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What is the difference between CAPD and APD?

CAPD (Continuous Ambulatory Peritoneal Dialysis) is the most common type of PD. C stands for Continuous, meaning all the time, and A for Ambulatory, because you can walk around while the dialysis takes place. When people are on CAPD, they do their own fluid exchanges. They drain 1.5 to 3 litres of dialysis fluid into their abdomen, leave it there for 4 to 8 hours, and then drain it out. This is done four to five times a day – every day. Exchanges are simple to do and can be performed in any clean area, almost anywhere – even in the car by the roadside for people who drive long distances.

APD (Automated Peritoneal Dialysis) uses a machine to do the dialysis fluid exchanges. The machine is usually placed in the bedroom and does the exchanges while the person is asleep. Some new APD machines are only the size of a video recorder, and make it possible for people to do exchanges in different places. Most people need to spend 8 to 10 hours attached to the machine every night. This enables the machine to perform an average of three to five exchanges of 1.5 to 3 litres of dialysis fluid each night. The length of time that PD fluid is left in the abdomen before it is exchanged by the machine, varies from between about 30 minutes to 3 hours. After spending the night on the machine, most people on APD keep fluid inside their peritoneum during the day. Some people will need to do one exchange during the daytime, or around teatime. A small number of people use a simple automated machine (sometimes called Quantum) to do one exchange during the middle of the night and then do CAPD by day.

Some kidney units offer help at home for people who cannot set the machines up themselves because they do not have the necessary strength or dexterity. A nurse visits the home in the evening to set the machine up, and again in the evening to take the bags off the machine. The person on dialysis just needs to connect themselves onto the machine before going to bed, and disconnect in the morning. This is called “Assisted APD”. It can help some people where they cannot do all the dialysis themselves, but would prefer not to come into the hospital three times a week for haemodialysis.

Will I have a choice of treatment?

In many Units people are offered CAPD or APD. Currently, about 80-90% of people on PD do CAPD, and 10 -20% do APD. However, the number of people doing APD is growing all the time. Different people may be better suited to either CAPD or APD for a number of reasons. Very few Units in the UK offer a free choice between CAPD and APD. This is because APD works best if the peritoneum clears waste quickly. In some Units the amount of money set aside for APD and/or CAPD may be a factor affecting which treatment a patient is offered. The main factors which are discussed when a person is initially choosing their PD treatment include:

• Person’s lifestyle
• Who will be doing the exchanges
• Person’s size – bigger people will need more dialysis

Tests can be carried out to see how the peritoneum works, and which type of PD would give the best dialysis. This is called a PET Test.

Peritoneal Dialysis - PET test

A Peritoneal Equilibration Test known as a PET test, is used to find out whether waste toxins pass quickly or slowly from the person’s bloodstream and into the dialysis fluid. If the toxins move quickly, the person is called a ‘high or fast transporter’. If the toxins move slowly, the person is a ‘low or slow transporter’.

Transporter	Waste removal	Water removal	Best type of PD
High or Fast	Fast	Poor	Frequent exchanges, short dwells – APD
Average	Okay	Okay	CAPD or APD
Slow or Low	Slow	Good	CAPD, 5 evenly spaced exchanges – 1 exchange at night using a small machine

The reason why ‘high Transporters’ have poor water removal is because the water and glucose from the PD fluid are absorbed into the body, if the fluid is not drained out after a 2-3 hour dwell. Waste, water and glucose can move quickly in both directions.

A PET test helps PD staff to decide whether your dialysis would be improved by leaving your PD fluid in the abdomen for longer or shorter periods of time. A nurse usually performs the test in hospital. A full PET test takes around 5 hours to complete and involves doing a CAPD exchange using a 2.27% glucose bag. Samples of PD fluid, and blood are taken at set times. Some Units do a mini PET test, and only one blood and PD sample are taken when the PD fluid is drained out after a 4-hour dwell in the abdomen. The person is usually asked to save all the PD fluid they drain out and all the urine they pass in the 24-hour period prior to the PET. The waste from the 24-hour period is tested and this is known as a Peritoneal Function Test. 

How will I know which type of PD would be best for me?

The main medical reason why a doctor or nurse may recommend either CAPD or APD for a person relates to the way the person’s peritoneum works during dialysis. In some patients the peritoneum allows waste and water to pass from the blood into the dialysis fluid quickly. These people are called ‘high or fast transporters’. High transporters are usually more suited to APD, because the machine is able to do rapid exchanges of dialysis fluid whilst they sleep. Other people called ‘low or slow transporters’, will get more dialysis if the fluid is left inside them for longer periods. Low transporters are generally better suited to CAPD.

