We're still learning about all the terrible things the novel coronavirus does to human bodies. Doctors are unsure of the best way to treat it.

In the sickest patients who have COVID-19, the disease caused by the coronavirus, the disease affects more than just the lungs.

Doctors have reported kidney problems, heart problems, and an unusually high rate of blood clots in severely ill patients.

Doctors aren't sure how to treat the blood clots. Some are proposing heavy doses of blood-thinning medications, while others are exploring clot-busting drugs otherwise used to treat strokes.

Trials to evaluate whether those options help patients recover are ongoing, but results are still months away, leading doctors to make decisions on their own.

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Dr. Alex Spyropoulos is focused on one big question when it comes to patients who are severely ill with COVID-19, the disease caused by the novel coronavirus.

The question: If doctors give therapeutic doses of the blood-thinner heparin, do those doctors save the lives of COVID-19 patients?

Spyropoulos is an expert on blood clots at Northwell Health in New York, and he said that's the single most important question he's come across in the time he's spent in his field.

As the coronavirus pandemic has spread, infecting millions globally and killing hundreds of thousands, doctors have been forced to rapidly figure out how to treat it without a vaccine or medicines that cure it. They're still not sure of the best way to get oxygen into the damaged lungs of the most severely ill patients, for example.

COVID-19 spread globally Confirmed Cases Deaths Global United States Last updated using data from JHU CSSE

And now, they're learning that the respiratory disease appears to be impacting more than just the lungs, particularly in severe cases. Doctors have reported kidney issues, heart problems, and more recently, issues with blood clots appearing in different parts of the body. Not helping the matter, researchers have also reported complications that might make it harder for people to form blood clots in other hospitalized COVID-19 patients.

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Typically, doctors like Spyropoulos prefer to rely on results from robust clinical trials to figure out whether treatments are effective. But because the virus is so new, trials are just getting started, with results expected in the coming months.

"We're driving blindly based on what we call very weak but very compelling data," Spyropoulos said.

In the absence of sturdy scientific evidence backing up whether one approach or another is best, doctors are debating the best way to treat the disease. Studies are ongoing looking into the answer to Spyropoulos's question, as well as whether clot-busting drugs — typically used to treat strokes — are a better approach.

Read more: There are more than 70 potential coronavirus vaccines in the works. Here are the top efforts to watch, including the 16 vaccines set to be tested in people this year.

'Remarkably high' incidence of clotting in COVID-19 patients

Early on in the coronavirus outbreak, doctors in the US were hearing from colleagues in China about the virus' effects on the heart, Dr. Thomas Maddox, who serves as chair of the science and quality committee of the American College of Cardiology, told Business Insider.

The effects seemed to be related to both a viral invasion of the heart and to the stress of fighting the respiratory disease.

With COVID-19, doctors are also seeing blood clots occurring in the lungs and bodies of patients on ventilators, Dr. Greg Martin, a professor of pulmonary critical care at Emory University and president-elect of the Society of Critical Care Medicine, told Business Insider.

Clots can wreak havoc on the body, traveling and blocking up veins, leading to strokes, heart attacks, kidney problems, lung problems, and more.

Blood clots aren't uncommon in severely ill patients in intensive care units, or even in hospitalizations. While in the ICU, patients are typically on breathing support and sedated, which limits their ability to move as they heal. Staying still raises the risk of clots.

Read more: The US is sprinting to develop a coronavirus vaccine or treatment. Here's how 19 top drugmakers are racing to tackle the pandemic.

Northwell Health's Dr. Alex Spyropoulos with a colleague. Northwell Health

Other viruses also raise the risk of clots

But doctors have a hunch that the virus might play a role in increasing the risk of clots, too.

"We think it's one of the most important, if not the most important, issue with respect to monitoring sick hospitalized COVID-19 patients," Spyropoulos said.

It wouldn't be the first time, he said. Doctors noted an elevated risk of clotting in patients with H1N1, or swine flu, as well as in the SARS outbreak, another coronavirus, in the early 2000s.

With the novel coronavirus, more clots seem to be originating in the lungs, Spyropoulos said.

In a recent observational study, Dutch researchers looked at 184 patients in the intensive care unit with coronavirus. About a third of them had a complication associated with a clot, such as clots in the lungs, clots in the legs, stroke, heart attack, or other clots cutting off blood to other parts of the body.

All were on preventive treatment intended to prevent clotting in the body.

That's a higher rate of problems than might typically be seen in patients in intensive care units, Spyropoulos said. Typically, he said, clotting rates in ICUs for different conditions are between 5-10%. The researchers called it "remarkably high," recommending increasing doses of the blood-thinning medication.

To figure out the best way to treat patients, researchers are evaluating two options:

Heparin, a blood thinner that prevents clotting complications in patients.

Clot-busting drugs, typically used to treat strokes.

Making decisions in the absence of evidence

For now, most of the evidence doctors are using isn't much better than anecdotes. The ACC is currently reviewing recommendations on how to take care of the clotting complications seen in COVID-19 patients, but heart doctors won't have much evidence to guide them until they get clinical trial results, likely, in the second half of this year.

"Our journals are inundated with essentially opinion pieces," Maddox said.

To prevent clotting complications in COVID-19, Northwell's Spyropoulos has been relying on his clinical experience to make decisions.

"If you have a deep experience in this field you can make reasonable assumptions and extrapolations and observations from indirect data," Spyropoulos said.

For the past month — partially while out sick with COVID-19 himself — he been working with the health system to adjust guidelines, stepping up preventive dosing of blood thinners and making sure doctors are more sensitive to signals that might indicate clotting, such as swelling in the legs of patients.

The health system has also been keeping tabs on signs of clotting once patients leave the hospital and prescribing blood thinners when patients are discharged.

Clotting complications outside the hospital in COVID-19 patients

For now, most of the conversation is about what to do with blood clot complications in hospitalized patients.

But, Spyropoulos said, there are two additional questions he's keeping his eye on.

One is whether clotting complications are happening in milder COVID-19 cases that don't require a hospital stay. That's a question that's far from being answered, particularly given the lack of routine testing available.

The other question is whether blood clots are causing problems after COVID-19 patients leave the hospital, leading to sudden deaths. Spyropoulos said he's concerned about complications arising after patients are discharged. In early autopsy data from Northwell, there appear to be major clotting events like a massive heart attack or lung clots in 40% of patients who have died after leaving the hospital.