- Enter the pre-HD plasma sodium (mEq/L) — the patient's anchor value. Net diffusive Na flux is referenced to this number.
- Enter the start and end dialysate Na (mEq/L) — typical ranges 148–152 (start) and 136–138 (end) for a descending ramp. For an iso-natric prescription, set start = end = pre-HD plasma Na.
- Enter the session duration (min) and select the profile shape (linear descending, three-tier step, or ascending–descending).
- The time-averaged dialysate Na (TAD-Na), the plasma–TAD delta, and the salt-loading classification update automatically once all fields are valid.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this tool when prescribing or troubleshooting a hemodialysis sodium profile — particularly for patients with interdialytic weight gain, refractory hypertension, intradialytic thirst, or planned sodium/UF ramping. The classification estimates whether the chosen dialysate Na profile pushes salt into the patient (positive diffusive Na flux) or extracts it (net-negative). Pair the result with the iso-UF & Na/UF ramping guide.
Appropriate population
Maintenance hemodialysis patients on a fixed or profiled dialysate Na. Especially useful when reviewing prescriptions in patients with high interdialytic weight gain (>3% of dry weight), poorly controlled volume-dependent hypertension, or persistent intradialytic thirst and cramping despite acceptable UF rate.
When NOT to rely on it
This is a steady-state diffusive-flux estimate that ignores convective Na losses with UF, residual kidney function, and intradialytic plasma-Na changes from osmolar shifts. Do not use it to justify a high dialysate Na in patients with the osmolar-collapse phenotype (frequent symptomatic intradialytic hypotension with documented plasma osmolarity drop) — those decisions belong with the prescribing nephrologist after a complete osmolar review.
Pearls & Pitfalls
The plasma anchor is the rule
Net diffusive Na flux is governed by the gradient between time-averaged dialysate Na and the patient's pre-HD plasma Na. A TAD-Na within ±1 mEq/L of the plasma anchor delivers a near-zero net diffusive load (the iso-natric target). A delta of +2 to +3 already constitutes mild salt loading; >+3 reliably increases interdialytic thirst, weight gain, and predialysis BP.
Profile shape changes the average, not just the peak
A linear 148→138 ramp and a flat 143 prescription deliver the same TAD-Na (143) — but the ramp may protect intradialytic stability via an initial osmolar buffer. A three-tier step from 150→144→138 (equal-time tiers) averages to ~144. An ascending–descending (A/D) profile usually averages between its peak and trough. Match TAD-Na to plasma, not just the start or end value.
Pitfalls
(1) Reading the start dialysate Na in isolation — a 152 start with a 136 end still averages to 144. (2) Using a high (>155) start dialysate Na as routine practice — this drives chronic salt loading in most patients and should be reserved for the documented osmolar-collapse phenotype. (3) Ignoring convective Na losses with UF (the diffusive estimate here is conservative for the typical clinical question of "is this prescription net-loading?"). (4) Treating a single pre-HD plasma Na as immutable — re-check across multiple sessions before re-prescribing the profile.
Why Use It
Dialysate sodium is one of the most under-appreciated dials in the hemodialysis prescription. A dialysate Na far above the patient's plasma anchor diffuses sodium into the patient over each session — the patient drinks more between sessions, gains more interdialytic fluid, raises blood pressure, and ultimately pays for the cumulative salt load with left-ventricular hypertrophy and worse cardiovascular outcomes. Conversely, an inappropriately low dialysate Na produces rapid plasma osmolar drops, intradialytic hypotension, and cramping. The iso-natric (plasma-anchored) prescription — TAD-Na within ±1 mEq/L of the pre-HD plasma Na — preserves intradialytic stability while keeping net diffusive Na flux near zero. This calculator makes the gradient explicit so you can audit, adjust, or document the prescription.
Dialysate Sodium Balance Calculator
Enter the pre-HD plasma Na, the start and end dialysate Na, the session duration, and select the profile shape. The time-averaged dialysate Na (TAD-Na), the plasma–TAD delta, and the salt-loading classification update live as you type.
