 open degrees. Now imagine the disk turning. It will put pressure on the tubing, and the pressure point will roll around through the 270 degrees, forcing the fluid to move. It is characteristic of dialysis machines that most of the blood out of the patients body at any given time is visible. This facilitates troubleshooting, particularly detection of clotting.
  3. The patient arrives and is carefully weighed. Standing and sitting blood pressures are taken. Temperature is taken.
  4. Access is setup. For patients with a fistula (a surgical modification to an arm or leg vein to make it more robust, and therefore usable for high capacity blood movement required by dialysis) this means inserting two large gauge needles into the fistula. (Yes, it hurts but there are various methods of numbing the entry sites before the needles are inserted - the two most common are lignocaine(lidocaine) a local anaesthetic injected under the skin and there is also available a cream called EMLA which is applied to the skin 45 minutes before the needles are inserted). Fistulas are widely considered the desirable way to get access for hemodialysis, but they take time to setup and mature (anywhere between 5 weeks to 15 weeks). For other patients, access may be via a catheter installed to connect to large veins in the chest. (This means no needles, but there are other severe downsides to a catheter). There are some other arrangements that can be made as well.
  5. When access has been setup, the patient is then connected to the preconfigured plumbing - creating a complete loop through the pump and filter. The pump and a timer are started. Hemodialysis is underway.
  6. Periodically (every half hour, nominally) blood pressure is taken. As a practical matter, fluid is also removed during dialysis. Most dialysis patients are on moderate to severe fluid restrictive diets (in addition to other dietary restrictions). This is because kidney failure usually includes an inability to properly regulate fluid levels in the body. A session of hemodialysis may typically remove 2-5 kilograms (5-10 pounds) of fluid from the patient. The amount of fluid to be removed is set by the dialysis nurse according to the patient's "estimated dry weight." . This is a weight that the care staff believes represents what the patient should weigh without fluid built up because of kidney failure. Removing this much fluid can cause or exacerbate low blood pressure. Monitoring is intended to detect this before it becomes too severe. Low blood pressure can cause cramping, nausea, shakes, dizziness, lightheadedness, and unconsciousness.
  7. At the end of the prescribed time, the patient is disconnected from the plumbing (which is removed and discarded, except perhaps for the filter, which may be sterilized and reused with the same patient at a later date). Needle wounds (in case of fistula) are bandaged with gauze, held for up to 1 hour with direct pressure to stop bleeding, then the taped in place. It's just like getting blood draw, only it takes a lot longer, and more fluid/Blood is lost.
  8. Temperature, standing and sitting blood pressure, and weight are all measured again. Temperature changes may indicate infection. BP discussed above. Weighing is to confirm the removal of the desired amount of fluid.
  9. Care staff verifies that the patient is in condition suitable for leaving. The patient must be able to stand (if previously able), maintain a reasonable blood pressure, and be coherent (if normally coherent). Different rules apply for in-patient treatment - in those cases the patient isn't leaving the facility.


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