In the shadow of a pandemic that confounded prediction, researchers at Massachusetts General Hospital found an unlikely ally in an old and familiar tool: a routine blood test, long used to detect anaemia and heart disease, that quietly revealed which COVID-19 patients were most likely to die. The red cell distribution width, or RDW, proved nearly three times more predictive of mortality than chance alone — and held its power even when age and underlying conditions were accounted for. Science, in this moment, did not arrive with something new, but with a deeper reading of something already in h
Blood test could predict Covid-19 mortality risk, study finds
Elevated RDW values correlate with increased mortality risk in COVID-19 patients, indicating potential for identifying those at higher risk of death.
Nearly three times as likely to die
Mark
Why would a test that measures red blood cell size tell you anything about who's going to die from a virus?
Mimi
That's the question the researchers were asking too. RDW doesn't directly measure viral load or immune response. But severe Covid seems to cause changes in the blood itself—inflammation, clotting, cellular stress. RDW might be capturing that cascade.
Mark
So it's not that the test is predicting death. It's that it's detecting something happening in the body that leads to death.
Mimi
Exactly. It's a marker of what's already in motion. The virus damages the lungs, the immune system overreacts, the blood gets stressed. RDW rises. And if it keeps rising after you're in the hospital, you're in trouble.
Mark
Could this have changed how doctors treated people in 2020?
Mimi
Potentially. If you could flag high-risk patients on day one or two, you could escalate care earlier—oxygen, ventilators, experimental drugs. Instead of waiting to see who got worse, you'd know.
Mark
And they still don't know why it happens?
Mimi
Not yet. That's the work ahead. Understanding the mechanism might lead to treatments that actually prevent the RDW from rising in the first place.
O Pulso
- Hospitals overwhelmed by COVID-19 in spring 2020 had no reliable early tool to separate patients who would survive from those who would not — a gap that cost critical hours and lives.
- A team at Massachusetts General Hospital discovered that RDW, a standard blood marker already in clinical use, correlated so strongly with COVID-19 death that patients with elevated values were nearly three times more likely to die.
- The finding resisted easy dismissal: even after controlling for age, pre-existing conditions, and other lab results, RDW's predictive power remained intact — and worsened when values rose after hospital admission.
- Clinicians now see a practical path: routine RDW monitoring during hospitalisation could flag deteriorating patients early, allowing aggressive intervention before the window for effective treatment closes.
- The deeper question — why severe COVID-19 causes red blood cell variation to increase — remains unanswered, and researchers are now pursuing that biological mechanism as the key to new treatments.
In the shadow of a pandemic that confounded prediction, researchers at Massachusetts General Hospital found an unlikely ally in an old and familiar tool: a routine blood test, long used to detect anaemia and heart disease, that quietly revealed which COVID-19 patients were most likely to die. The red cell distribution width, or RDW, proved nearly three times more predictive of mortality than chance alone — and held its power even when age and underlying conditions were accounted for. Science, in this moment, did not arrive with something new, but with a deeper reading of something already in hand.
In the spring of 2020, as Boston-area hospitals filled with COVID-19 patients, researchers at Massachusetts General Hospital turned to a question both simple and urgent: could an ordinary blood test predict who would die?
The test they examined was the red cell distribution width, or RDW — a long-established marker used to diagnose anaemia, heart disease, and blood disorders. It measures variation in the size of red blood cells, and pathologists have relied on it for years. But when the Massachusetts General team reviewed records from 1,641 hospitalised COVID-19 adults, they found that RDW values tracked closely with mortality — a pattern that held even after accounting for age, pre-existing conditions, and other laboratory results.
Patients with elevated RDW were nearly three times as likely to die as those with normal values. More telling still, patients whose RDW rose after admission faced even greater risk — suggesting the test could serve not just as an initial screen, but as an ongoing signal of whether a patient was improving or declining.
Dr. John M. Higgins and colleagues framed the clinical stakes plainly: identify high-risk patients early, before deterioration sets in, and direct the most aggressive care toward those who need it most. The findings, published in JAMA Network Open, pointed toward a more disciplined triage — and potentially toward new ways of measuring and treating disease severity.
What remains unresolved is the biological question beneath the data: why does severe COVID-19 cause red blood cell variation to increase at all? That mechanism is now the focus of the next phase of research — one that may, in time, open new doors to treatment.
In the spring of 2020, as hospitals in the Boston area filled with Covid-19 patients, researchers at Massachusetts General Hospital began asking a straightforward question: could a simple blood test tell them which patients were most likely to die?
The test they focused on was not new. The red cell distribution width, or RDW, has been used for years to help diagnose anaemia, heart disease, blood disorders, and diabetes. It measures the variation in size and volume of red blood cells circulating in the bloodstream—a routine marker that pathologists have long understood. But when the Massachusetts General team analyzed medical records from 1,641 adults hospitalized with Covid-19 between early March and late April 2020, they found something that surprised them: RDW values correlated strongly with who lived and who died.
The correlation held even when researchers accounted for the factors doctors already knew mattered—patient age, other laboratory results, existing medical conditions. A patient with an elevated RDW was nearly three times as likely to die from Covid-19 as a patient whose RDW fell within normal range. More striking still, if a patient's RDW rose after admission to the hospital, the risk climbed higher.
Dr. Jonathan Carlson, one of the study's authors, described the persistence of this pattern across different patient populations and risk profiles. His colleague Dr. John M. Higgins, a pathology investigator, framed the practical stakes: the goal was to spot high-risk patients early, before they deteriorated, so that doctors could offer aggressive treatment to those most likely to benefit. Regular monitoring of RDW during a patient's hospital stay could signal whether treatment was working or whether the patient was declining.
The findings, published in JAMA Network Open, suggested a path forward. If RDW could reliably identify severe cases, it might point toward new ways to treat Covid-19 or at least new markers to measure disease severity. Dr. Aaron Aguirre, a cardiologist and critical care physician at the hospital, noted that such discoveries could reshape how clinicians approached the disease.
The next phase of the work was already underway: understanding why RDW elevation occurred in severe Covid-19 cases in the first place. That biological mechanism—the chain of events in the body that caused red blood cell variation to increase—remained a mystery. But solving it might unlock new treatment strategies, or at minimum, give doctors a clearer window into which patients needed the most urgent intervention.
Citações Notáveis
The correlation persisted when controlling for other identified risk factors like patient age, some other lab tests, and some pre-existing illnesses.— Dr. Jonathan Carlson, Massachusetts General Hospital
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.— Dr. John M. Higgins, Massachusetts General Hospital
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