⚕ Song JH et al. JASN 2005;16(1):237–246 · Oliver MJ et al. JASN 2001;12(1):151–156 · Sang GL et al. AJKD 1997;29(5):669–677 · Flythe JE et al. Kidney Int 2020;97(5):861–876. Diffusive estimate; ignores convective Na losses with UF and intradialytic plasma-osmolar shifts. The A/D average is approximated as (peak + trough)/2. For licensed clinicians; not a substitute for individualized assessment.
How to Interpret
The classification turns the plasma–TAD delta into a salt-loading verdict:
| Plasma–TAD delta (mEq/L) | Classification | Action |
|---|---|---|
| |delta| ≤ 1 | Net-zero (safe) | Iso-natric prescription. Keep as is. Cumulative diffusive Na flux is near zero. |
| 1 < |delta| ≤ 3 | Mild salt loading (caution) | Plausible driver of interdialytic weight gain and BP. Trend pre-HD plasma Na across sessions and consider lowering TAD-Na by 1–2 mEq/L (drop start or end of the ramp). |
| |delta| > 3 | Salt loading (STOP unless osmolar-collapse phenotype documented) | Rework the profile toward the plasma anchor. The only routine exception is the documented osmolar-collapse phenotype (recurrent symptomatic intradialytic hypotension with measured plasma-osmolarity drop), where a small positive delta may be intentional. |
Note: a negative delta of similar magnitude (TAD-Na < plasma) signals diffusive Na removal — typically well tolerated when small, but watch for plasma osmolarity drops, cramping, and intradialytic hypotension at larger negative deltas.
Next Steps
Use the salt-loading classification to guide the next prescription change.
- Net-zero: retain the profile. Re-audit at the next pre-HD plasma Na drift > 2 mEq/L or any change in interdialytic weight gain.
- Mild salt loading: drop the dialysate Na ramp by 1–2 mEq/L (typically lower the end value first), reassess interdialytic weight gain and pre-HD BP after 2–3 weeks.
- Salt loading: rework the prescription toward the plasma anchor. Pair with UF profiling (see the iso-UF & Na/UF ramping guide) and reinforce dietary sodium restriction (see the sodium & salt reduction guide).
- Document the rationale in the dialysis order. For the osmolar-collapse phenotype, record the documenting plasma-osmolarity drop and the planned re-audit interval.
Evidence & References
Profile averaging conventions
| Profile shape | TAD-Na formula (this tool) | Note |
|---|---|---|
| Linear descending | (start + end) / 2 | Exact mean for a linear ramp over fixed time. |
| Step (3 equal-time tiers) | Mean of (start, midpoint, end) | Tiers held for equal thirds of session duration. |
| A/D (ascending–descending) | (peak + trough) / 2 | Approximation — the true average depends on tier durations and ramp slopes. |
Classification thresholds
| Plasma–TAD delta | Classification |
|---|---|
| |delta| ≤ 1 mEq/L | Net-zero (safe) |
| 1 < |delta| ≤ 3 mEq/L | Mild salt loading (caution) |
| |delta| > 3 mEq/L | Salt loading (STOP unless osmolar-collapse phenotype documented) |
The ±1 mEq/L band approximates measurement uncertainty in plasma Na and the iso-natric target reported in the sodium-balance literature. Profile flag fires when start dialysate Na > 155 mEq/L.
References
- Song JH, Lee SW, Suh CK, Kim MJ. Time-averaged concentration of dialysate sodium relates with sodium load and interdialytic weight gain during sodium-profiling hemodialysis. J Am Soc Nephrol. 2005;16(1):237–246. PMID: 15563561.
- Oliver MJ, Edwards LJ, Churchill DN. Impact of sodium and ultrafiltration profiling on hemodialysis-related symptoms. J Am Soc Nephrol. 2001;12(1):151–156. PMID: 11134261.
- Sang GL, Kovithavongs C, Ulan R, Kjellstrand CM. Sodium ramping in hemodialysis: a study of beneficial and adverse effects. Am J Kidney Dis. 1997;29(5):669–677. PMID: 9159299.
- Flythe JE, Chang TI, Gallagher MP, et al. Blood pressure and volume management in dialysis: Conclusions from a KDIGO Controversies Conference. Kidney Int. 2020;97(5):861–876. PMID: 32278617.
