Dialysis is not safe. Accidents will happen unexpectedly.
The nurse is the last line of defense. These pages are your preparation.

Vigilance — Critical Thinking — Rapid Action

TDS Monitoring & Patient Impact – Dialysis Safety

Back to Safety Series Home

The Key Concept

"If TDS suddenly rises more than 5 in a day — that indicates malfunction and needs assessment and management — regardless of the number."

Trust the Trend — Not Just the Number

What is TDS? Total Dissolved Solids

Total Dissolved Solids (TDS) measures the total concentration of dissolved ions in water — including sodium, calcium, magnesium, potassium, bicarbonate, and other minerals.

mg/L

Measured In

Milligrams per liter — also known as ppm (parts per million)

µS/cm

Alternative Unit

Microsiemens per centimeter — conductivity measurement

≈ 1.5

Conversion

1 mg/L ≈ 1.5 µS/cm (approximate conversion)

The Confusion — What is "Normal"? Different Standards

< 25

Strict Standard

Some guidelines recommend this — often used for high-risk units

< 50

Relaxed Standard

Some units use this — still considered acceptable

< 100

ISO 13959 Standard

Maximum allowable for hemodialysis water — absolute limit

The Correct Approach: The absolute number matters less than the change. A TDS of 20 mg/L is "acceptable" — but if it was 12 mg/L yesterday, that is a 67% increase and indicates a problem.

The Sodium Pathway From Water to Hypertension

How High TDS Causes Unexplained Hypertension

High TDS in Water High Sodium (Na⁺) RO Cannot Remove All Sodium Dialysate Sodium is High Sodium Moves to Patient's Blood Volume Expansion → Hypertension

The Machine May Say "PASS" — But the Patient's Blood Pressure Tells the Truth.

Why Not Calcium? The Role of the Softener

Sodium (Na⁺)

Main contributor to TDS. Passes through RO membrane easily. This is what causes hypertension.

Calcium (Ca²⁺)

Softener removes it. Calcium is exchanged for sodium in the softener. Less calcium in water = less calcium in dialysate.

Magnesium (Mg²⁺)

Softener removes it. Magnesium is exchanged for sodium in the softener. Less magnesium in water = less magnesium in dialysate.

The Result

TDS may stay the same but the composition changes — more sodium, less calcium/magnesium.
The machine measures TOTAL ions — not individual ions.

The Double Check Sensor A False Sense of Security

Conductivity CANNOT Detect Sodium Substitution

The double check sensor measures total conductivity — it cannot distinguish sodium from calcium from magnesium from potassium.

High sodium = high conductivity = machine says "PASS"
But the patient gets a sodium load → hypertension

Effect on Patients Clinical Presentation

Unexplained Hypertension

BP is higher than expected — not responding to usual interventions. Most common presentation.

Interdialytic Weight Gain

Higher than expected — sodium drives fluid retention between sessions.

Excessive Thirst

Patients report feeling extremely thirsty during and after dialysis.

Headache

Due to hypertension and fluid shifts during dialysis.

Intradialytic Hypertension

BP rises during dialysis — a paradoxical response to sodium load.

Worsening Heart Failure

Fluid overload from sodium retention worsens heart failure symptoms.

Who is Most Affected? High-Risk Patients

Hypertensive Patients

Most affected. Already have labile blood pressure. Sodium load pushes them into dangerous hypertension.

Patients with Heart Failure

Fluid overload → worsening heart failure symptoms. Can lead to pulmonary edema.

Sodium-Sensitive Patients

Unpredictable response to sodium load. BP may rise dramatically.

Diabetic Patients

Often have autonomic dysfunction. Unpredictable BP response.

Action Levels When to Act

Change in TDS	Interpretation	Action
0–2 mg/L	Normal fluctuation — acceptable	Monitor — continue routine checks
2–5 mg/L	Minor change — investigate	Investigate — check logs, check pre-treatment components (sand filter, softener, carbon tanks)
> 5 mg/L	Significant change — ALERT Indicates malfunction	Immediate assessment • Check RO membranes • Check sand filter • Check softener • Check incoming water TDS • Notify biomedical team
> 25 mg/L	Critical — ACTION REQUIRED Above ISO standard	Immediate action • Flush RO membranes • Consider membrane replacement • Check all components urgently • Do NOT use water for dialysis until resolved

Pattern Recognition — The Nurse's Alarm

Unexplained hypertension in MULTIPLE patients = SUSPECT WATER QUALITY.

This is the most important clinical clue that TDS may be rising. Even if the TDS number is still "acceptable" — multiple patients with hypertension is a red flag.

🧠 Think: "Why are so many patients hypertensive today?"
📊 Check: TDS logs for the past 3–5 days
🛑 Act: Notify biomedical team immediately
📋 Document: BP trends, TDS values, patient symptoms

The Nurse's Role Early Detection & Action

Check TDS Every Shift

Record the number. Compare to previous day. A 5+ increase is an ALARM — regardless of the absolute number.

Monitor BP Trends

Look for unexplained hypertension across multiple patients. Compare BP readings to previous sessions.

Compare Patients

If several patients have unexplained hypertension — suspect water quality.

Notify Biomedical Team

Report immediately if TDS is rising or hypertension is unexplained. Do NOT wait for the number to reach 25.

Document Everything

Record TDS values, BP trends, patient symptoms, and actions taken. Documentation is essential for tracking trends and preventing future incidents.

Ask the Right Questions

Ask patients: "Do you feel more thirsty than usual?" "Have you noticed any swelling?" "Is your BP higher at home?" Patients often notice changes before we do.

Case Example Real Scenario

✅ Normal TDS Trend

⚠️ TDS Rising — Hypertension Appears

Day	TDS (Post-RO)	Change	Patient BP Trends	Action
Monday	8 mg/L	—	Stable	Routine
Tuesday	9 mg/L	+1	Stable	Routine
Wednesday	10 mg/L	+1	Stable	Routine
Thursday	16 mg/L	+6	Several patients hypertensive	⚠️ ALERT — Investigate
Friday	22 mg/L	+6	Multiple patients hypertensive — not responding to treatment	🚨 CRITICAL — Flush RO membranes

What happened? The sand filter was not backwashed properly — suspended solids reached the RO membrane, causing it to foul. The softener resin was also exhausted — hardness reached the RO membrane and caused scaling. The TDS rose by 6 mg/L in one day — the nurse recognized the trend and acted.

Trust the Trend — Not Just the Number

TDS monitoring is not about hitting a target — it is about detecting change.

A 5+ increase in one day is more important than the absolute number. Unexplained hypertension in multiple patients is the clinical alarm.

The machine may say "PASS" — but the patient's blood pressure tells the truth.

When TDS rises — investigate. When patients are hypertensive — suspect water quality. When both happen — ACT.

✍️ Author: Ahmed Mohmad Rashyd Musleh Registered Staff Nurse