Balanced Fluids Versus Saline: What the Big Trials Show

The largest randomized trials comparing balanced crystalloids with 0.9% saline show, at most, a small kidney and survival advantage for balanced fluids, and the two most rigorous blinded trials found no difference in death. In critically ill and emergency-department patients, balanced solutions such as lactated Ringer's or Plasma-Lyte are a defensible default, but the case rests on modest, sometimes fragile composite-outcome signals rather than a dramatic effect. Reading these trials well means understanding how composite endpoints, cluster designs, and confidence intervals shape what the word "positive" actually means.

Why the choice of fluid is even a question

Physiologic plasma carries roughly 100 mmol of chloride per liter. So-called normal saline carries 154 mmol per liter of both sodium and chloride, well above physiologic levels. Large saline volumes can produce a hyperchloremic metabolic acidosis, and animal and human physiology studies have linked high chloride loads to renal vasoconstriction and reduced kidney blood flow. Balanced crystalloids substitute part of the chloride with buffers such as lactate, acetate, or gluconate, bringing the electrolyte profile closer to plasma. The clinical question is whether that closer match translates into fewer deaths, less kidney injury, or shorter hospital stays.

The Vanderbilt pair: SMART and SALT-ED

Two 2018 trials from a single academic center put numbers to the debate. The SMART trial enrolled 15,802 critically ill adults across five intensive care units and used a cluster-randomized, multiple-crossover design, in which whole units alternated between balanced fluids and saline by calendar month. Its primary endpoint was MAKE30, a composite of death, new renal-replacement therapy, or persistent renal dysfunction within 30 days. MAKE30 occurred in 14.3 percent of the balanced-crystalloid group and 15.4 percent of the saline group, a marginal odds ratio of 0.91 with a 95 percent confidence interval of 0.84 to 0.99. The companion SALT-ED trial applied the same monthly crossover to 13,347 non-critically ill emergency-department patients. Its primary outcome, hospital-free days, was identical between groups (median 25 days each), while the secondary MAKE30 endpoint favored balanced fluids at 4.7 versus 5.6 percent.

Both results point in the same direction, and both are worth taking seriously. But the effects are small. The absolute MAKE30 difference in SMART was about one percentage point, corresponding to a number needed to treat near 94, and the upper confidence bound sat just under 1.0.

The blinded multicenter trials: BaSICS and PLUS

Larger, double-blind trials followed and complicate the story. The BaSICS trial, reported in 2021 across 75 Brazilian ICUs, randomized critically ill patients to Plasma-Lyte 148 or saline and found no significant difference in 90-day mortality. A subgroup of patients with traumatic brain injury showed a signal that, if anything, favored saline, a reminder that hypotonic balanced fluids can be a concern where cerebral edema is in play. The 2022 PLUS trial randomized roughly 5,000 patients in 53 ICUs across Australia and New Zealand under blinded conditions. At 90 days, mortality was 21.8 percent with balanced solution and 22.0 percent with saline, with no meaningful effect on kidney function. Saline predictably raised serum chloride and lowered pH, but those biochemical changes did not convert into worse patient outcomes.

How to read a small composite difference

The gap between the two pairs of trials is instructive, and it is mostly about method rather than contradiction.

Composite endpoints hide their drivers

MAKE30 bundles death with dialysis and a creatinine threshold. A composite is usually moved by its most frequent component, and a one-point shift in the composite can reflect changes in the least patient-centered part of it. When a composite is positive, the honest next question is which component moved and whether that component matters to patients.

Confidence intervals near 1.0 are fragile

SMART's interval reached 0.99. Results that brush against the null are sensitive to a handful of events; reclassifying a few patients can flip statistical significance. That fragility is not fraud, but it is a caution against treating a barely-significant p-value as a settled biological truth.

Design shapes credibility

The Vanderbilt trials were pragmatic and unblinded, with treatment assigned by the calendar rather than by the individual patient. That approach is efficient and real-world, but it is more vulnerable to secular trends and unmeasured differences than the patient-level, blinded randomization used in BaSICS and PLUS. When a large blinded trial and a large pragmatic trial disagree, the blinded design generally deserves more weight on the specific question of effect size.

Meta-analysis narrows, it does not erase

Pooled analyses that combine these trials suggest a high probability of a small benefit from balanced fluids, particularly for kidney outcomes and in sepsis, while the credible range still includes no effect. The reasonable summary is a probable modest advantage, not a proven large one.

What this means at the bedside

For most critically ill and general medical patients receiving several liters of crystalloid, balanced fluids are a sensible default, inexpensive, and unlikely to cause the hyperchloremia that saline can. Saline retains clear roles, including hypochloremic or hyponatremic states, certain neurologic patients, and situations where a specific electrolyte target matters. The trials do not license the claim that switching fluids will noticeably change survival for an individual patient. They support a low-cost, low-risk preference at the population level, held with appropriate humility about the size of the effect. This article is educational and not a substitute for individualized medical advice.

The larger lesson travels beyond fluids. A statistically significant composite outcome from a single pragmatic center is a starting hypothesis, not a verdict, and the discipline of asking which component moved, how wide the interval is, and whether blinded trials agree is what separates evidence literacy from headline reading.