The sister studies -- one on patients in the intensive care unit
and one on adults who were not critically ill
-- showed that "balanced crystalloids," solutions that are more similar to what's in human blood, were less likely to result in major kidney problems in the month after they were enrolled in the study.
ICU patients on these fluids in the hospital also died at lower rates within the 30 days. The study authors said that making the switch could save tens of thousands of lives among the millions who get these fluids in the hospital every year.
"Saline has been used in practice for over a century," said study author Dr. Todd W. Rice
, director of the medical intensive care unit at Vanderbilt University Medical Center and an associate professor of medicine in its Division of Allergy, Pulmonary, and Critical Care Medicine.
"We actually heard from a number of people that there couldn't have been much difference between these two (intravenous fluids), because we would have already known it if there was."
Of nearly 16,000 critically ill patients, 10.3% of those given balanced crystalloids and 11.1% of those given saline died in the hospital within a month. Serious kidney events arose in 14.3% of the "balanced" group and 15.4% in the saline group.
On the other hand, the study of more than 13,000 patients who were not critically ill found that there was no difference in how early surviving patients were discharged in the four weeks after they came to the emergency room. But the researchers did find a slightly lower incidence of serious kidney events: 4.7% for the "balanced" group versus 5.6% for those receiving saline. This means 111 saline patients would need to be treated with balanced crystalloids instead in order to prevent one adverse kidney event.
Though these numbers may be substantial when looking at the country as a whole, Rice said it's not a reason to worry for an individual who finds themselves attached to a saline drip.
"For any one patient, the risk is pretty small," he said.
IV bags can contain any number of ingredients, such as fluids, sugars and medications. But many patients are dehydrated, and that's where saline and "lactated ringers," the most common balanced crystalloid, come in handy.
"We've tried to do a lot of history-looking to figure out why saline became the default," Rice said. "It's not entirely clear."
Neither IV fluid tends to cost more than the other, and both have been widely available for decades, the researchers said.
These fluids are meant to mirror salt concentrations in human blood, keeping water where it needs to be in order to better hydrate the body, according to Rice. (This is partly why chicken soup is often recommended for those who are sick, experts say.)
Saline consists of sodium chloride -- the main ingredient in common table salt -- dissolved in water. Lactated ringers contain several additional ions, such as calcium and potassium.
Research has suggested that a higher chloride concentration in saline can affect kidney function
and lead to metabolic problems. However, a couple of smaller studies
recently failed to show a difference
between the two fluids, Rice said.
"This study makes me think again about what choice of fluids I'll use now," said Dr. David Hager
, associate director of the medical ICU at the Johns Hopkins Hospital. Hager was not involved in the new studies.
Hager said he believes that other studies will spin off from this research, which was conducted at a single medical center. He also noted that the research was unblinded, meaning that doctors "knew they were giving one fluid versus another, and that may have an impact on other decisions that they make."
Patients who were not critically ill -- who received IV fluids in the emergency room -- did not necessarily receive the same fluid after being admitted to the hospital, the researchers said.
"That was actually quite a surprise to us," Rice said, and this could be because some patients receive the majority of these IV fluids in the emergency department.
It could also have something to do with being more vulnerable in the early stages of an illness, he said.
Doctors may still opt for saline if, for example, a patient has low concentrations of sodium or chloride, Rice said. Many of his colleagues who treat brain injuries are also wary of using lactated ringers over saline because of its potential impact on the pressure within the skull, he added.
"They get very very worried about increased swelling," Rice said. More research could uncover which types of patients are most sensitive to the effect of one fluid over another, he said.
Though Vanderbilt's hospital system has made changes to prioritize the balanced crystalloid, Rice said it could be a long time before this becomes standard practice.
"Change is slow in medicine -- slower than any of us believe it should be," he said. "We're trying to get the message out."