Balanced Crystalloids or Saline? Reading the Big Fluid Trials Honestly

If you are choosing a resuscitation fluid for an adult who needs several liters of crystalloid, the honest bottom line is this: balanced solutions such as lactated Ringer's or Plasma-Lyte are a reasonable default, but the evidence behind that preference is modest rather than dramatic. The biggest randomized trials point, at most, to a small advantage for balanced fluids on kidney and survival endpoints, and the two most carefully blinded trials found no difference in death at all. The interesting part is not just the answer. It is what these four trials teach you about reading a "positive" result when the outcome is a bundle of events rather than a single one.

Key points#

• Saline delivers far more chloride than plasma normally carries, and a high chloride load has been tied to worse kidney blood flow in physiology studies.
• Two large pragmatic trials, SMART and SALT-ED, nudged a composite kidney outcome in favor of balanced fluids, with small absolute differences.
• Two large blinded trials, BaSICS and PLUS, found no mortality difference, so the enthusiasm for balanced fluids rests on the softer of the two designs.
• The signal is easy to overstate because the primary endpoint, MAKE30, packages death together with dialysis and a creatinine threshold.
• Balanced fluids are a sensible, inexpensive default at the population level; saline keeps clear roles in specific situations.

The chemistry behind the debate#

Blood plasma runs about 100 mmol of chloride per liter. So-called normal saline is neither normal nor plasma-like: it carries 154 mmol per liter of sodium and the same of chloride, both above physiologic values. Pour in enough of it and the extra chloride can drive a hyperchloremic metabolic acidosis. Laboratory and human physiology work has linked that chloride burden to constriction of the renal vessels and a drop in kidney blood flow. Balanced crystalloids swap some of the chloride for buffers such as lactate, acetate, or gluconate, so the bag looks more like the fluid the body already runs on. The clinical question is whether that closer chemical match actually buys fewer deaths, less kidney injury, or shorter stays. Physiology suggests it should. Trials are how we find out whether it does.

Four trials, two very different designs#

It helps to sort the landmark trials by how tightly they controlled bias, because that, more than any single result, explains why smart clinicians still disagree.

The pragmatic pair: SMART and SALT-ED#

Two 2018 trials from one academic center put numbers on the debate. SMART enrolled 15,802 critically ill adults across five intensive care units using a cluster design in which whole units switched between balanced fluid and saline month by month. Its primary endpoint was MAKE30, a composite of death, new dialysis, or persistent kidney dysfunction within 30 days. MAKE30 struck 14.3 percent of the balanced group and 15.4 percent of the saline group, an odds ratio of 0.91 (95 percent confidence interval 0.84 to 0.99). The companion SALT-ED trial applied the same monthly switch to 13,347 less acutely ill emergency-department patients. Its primary outcome, hospital-free days, was identical between groups, while the secondary MAKE30 figure favored balanced fluid, 4.7 versus 5.6 percent.

Both trials lean the same way, and both deserve to be taken seriously. But the effects are small. The absolute MAKE30 gap in SMART was roughly one percentage point, a number needed to treat near 94, with the upper edge of the interval sitting just short of 1.0.

The blinded pair: BaSICS and PLUS#

Then came larger trials designed to remove the bias that a calendar-based switch can let in. BaSICS, 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. Among patients with traumatic brain injury the signal, if anything, tilted toward saline, a useful reminder that a more hypotonic balanced fluid is a poor fit where cerebral swelling is a concern. The 2022 PLUS trial randomized about 5,000 patients across 53 ICUs in 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 separation in kidney function. Saline raised serum chloride and lowered pH as predicted, but those biochemical shifts did not translate into worse patient outcomes.

How to read a "positive" composite#

The two pairs do not really contradict each other. They mostly differ in method, and unpacking why is where the general lesson lives.

A composite is only as good as its parts. MAKE30 ties death to dialysis to a creatinine cutoff. Composites usually move on the back of their most common component, so a one-point shift can reflect the least patient-centered piece of the bundle. Whenever a composite reads as positive, the fair next question is which component actually moved, and whether that component matters to the person in the bed.

Intervals that graze 1.0 are fragile. SMART's interval reached 0.99. A result that brushes the null hangs on a handful of events; reclassify a few patients and significance can flip. That is not a flaw in the trial, but it is a reason not to treat a barely-significant p-value as settled biology.

Design earns trust. The pragmatic trials assigned fluid by the calendar rather than the individual patient. That is efficient and close to real-world practice, but it is more open to secular trends and hidden differences than the patient-level blinded randomization in BaSICS and PLUS. When a big blinded trial and a big pragmatic trial disagree on effect size, the blinded design generally deserves the greater weight in your reading.

Pooling narrows, it does not erase. Meta-analyses that combine these trials suggest a high probability of a small benefit from balanced fluids, strongest for kidney outcomes and in sepsis, while the credible range still touches no effect. A probable modest advantage is the honest summary, not a proven large one.

What it means at the bedside#

For most patients receiving several liters of crystalloid, balanced fluids are a defensible default: cheap, physiologic, and unlikely to produce the hyperchloremia saline can. Saline keeps real roles, including hypochloremic or hyponatremic states, some neurologic patients, and any case where a specific electrolyte target drives the plan. What the trials do not license is the claim that switching bags will visibly change survival for the patient you are treating. They support a low-cost, low-risk preference across a population, held with humility about the size of the effect.

The wider point outlasts the fluid question. A significant composite from a single pragmatic center is a starting hypothesis, not a verdict, and the habit of asking which component moved, how wide the interval runs, and whether blinded trials agree is exactly what separates reading evidence from reading headlines.