In a patient with acute kidney injury, which crystalloid solution is preferred: normal saline (0.9% sodium chloride) or lactated Ringer's solution?

Lactated Ringer's Solution is Preferred Over Normal Saline for Acute Kidney Injury

For patients with acute kidney injury, lactated Ringer's solution (or other balanced crystalloids) should be used preferentially over normal saline for fluid resuscitation. The most recent and highest quality evidence demonstrates that balanced crystalloids reduce major adverse kidney events and mortality compared to normal saline 1.

Evidence-Based Rationale

Primary Guideline Recommendations

The 2022 French guidelines for critically ill patients provide a GRADE 2+ recommendation to use balanced crystalloids rather than 0.9% NaCl as first-line fluid therapy to reduce mortality and/or adverse renal events 2. While this recommendation was made in the context of hemorrhagic shock, the underlying mechanism—that high-volume chloride-rich solutions have potentially deleterious effects on renal function and survival—applies directly to AKI patients 2.

The 2014 KDIGO guidelines recommend isotonic crystalloids over colloids for AKI prevention and management, though they don't specify between balanced and unbalanced crystalloids 3. However, the guideline acknowledges the emerging debate about crystalloid choice and notes that normal saline's high chloride content (153 mEq/L) induces hyperchloremic metabolic acidosis 3.

Landmark Clinical Trial Evidence

The SMART trial (2018) provides the strongest direct evidence: among 15,802 ICU patients randomized to balanced crystalloids (lactated Ringer's or Plasma-Lyte) versus saline, those receiving balanced crystalloids had:

• Lower rate of major adverse kidney events (MAKE30): 14.3% vs 15.4% (OR 0.91,95% CI 0.84-0.99, p=0.04) 1
• MAKE30 is a composite of death, new dialysis, or persistent renal dysfunction—the exact outcomes that matter most
• Trend toward lower 30-day mortality: 10.3% vs 11.1% (p=0.06) 1

Supporting Evidence Across Clinical Contexts

In sepsis-induced hypotension (a common cause of AKI), a 2025 secondary analysis of the CLOVERS trial found that lactated Ringer's compared to saline resulted in:

• Reduced mortality: 12.2% vs 15.9% (adjusted HR 0.71,95% CI 0.51-0.99, p=0.043) 4
• More hospital-free days at 28 days 4

In critically ill patients, a 2016 retrospective analysis of over 10,000 patients showed higher percentages of lactated Ringer's were associated with:

• Lower mortality (effect increased with total fluid volume)
• Less acute kidney injury from days 3-7 5
• For patients receiving >7L of fluid, the mortality benefit was substantial (OR 0.50 for 75% vs 25% lactated Ringer's) 5

Physiologic Mechanisms

Why Normal Saline is Problematic in AKI:

• Hyperchloremic metabolic acidosis: Saline contains supraphysiologic chloride (154 mEq/L vs plasma 98-106 mEq/L), worsening acidosis in already compromised kidneys 3, 6
• Reduced renal blood flow: Normal saline decreases renal perfusion 7
• Progressive chloride accumulation: Patients receiving saline show progressively greater plasma chloride concentrations across all volume categories 7

Why Balanced Crystalloids are Superior:

• Physiologic electrolyte composition: More closely matches plasma 6
• Metabolic alkalinizing effect: Lactate is metabolized to bicarbonate, consuming hydrogen ions and improving acid-base balance 8, 6
• Better acid-base profile: Studies consistently show better pH improvement and anion gap correction with lactated Ringer's 6

Practical Implementation

First-Line Choice:

• Use lactated Ringer's solution or Plasma-Lyte for all fluid resuscitation in AKI patients
• This applies whether AKI is prerenal, sepsis-related, or from other causes

Volume Considerations:

• The benefit of balanced crystalloids increases with larger volumes 5
• This is particularly important in AKI where substantial volume resuscitation may be needed
• Even modest volumes (1-2L) show benefit, but the effect is most pronounced with >4L 5

Specific Contraindications to Lactated Ringer's:

Per FDA labeling, avoid or use with extreme caution in 8:

• Severe hyperkalemia (lactated Ringer's contains 4 mEq/L potassium)
• Severe metabolic alkalosis (lactate metabolism produces bicarbonate)
• Severe hepatic failure (impaired lactate metabolism)
• Lactic acidosis (not indicated for treatment)

When Normal Saline May Be Acceptable:

• Concurrent severe hyperkalemia (K+ >6.5 mEq/L)
• Severe alkalosis (pH >7.55)
• Traumatic brain injury with concern for cerebral edema (though evidence is mixed)
• Simultaneous blood transfusion through same line (citrate interaction risk) 8

Common Pitfalls to Avoid

1. Don't assume "normal" saline is physiologically normal—it's actually supraphysiologic in chloride content and causes predictable metabolic derangements

2. Don't worry about lactate levels from lactated Ringer's—the lactate in the solution is rapidly metabolized and does not significantly elevate serum lactate or interfere with lactate monitoring

3. Don't use colloids (albumin, starches) as first-line—guidelines clearly recommend crystalloids over colloids for AKI 3

4. Don't mix crystalloid types unnecessarily—pick one and use it consistently to maximize benefit 5

5. Monitor potassium closely when using lactated Ringer's in patients with baseline renal impairment, though the 4 mEq/L potassium content is similar to plasma 8

Clinical Bottom Line

Lactated Ringer's solution should be the default crystalloid for acute kidney injury based on consistent evidence showing reduced major adverse kidney events and mortality compared to normal saline. The only exceptions are specific contraindications like severe hyperkalemia or alkalosis. The evidence is strongest and most recent from the SMART trial 1, supported by multiple other studies across different clinical contexts 5, 4, and endorsed by current guidelines 2.