During an average week of hemodialysis, a patient can be exposed to 300-600 liters of water, providing multiple opportunities for potential patient exposure to waterborne pathogens. Adverse patient outcomes including outbreaks associated with water exposure in dialysis settings have resulted from patient exposure to water via a variety of pathways; including improper formulation of dialysate with water containing high levels of chemical or biological contaminants, contamination of injectable medications with tap water, and reprocessing of dialyzers with contaminated water. For the health and safety of hemodialysis patients, it is vital to ensure the water used to perform dialysis is safe and clean.
AAMI Water Standards
The Association for the Advancement of Medical Instrumentation (AAMI) external icon in conjunction with the International Standards Organization (ISO) have established chemical and microbiological standards for the water used to prepare dialysate, substitution fluid, or to reprocess hemodialyzers for renal replacement therapy. The AAMI standards address:
- Equipment and processes used to purify water for the preparation of concentrates and dialysate and the reprocessing of dialyzers for multiple use.
- The devices used to store and distribute this water.
- The allowable and action threshold levels of water contaminants, bacterial cell counts, and endotoxins. Refer to specific reference listed for full details on maximum allowable chemical contaminates and bacterial/endotoxin limits.
What are the potentially harmful water contaminants?
- Aluminium – added as a flocculating agent by many municipal water systems to remove nonfilterable suspended particles. It is toxic to dialysis patients, becoming sequestered in bone for long period of time, resulting in adynamic bone disease and osteomalacia. It can also cause the well-described dialysis encephalopathy syndrome.
- Chloramine – added to water to prevent bacterial proliferation. It can cause hemolytic anemia (see Table 1 for other causes of hemolysis in patients on dialysis).
- Fluoride – added to water to reduce tooth decay. Large amounts of fluoride can elute from an exhausted deionizer and cause pruritis, nausea, and arrhythmias.
- Copper and zinc – can leach from metal pipes and fittings. Another cause of hemolytic anemia.
- Bacteria and endotoxin – the substances added to municipal water to suppress bacterial proliferation are removed in the water purification process for dialysis treatment. Passage of endotoxin, endotoxin fragments and other bacterial products across the dialyzer membrane and into the bloodstream can lead to pyrogenic reactions.
METHODS OF PURIFICATION
- Pre-treatment
This involves a valve to blend hot and cold water to a constant temperature, preliminary filtration, softening, and adsorption with activated carbon. Injection of hydrochloric acid to correct the pH in the case of excess alkalinity is sometimes required as this can disturb the carbon adsorption beds and the reverse osmosis (RO) membrane.
Water softeners exchange calcium and magnesium for sodium that has been affixed to a resin bed. Although these ions are also removed by RO, water softeners in regions with ‘hard water’ reduce accumulation of calcium and magnesium salts, thereby prolonging the life of the RO membrane.
Carbon adsorption by activated carbon removes chlorine, chloramines, and other dissolved organic contaminants. This step is essential and generally includes two carbon beds to ensure complete removal of chloramine
- Primary purification process
A filter is placed just upstream to the RO membrane to catch any carbon particles and resin beads that have been inadvertently released from the pre-treatment system.
RO is the mainstay of dialysis water purification. Hydrostatic pressure drives water across a semipermeable membrane and excludes >90% of the contaminants. This strategy removes ionic contaminants, bacteria, and endotoxin.
De Mineralisation (DI) removes ionic contaminants by exchanging cations for H+ and anions for OH–. The exchanged H+ and OH– ions then combine to become water. DI is usually used for water purification when the RO membrane fails or as an additional purification process. Water processing through DI requires bacterial control filters (ultrafilters) after DI purification because DI systems promote bacterial growth. Finally, the DI membrane is saturable and requires continuous monitoring of conductivity with mechanisms for diverting RO water when conductivity exceeds 1 microS/cm to avoid breakthrough of fatal contaminants such as fluoride.
- Distribution of purified water
Purified water for hemodialysis is then distributed to individual dialysis machines to produce dialysate solution that remains free of contaminants. Inert material such as plastics are used to avoid chemical contaminants. Water piping systems must be carefully designed and constructed to avoid bacterial contamination with regular disinfection to prevent bacterial colonization of the system and to minimize formation of biofilm.
What monitoring is required?
There are safety standards for equipment used to purify water for dialysis as outlined by the AAMI and European Best Practices Group. Essentially, these include monitoring of the water and dialysis solution for chemical or microbiologic contaminants on a regular basis. Chloramines, for example, should be checked for at least twice daily. Dialysis patients should also be carefully monitored for any evidence of a hemolytic, pyrogenic, or other reaction, that may suggest contamination.