Treating coronavirus patients with blood thinners could help boost their prospects for survival, according to preliminary findings from physicians at New York City’s largest hospital system that offer another clue about treating the deadly condition.

The results of an analysis of 2,733 patients, published Wednesday in the Journal of the American College of Cardiology, are part of a growing body of information about what has worked and what has not during a desperate few months in which doctors have tried dozens of treatments to save those dying of COVID-19, the disease caused by the novel coronavirus.

Valentin Fuster, a physician in chief at Mount Sinai Hospital and one of the study’s authors, said in an interview that the observations are based only on a review of medical records and that more rigorous, randomized studies are needed to draw broader conclusions, but that the results are promising.

“My opinion is cautious, but I must tell you I think this is going to help,” he said. “This is the opening of the door for what drugs to use and what questions to answer.”

Since March, when the pandemic hit Europe and the United States, doctors have been reporting mysterious blood clots, which can be gel-like or even semisolid, in a significant subset of coronavirus patients. Autopsies of patients who died of respiratory arrest have shown that some had unusual microclots in their lungs rather than the typical damage expected. And last month, doctors reported in the New England Journal of Medicine on five unusual cases of COVID-positive people in their 30 and 40s experiencing large strokes.

The Mount Sinai study focused on hospitalized patients treated at its five branches from March 14 through April 11. Among patients who were not on ventilators, those treated with blood thinners died at similar rates to those who did not get blood thinners. But they lived longer – a median of 21 days compared to 14 days.

For patients on ventilators, the difference was more significant. About 63 percent of patients who did not receive the medications died compared with 29 percent who received the treatment.

Another critical finding of the study is that giving blood thinners to these patients appears to be relatively safe. There was not a significant difference in the most dangerous side effect of anticoagulants – bleeding – in those who were on the drugs vs. those who were not.

As a result of the analysis, Fuster said, the hospital system changed its treatment protocols several days ago to begin giving patients with COVID-19 higher doses of blood thinners.

Deepak Bhatt, a professor at Harvard Medical School who specializes in interventional cardiology, called the paper “a very important study” with the blood issues in COVID-19 patients having evolved from just a suspicion to a well-recognized complication of the virus. “What we are figuring now is what do we do now that we know” in terms of treatments, he said.

Thomas Wakefield, head of vascular surgery for Michigan Medicine, said the anticoagulant heparin, which was used in some patients, probably has “two mechanisms among others that contribute to the good results seen in the study.” He said some data suggest heparin may interfere with entry of the virus into cells through spike proteins, and that heparin may also be able to decrease the inflammatory effects of the “cytokine storm” in patients with severe infection.

Doctors caring for the sickest coronavirus patients confront a limited arsenal of treatments. On May 1, the Food and Drug Administration issued an emergency use authorization for the antiviral drug remdesivir in patients who are hospitalized and seriously ill.

But trials of other treatments, including those involving hydroxychloroquine, a malaria drug touted by President Trump, have been stopped because of a lack of efficacy and concerns about toxicity. In late April, scientists reported that an arthritis drug made by Regeneron and Sanofi that had drawn enthusiasm from investors early on produced disappointing results in clinical trials.

Anticoagulants – first discovered more than 100 years ago and available in pills, injections and IVs – have been a key building block of treatment plans for weeks at many medical centers, prompted by discoveries about how the virus attacks the human body and an increasing recognition of blood-related complications in coronavirus patients.

A number of medical societies, including the International Society on Thrombosis and Haemostasis and the American Society of Hematology, have put out guidance recommending use of blood thinners for some COVID-19 patients, but the advice has taken a conservative approach. “It’s a delicate balance between clotting and bleeding, especially when patients are as sick as some of the ones who have COVID-19,” said Geoffrey Barnes, an assistant professor at the University of Michigan who works in cardiovascular medicine.

“A week ago, we were making some educated guesses on how to prevent blood clots,” he said. “This is the first time we have seen data that says higher doses may possibly be effective and safe.”

Mitchell Elkind, a professor of neurology at Columbia University College of Physicians and Surgeons and president-elect of the American Heart Association, agreed the Mount Sinai study was “encouraging” but urged caution. He said there may be other explanations for why the treated group had better outcomes unrelated to the blood thinning drugs.

Elkind said “people are rushing to get answers for the good reason that we are in the midst of a crisis, but we need to make sure we are not jumping ahead of the evidence.”

Fuster said Mount Sinai is beginning a trial this week that will include 5,000 patients who will be randomized into treatment groups to try to get more definitive information. Many unknowns remain about the blood thinners, including the best dosage and timing, and whether patients with COVID-19 who are not sick enough to be hospitalized and are dealing with their illness at home would benefit from taking the medication.