The analysis revealed that patients with red cell distribution width, or RDW, above the normal range were nearly three times as likely to die from the disease, the data showed.
Thirty-one percent of COVID-19 patients with above-normal RDW values died from the disease, while the mortality rate for those with normal levels was 11%, the researchers said. The normal range for RDW is 12.2% to 16.1% in adult women and 11.8% to 14.5% in adult men.
"We found that a single test that's routinely done as part of a complete blood count is associated with a significant elevation in mortality risk from COVID-19," study co-author Dr. John M. Higgins, a pathologist at Massachusetts General Hospital, told UPI.
The measure is taken as part of a standard complete blood count at virtually every hospital, clinic and medical practice.
If doctors pay attention to the measure, researchers say, they could use it to identify patients with greater risk for more severe disease -- and be ready to treat worse reactions to COVID-19, if not prevent them.
In severely ill patients, COVID-19 causes a "really intense inflammatory response that's going to leave all sorts of signals that it's going on -- and this test is picking up one of those signals," said Higgins, who also is an associate professor of systems biology at Harvard Medical School.
Measuring RDW would not replace the tests used to diagnose COVID-19 or the blood antibody screenings that identify people who were sick and recovered, Higgins and his colleagues said.
Instead, it could serve as a way to help hospitals triage patients who may need treatment in the intensive care unit and ventilator support, they said.
It could also lead clinicians to start patients on corticosteroids earlier -- based on their risk for severe illness. Corticosteroids can help some patients with severe COVID-19, research suggests.
For this study, Higgins and his colleagues, analyzed blood test findings on 1,641 patients with the virus who were admitted to four hospitals in the Boston area between March 4 and April 28.
Those with RDW values above the normal range when they were admitted to the hospital were 2.7 times more likely to die than those with values in the normal range, the data showed.
Thirty-one percent of COVID-19 patients with above-normal RDW values died from the disease, while the mortality rate for those with normal levels was 11%, the researchers said.
Older patients with elevated RDW values were at highest risk for death from COVID-19, the data showed. For example, 46% of patients 80 years old and older who had elevated RDW died from the disease -- compared to 29% of those with normal levels.
In addition, an increase in RDW after hospital admission was associated with an even higher risk of dying, suggesting that RDW could be tracked during hospitalization to help determine whether patients are responding to treatment or getting worse, they said.
The mortality rate for those whose RDW values increased from normal to elevated levels while in the hospital was 24%, while patients who were admitted with elevated levels that continued to go up had a mortality rate of 40%, the researchers said.
The researchers still aren't sure why RDW values are elevated in some COVID-19 patients and not in others.
They think, however, that the link between RDW and death in those with the disease is compelling enough for clinicians to begin "paying attention" to it as a sign of severe illness, Higgins said.
"The good news is that this is a test that is most likely already being done in most hospitals," he said. "We hope [our findings] will lead more clinicians to pay attention to RDW in COVID-19 patients, and use [the measure] as a way to identify patients they need to be most worried about."