APD can also be particularly good for people who require a lot of dialysis – for example, large people, especially those who no longer pass urine. This is because the machine can do more fluid exchanges than a person would be able to do himself or herself with CAPD. Also, as the person is lying down, they may be more able to tolerate bigger volumes of dialysis fluid. In these ways, APD can remove more waste toxins than CAPD. Even so, for some very large people, APD during the night may not be enough, and an additional exchange at teatime is needed.

Since APD exchanges are done during the night, this form of dialysis can be particularly suitable for patients who work or who are in full-time education.

APD is a possible treatment option for patients who need a carer to perform dialysis for them, such as the elderly, infirm or very young.

How will I know which bags of PD fluid to use?

Whatever the type of PD (either CAPD or APD), the ability to remove waste can be raised by increasing either the volume of fluid used, the number of exchanges, or both. A larger bag will remove more toxins (and a little more water) than a smaller bag. The dialysis needs of people depend partly on their body size. Big people usually need big bags (2.5 or 3 litres of dialysis fluid).

The ability of PD fluid to remove water is affected by the amount of glucose (sugar) in the bag – the more glucose in the bag, the more water is removed. There are three different strengths: a ‘strong’ bag (3.86% glucose solution), a ‘medium’ bag (2.27% glucose) and a ‘weak’ bag (1.36% glucose).

The strength of the bag is different from the size of the bag. A strong bag has more glucose in it than a weak bag, but it is no larger. People are advised to consider the weak bag as their ‘standard’ bag, and to try to use a minimum number of strong bags.

Types of PD fluid

Standard PD fluid can contain different amounts of glucose or calcium. In addition to these, there are a number of new ‘special’ dialysis fluids, which are sometimes prescribed for people with particular problems.

Strong, Medium and Weak Glucose bags:

The ability of PD fluid to remove water is affected by the amount of glucose (sugar) in the bag – the more glucose in the bag, the more water is removed. Because sugar is absorbed into the body, people are advised to limit the number of strong bags they use each day. People who use 2 strong bags each day may find that they put on weight as a result of the extra calories they get from the PD fluid. Using a lot of ‘strong bags’ for a long period of time may also affect the life span of the peritoneum (adversely). There are three different strengths of glucose bags:

• Strong bags (3.86% or 4.25% glucose solution)
• Medium bags (2.27% or 2.25% glucose)
• Weak bags (1.36% or 1.5% glucose)

High, Medium and Low Calcium bags:

In order to help maintain the calcium level in the blood at the normal level of 2.2-2.6 mmols, the level of calcium in the PD fluid can be altered by using different bags. PD fluid can contain the following levels of calcium:

• 1.75 mmols
• 1.25 mmols
• 1 mmol

The level of calcium in the PD fluid is written on the bag, and in the information sheet in the box of fluid.

Icodextrin: This fluid (also known as Extraneal) is sometimes used to help with people who have difficulty with fluid removal, particularly when their PD fluid is left in for a long time. This fluid contains a glucose polymer (in which the glucose particles are stuck together), rather than ordinary glucose. Icodextrin is less likely than ordinary glucose to be absorbed into the body. Icodextrin has also been shown to benefit people who have been on PD for a long time and whose peritoneums do not work very well for dialysis. It is used once each day, and is left in the peritoneum overnight for people on CAPD and during the daytime for people on APD.

Amino Acid: This is a PD fluid containing Amino acids instead of glucose (sometimes known as Nutrineal). As amino acids are the building blocks of protein, and as some of the amino acids are absorbed into the blood, it is thought that these dialysis fluids might also act as food supplements. Dialysis fluids containing amino acids can be useful for patients who do not eat well or who have malnutrition. In some Units people on PD are taking part in a research study to help test the benefits of Nutrineal in preventing poor nutrition. Amino Acid fluids are usually used only once each day, otherwise the blood might become too acid.

Bicarbonate: This fluid is relatively new, and has been developed to help people who have problems regulating the level of acid in their bodies. This fluid sometimes helps people who get a lot of pain when they have the standard PD fluid in their peritoneum. A similar product, known as Physioneal, contains a mixture of Bicarbonate and normal lactate to help control acidity in the body.

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More information about Peritoneal Dialysis click the links below

What is Peritoneal Dialysis?

Introduction to Peritoneal Dialysis

Peritoneal Dialysis – Frequently Asked Questions

The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.