Dialysate Concentrates and Powder Mixing
Key Takeaways
- Dialysate is proportioned from three streams: treated water plus ACID concentrate plus BICARBONATE concentrate; the two concentrates are kept separate until dilution to prevent calcium/magnesium carbonate from precipitating.
- The machine verifies the bath two ways: CONDUCTIVITY (confirms total electrolyte concentration, normal ~13-15 mS/cm or 13.5-14.5 depending on prescription) and pH (normal ~6.9-7.6); both must be in range before patient connection.
- An independent (handheld) conductivity meter check is the technician's safeguard against a machine that is itself miscalibrated - a wrong concentrate or proportioning fault can hide behind a 'normal' machine display.
- Color coding (e.g., red acid, blue bicarbonate) is a backup, NOT a substitute for reading the label and verifying the concentrate matches the prescription's potassium, calcium, sodium, and bicarbonate values.
- Wrong concentrate, wrong connector, incomplete powder dissolution, or expired concentrate produces unsafe dialysate; if conductivity or pH fails, do NOT start treatment - recheck, correct, and notify staff.
How Dialysate Is Made
Dialysate is the prescribed fluid bathing the dialyzer fibers. The machine produces it continuously by proportioning three streams: purified water, acid concentrate, and bicarbonate concentrate. Concentrates are shipped 35x or 45x strength and the machine dilutes them with water to the final 1x bath.
Why two separate concentrates? If you combined the calcium and magnesium (in the acid concentrate) with bicarbonate in concentrated form, they would precipitate as insoluble calcium and magnesium carbonate - a chalky scale that fouls the machine and changes the bath. Keeping acid and bicarbonate apart until they are simultaneously diluted with a large volume of water prevents precipitation. The acid concentrate also contains a small amount of acetic or citric acid that, when mixed, converts to maintain the correct final pH.
A typical final dialysate composition (the prescription individualizes potassium, calcium, and bicarbonate):
| Component | Typical dialysate value |
|---|---|
| Sodium | 135-145 mEq/L |
| Potassium | 0-4 mEq/L (commonly 2-3) |
| Calcium | 2.0-3.0 mEq/L |
| Magnesium | 0.5-1.0 mEq/L |
| Bicarbonate | 30-40 mEq/L |
| Glucose (dextrose) | 100-200 mg/dL (if present) |
Concentrate selection matters because potassium, calcium, sodium, bicarbonate, glucose, and acid components differ by container. A 2K bath and a 3K bath look identical on the shelf, yet delivering the wrong one can drive a patient toward hyperkalemia or hypokalemia. Color coding helps but never replaces reading the label and matching it to the prescription.
Conductivity and pH: The Two Independent Checks
The machine monitors the finished bath continuously with two complementary sensors:
- Conductivity measures the water's ability to carry electric current, which is proportional to total dissolved electrolyte concentration. A normal dialysate conductivity is roughly 13-15 mS/cm (the exact target depends on the prescribed sodium). High conductivity = too concentrated (hypertonic bath -> hypernatremia risk); low conductivity = too dilute (hypotonic bath -> hemolysis and hyponatremia risk).
- pH confirms acid-base correctness, normally about 6.9-7.6. An out-of-range pH suggests the wrong concentrate ratio or a bicarbonate problem.
The critical safeguard is the independent conductivity check. The machine's own conductivity reading can be normal while the bath is actually wrong if the machine's cell is miscalibrated or the wrong concentrate was hung. Verifying with a separate, calibrated handheld conductivity meter catches errors the machine cannot see itself. If conductivity or pH is out of range, do not connect the patient - recheck per policy, confirm the correct concentrates and connections, and notify the appropriate staff.
Mixing Bicarbonate From Powder
Many units mix bicarbonate concentrate on site from powder (or use single-patient powder cartridges) to avoid bacterial growth in pre-mixed liquid bicarbonate, which is an excellent microbial growth medium. Mixing is a controlled procedure:
- Use the correct water volume and powder amount for the mixer.
- Allow full dissolution and mixing time - powder must completely dissolve; undissolved powder yields a weak, non-prescription bath.
- Label the container for traceability: mix date/time, expiration or discard time, lot number, and initials.
- Bicarbonate is microbiologically fragile - observe the discard time (commonly within 24 hours of mixing) because bacteria multiply quickly.
Do not top off old concentrate with new, mix lots casually, use an unlabeled jug, or borrow concentrate from another station without verification. Each of these defeats traceability and can deliver the wrong or contaminated bath.
Worked example: A technician sets up a machine and the display shows conductivity 14.0 mS/cm - apparently normal. Following policy, the technician checks with a handheld meter and reads 15.8 mS/cm. The discrepancy means the machine's conductivity cell is reading low; the actual bath is hypertonic. The technician does not connect the patient, removes the machine from service, reports to biomed and the nurse, and documents. Trusting the machine display alone would have exposed the patient to a hypernatremic bath.
Connecting Concentrate Errors to Patient Harm
The whole point of concentrate vigilance is that the dialysate composition drives the patient's blood chemistry by diffusion across the membrane. Linking each error to its clinical consequence is exactly how the exam frames application questions.
| Concentrate error | Resulting bath | Patient consequence |
|---|---|---|
| Hung 0K instead of 2K bath | Too little potassium | Hypokalemia, arrhythmia |
| Hung 4K instead of 2K bath | Too much potassium | Hyperkalemia risk |
| Wrong/low calcium concentrate | Low dialysate calcium | Hypocalcemia, cramps, hypotension |
| Conductivity high (too concentrated) | Hypertonic bath | Hypernatremia, thirst, hypertension |
| Conductivity low (too dilute) | Hypotonic bath | Hemolysis, hyponatremia |
| Wrong bicarbonate / bad pH | Acid-base off | Acidosis or alkalosis symptoms |
The technician's defenses against all of these are the same five verifications, performed every setup in order:
- Read the prescription.
- Read each concentrate label and confirm it matches the prescribed K, Ca, Na, and bicarbonate.
- Confirm the connections - acid port to acid, bicarbonate port to bicarbonate.
- Verify conductivity is in range, ideally with an independent handheld meter.
- Verify pH and the expiration/discard time.
CCHT items often disguise one of these checks inside an otherwise routine setup scenario - the safe answer is the one that catches it before the patient is connected, never one that relies on color, memory, or habit.
Why are acid concentrate and bicarbonate concentrate stored and delivered as two SEPARATE solutions rather than premixed at full strength?
Before patient connection, the dialysate conductivity reads significantly BELOW the prescribed range. What is the primary patient risk if treatment proceeds?
The dialysis machine displays a normal conductivity, but the technician's independent handheld meter reads well outside the acceptable range. What should the technician do?
A technician finds a jug of bicarbonate concentrate mixed from powder yesterday with no label. The most appropriate action is to: