Normal kidneys have two jobs, to make urine and to produce certain hormones.
The kidneys help to get rid of waste products from the blood such as urea and creatinine, maintain a normal balance of water in the body, maintain a normal balance of salts in the body such as sodium and potassium and get rid of acid made in the body.
The kidneys filter the blood, removing the waste products and excess water which is excreted from the body as urine.
This role is known as an excretory function
The second role of the kidneys is to make a hormone called erythropoietin or EPO. This helps the body to produce and maintain a normal level of red blood cells in the bloodstream.
The kidneys also convert vitamin D into an active form that is important in maintaining a normal balance of calcium in the body and, as a result, keeping teeth and bones healthy.
This role is known as an endocrine function
When a patient has chronic kidney failure (or chronic renal failure - CRF), the kidneys’ ability to carry out these jobs gradually deteriorates. When so-called end-stage renal failure (ESRF) is reached dialysis is used to replace the work that the kidneys do. However, dialysis can only partly replace the excretory function, the removal of waste products and excess water from the blood. The endocrine function, the manufacture of hormones, has to be replaced when necessary in a different way by injections of EPO and tablets of active vitamin D.
When a patient is on dialysis it is important to know and ensure that they are receiving enough treatment. In general the rule is the more dialysis the better. This means that it is necessary to measure at intervals the adequacy of dialysis to find out if the treatment a patient is receiving is sufficient or adequate.
The doctors and nurses caring for the patient want to find out how well the treatment is replacing the excretory function of the kidneys, that is the removal of waste products and excess water.
There are three questions that need to be answered in order to find this out:
1. How well is the dialysis getting rid of the waste products e.g. urea and creatinine?
2. How normal is the fluid (water) and salt balance in the body? Fluid balance is closely related to salt balance.
3. How well is the acid being controlled?
Patients receiving haemodialysis (HD) will have this done during a treatment session. The level of urea in the blood is measured at the beginning and end of the dialysis session. The more the level drops, the more efficient has been the treatment.
Patients receiving peritoneal dialysis (PD) need to have all their bags collected for 24 hours. These are then sampled. In addition, the amount of urea and creatinine in the blood is also checked in order for the amount being removed to be calculated. If the patient is still passing reasonable quantities of urine this is also collected for 24 hours and the amount of waste products still being removed by the patients kidneys is measured also.
Because patients vary in height and weight, the results for both types of dialysis are usually corrected for a patient’s size. This allows comparisons between different patients to be made.
A blood test will tell the doctor how much sodium and potassium is in the blood. However, it is more important to examine the patient. If there is any swelling (oedema) this usually means that there is excess salt and water in the body.
Once a patient is having dialysis, blood pressure also provides a guide. High blood pressure often means an excess of salt and water in the body and low blood pressure might mean that the patient is dehydrated.
One of the best guides to fluid balance is the weight of the patient. An increase in weight over a short period of time, a few hours or days, means increasing fluid in the body. A falling body weight means the opposite, that there is less fluid in the body.
A simple blood test will quickly reveal the level of acid in the body.
If the tests show that the dialysis is adequate there will be no change in the treatment. For many patients, with time, there will be a need to alter dialysis. This usually means an increase is necessary, particularly if there is a need to remove more waste products.
It is important to realise that all the evidence suggests that more dialysis is better because it means fewer medical problems and a longer life. For patients receiving haemodialysis increased dialysis can mean increasing the hours of treatment or increasing the pump speed or increasing the size of the dialyser (artificial kidney) or increasing the number of sessions per week. Often a combination of these changes will be necessary. If the tests continue to get worse this usually means that there is a problem with access, the fistula or line, which will need to be sorted out.
Peritoneal dialysis patients have their dialysis increased by increasing the size of the bags or increasing the number of the bags. Sometimes a PD machine may be needed to give frequent cycles of treatment overnight. As with haemodialysis, a combination of these changes may be needed. To improve dialysis with PD it is essential to have fluid in the abdomen 24 hours a day, seven days a week.