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Infectious Diseases

Updated 30 September 2020

Blood test could spot those at highest risk for severe Covid-19

A common blood marker called 'red cell distribution width' may predict how severe one's Covid-19 might become, new research shows.

  • A new study wanted to find a way to identify high-risk Covid patients as early and as easily as possible
  • Researchers quickly discovered that a blood marker called 'red cell distribution width' fit the bill
  • This means that the greater the variance in the size of a Covid-19 patient's red blood cells, the poorer their prognosis


If you're unfortunate enough to be admitted to the hospital with Covid-19, a common blood marker may predict how severe your illness might become, new research shows.

The blood marker is called "red cell distribution width" (RDW) – basically, the greater the variance in the size of red blood cells, the poorer a patient's prognosis, the study authors explained.

A Covid-19 patient's RDW test result "was highly correlated with patient mortality, and the correlation persisted when controlling for other identified risk factors like patient age, some other lab tests and some pre-existing illnesses," said study co-author Dr Jonathan Carlson, of Massachusetts General Hospital (MGH) in Boston.

The new study was published online on 23 September in JAMA Network Open and was led by Dr John Higgins, a pathologist investigator at the hospital and associate professor of systems biology at Harvard Medical School.

A quick discovery

"We wanted to help find ways to identify high-risk Covid patients as early and as easily as possible – who is likely to become severely ill and may benefit from aggressive interventions, and which hospitalised patients are likely to get worse most quickly," Higgins said in a hospital news release.

To do so, they looked at blood tests for more than 1 600 adults diagnosed with SARS-CoV-2 infection who'd been admitted to one of four Boston-area hospitals in March and April 2020.

Higgins and his team had expected that they might have to ferret out some obscure blood marker that might predict poor outcomes from Covid-19. But they quickly discovered that RDW – already used in standard blood tests – easily fit the bill.

In fact, patients whose RDW values were above the normal range when they were first admitted to the hospital had a risk of death that was 2.7 times that of patients whose test results were in the normal range, the researchers found. Overall, 31% of patients with above-normal RDW test results died, compared to 11% of those with normal RDW test results.

And if a patient's RDW rate was normal upon admission but then slowly began to rise to above-normal levels, that correlated with a rise in the patient's odds for death as well, the study found.

Pinpointing at-risk patients

The next step for the Boston team is to discover why a high RDW score is tied to worse outcomes. "Such discoveries could point to new treatment strategies or identify better markers of disease severity," said study co-author Dr Aaron Aguirre, an MGH cardiologist and critical care physician.

Dr Teresa Murray Amato is chair of emergency medicine at Long Island Jewish Forest Hills, in New York City. Reading over the new study, she said that "although we know that advanced age and certain [health factors] such as diabetes and hypertension are associated with worse outcomes, there is still great variability of symptoms and severity within those groups."

A blood marker like RDW that could pinpoint at-risk patients would be very useful, Amato said.

Already, "an elevated RDW can be an indication of overall poor health" in any hospital patient, she noted. Knowing soon after hospital admission that a patient is or is not at a high risk of death, "we will be better at tailoring treatment in a scientific way, in order to give our patients the best possible outcomes," Amato said.

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