“If you define the problem correctly, you almost have the solution.”
– Steve jobs
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:
1. Equipment and processes used to purify
water for the preparation of concentrates and dialysate and the
reprocessing of dialyzers for multiple use.
2. The devices used to store and distribute this water.
3. 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?
1. 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.
2. 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).
3. Fluoride – added to
water to reduce tooth decay. Large amounts of fluoride can elute from an
exhausted deionizer and cause pruritis, nausea, and arrhythmias.
4. Copper and zinc – can leach from metal pipes and fittings. Another cause of hemolytic anemia.
5. 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
1.
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
2. 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.
3. 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.
