5.3 Dialysate Composition & Concentrate Formulations

Dialysate Composition & Concentrate Formulations

Dialysate is an aqueous electrolyte solution formulated to correct uremic imbalances, restore acid-base homeostasis, and normalize extracellular fluid composition. Circulating through the dialyzer shell space, dialysate exchanges solutes and water with blood across capillary fibers. Because 90 to 150 liters of dialysate interact with blood each session, formulation errors can rapidly cause life-threatening electrolyte shifts, arrhythmias, or acute hemolysis. The Certified Hemodialysis Nurse (CHN) must understand concentrate chemistry, electrolyte prescription targets, and mandatory independent verification protocols.

---

Automated Proportioning Systems

Modern hemodialysis delivery machines blend purified reverse osmosis (RO) water with two concentrates on demand:

1. Acid Concentrate ("A"): Contains electrolytes, dextrose, and an organic acid.
2. Bicarbonate Concentrate ("B"): Contains sodium bicarbonate and sodium chloride.

Standard Proportioning Ratios

Proportioning systems mix water and concentrates in calibrated volumetric ratios:

• 45X Formulation: 1 part acid : 1.72 parts bicarbonate : 42.28 parts RO water (Total = 45 parts).
• 35X Formulation: 1 part acid : 1.4 parts bicarbonate : 32.6 parts RO water (Total = 35 parts).

Concentrate formulations and machine calibration codes must match exactly to avoid severe hypo- or hypernatremia.

---

Chemical Rationale for Concentrate Separation

Concentrates cannot be pre-mixed into a single container due to chemical incompatibility between divalent cations and bicarbonate:

$$Ca^{2+} + 2HCO_3^- \longrightarrow CaCO_3 \downarrow + H_2O + CO_2$$ $$Mg^{2+} + 2HCO_3^- \longrightarrow MgCO_3 \downarrow + H_2O + CO_2$$

If concentrated calcium or magnesium contacts concentrated bicarbonate in an alkaline solution, insoluble calcium carbonate ($CaCO_3$, limestone) and magnesium carbonate ($MgCO_3$) immediately precipitate as white chalky crystals. These precipitates clog valves, flowmeters, and dialyzer fibers. Keeping concentrates separate and blending them with RO water under automated control prevents precipitation.

Acid Concentrate Chemistry and Citrate Buffers

• Acid Buffer: Acid concentrate is acidified to pH 2.0 to 3.0 using acetic acid (~4 mEq/L) or citric acid to maintain divalent cation solubility.
• Citrate-Buffered Dialysate: Formulations containing citric acid (~2.4 mEq/L) chelate free ionized calcium within the dialyzer, reducing available calcium locally; this is not automatically equivalent to a prescribed regional citrate anticoagulation protocol. This reduces dialyzer clotting, lowers systemic heparin requirements, and enhances solute clearance.

---

Delivered Dialysate Electrolyte Parameters

Final dialysate electrolyte concentrations are prescribed to meet individual patient needs:

• Sodium ($Na^+$: 135 to 145 mEq/L): Determines extracellular fluid osmolality. Setting dialysate sodium to match patient pre-dialysis serum sodium prevents acute osmolar shifts. Sodium modeling (ramping sodium from 145 to 138 mEq/L) temporarily reduces intradialytic cramps, but promotes post-dialysis thirst, high interdialytic weight gain (IDWG), and hypertension. Modern guidelines discourage routine sodium modeling.
• Potassium ($K^+$: 0, 1.0, 2.0, 3.0, 4.0 mEq/L): Prescribed based on pre-dialysis potassium. Most chronic patients dialyze against a 2.0 or 3.0 mEq/L bath. In patients taking digoxin, low potassium baths (0 or 1.0 mEq/L) cause rapid hypokalemia, enhancing digoxin binding and triggering fatal ventricular arrhythmias; the nurse should flag the interaction and confirm the individualized potassium prescription.
• Calcium ($Ca^{2+}$: 2.0, 2.5, 3.0 mEq/L): Standard 2.5 mEq/L matches normal serum ionized calcium (~1.25 mmol/L), but does not guarantee neutral whole-body calcium balance. KDIGO suggests 2.5–3.0 mEq/L for CKD G5D, with individualization. Very low calcium can increase hypotension and arrhythmia risk; neither hypercalcemia nor low bone turnover authorizes the nurse to choose an unprescribed low bath.
• Magnesium ($Mg^{2+}$: 0.75 to 1.0 mEq/L): Maintained slightly below normal serum levels to allow modest diffusive removal and prevent hypermagnesemia in ESRD.
• Chloride ($Cl^-$: 100 to 110 mEq/L): Major balancing counter-anion.
• Bicarbonate ($HCO_3^-$: 32 to 38 mEq/L): Diffuses into blood to correct chronic metabolic acidosis ($HCO_3^- < 22\text{ mEq/L}$). Excessive bath bicarbonate (> 38 mEq/L) causes post-dialysis metabolic alkalosis, provoking hypokalemic arrhythmias, reduced ionized calcium (tetany and cramps), and hypoventilation.
• Dextrose (100 to 200 mg/dL): Prevents hypoglycemia, reduces post-dialysis fatigue, and dampens rapid osmolar drops.

---

Independent Safety Testing: Conductivity, pH, and Temperature

Before initiating treatment, the nurse verifies prescription, concentrate formulation, residual clearance and fluid parameters through the device and facility procedure. The following numerical ranges are teaching examples, not universal limits:

Conductivity and Automated Bypass

Conductivity reflects total ionic concentration (primarily sodium), normally 13.0 to 15.0 mS/cm.

• Machines contain redundant internal conductivity sensors. If conductivity deviates outside preset limits ($\pm 0.2-0.4\text{ mS/cm}$), the machine alarms and diverts dialysate to the drain via the bypass valve.
• Hypotonic dialysate causes water to rush into red blood cells, causing acute hemolysis and hyperkalemic arrest.
• Hypertonic dialysate causes cellular crenation, hypernatremia, and intracranial hemorrhage.
• Independent testing: Before patient exposure, perform the delivery system’s required independent conductivity and pH verification using the approved calibrated method. Acceptance ranges depend on the formulation and device instructions; do not present one 6.9–7.6 pH range as the universal CMS requirement.

Temperature and Hemolysis Risk

Use the prescribed temperature and the delivery system’s validated alarm limits. Excessively hot fluid can injure blood cells, but 39°C is not a universal instantaneous hemolysis threshold. Temperature above the ordered range requires correction before exposure. Cool dialysate may improve hemodynamic tolerance but can cause discomfort and shivering; it is individualized rather than imposed on all patients.

Conductivity estimates total ionic content, not the identity of each ion. A wrong potassium or calcium concentrate may still yield plausible conductivity. Independently match the concentrate label, lot, mixing ratio and machine setting to the order, then perform the manufacturer’s required conductivity, pH and residual-disinfectant checks. Alarm bypass is a protective response, not permission to override the alarm and continue exposure. A bath change requires the same verification as initial setup.

Dialysate Formulation Reference

Sources checked 2026-10-10: NIDDK HD; current device and medication instructions govern product-specific details.

Device source checked 2026-10-11: Fresenius operator manuals, including 2008T revision AE. Examples explain principles; the actual device and chemical labeling govern operation.