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Instalab ResearchRDW stands for red cell distribution width. It measures how much variation exists in the size of your red blood cells. Normal values typically fall between 11% and 15%, depending on the lab.
When your red blood cells are mostly uniform in size, your RDW stays low. When you have a mix of unusually small and large cells, your RDW rises. This variation happens when something disrupts how your body makes or maintains red blood cells.
Common causes of elevated RDW include:
The research shows that RDW behaves like a general "stress signal" for your body. It doesn't point to any single disease, but it reflects whether something is off with your red blood cell production, inflammation levels, or nutritional status.
The research doesn't identify a single threshold where RDW becomes "dangerous." Instead, risk increases on a sliding scale. Here's what studies have found across different populations:
14-15%: Values in this range already correlate with higher long-term risk in population studies, even in people who seem otherwise healthy. One large study found that each 1% increase in RDW was associated with about 14% higher all-cause mortality in older adults.
15% and above: This is where many studies start to see clearly elevated risk. In hospitalized patients, those with RDW at or above 15% had roughly double the 30-day mortality compared to those below 15%. In patients at nutritional risk, high RDW (15% or above) strongly predicted worse outcomes.
Very high values (17% or above): In one large community study, people with RDW at or above 17% had 3-4 times the mortality risk compared to those below 13%.
Some disease-specific thresholds from the research:
Not necessarily. RDW is a marker, not a disease itself. An elevated reading signals that something deserves investigation, not that you're in immediate danger.
The research consistently shows that higher RDW is associated with worse outcomes across many conditions, including heart failure, liver disease, COPD, various cancers, and kidney disease. In people with non-alcoholic fatty liver disease, high RDW predicted a 3.6-fold higher cardiovascular mortality and 2.3-fold higher all-cause mortality risk.
But here's an important nuance: in "healthy" adults followed for up to 9 years, higher RDW predicted increased later development of coronary artery disease, heart failure, atrial fibrillation, stroke, hypertension, and cancer. This suggests RDW can pick up on problems before they become obvious.
The research identifies several processes that drive RDW higher:
One study found that RDW in heart failure patients correlated with inflammatory cytokines, iron status, markers of poor nutrition, and kidney function. This illustrates why RDW works as a general vulnerability marker rather than pointing to any single problem.
First, don't interpret it in isolation. Your doctor needs to look at RDW alongside other blood test results, including your hemoglobin (are you anemic?), MCV (are your cells too small or too large?), and iron studies.
The research suggests these evaluation steps:
If your RDW is high but your iron tests look normal, be cautious. Ferritin rises in inflammation, liver disease, and kidney disease, so "normal" ferritin with high RDW might still mean iron deficiency.
Based on the research, elevated RDW carries particular significance for:
If your RDW comes back elevated, treat it as a prompt to dig deeper, not a cause for panic. Ask your doctor whether additional testing makes sense, especially for iron status, vitamin deficiencies, and organ function.
The research suggests that many drivers of high RDW are at least partly modifiable: nutrition, inflammation, metabolic factors, and lifestyle. Addressing these underlying issues may reduce long-term risk, though the studies don't establish that lowering RDW itself improves outcomes.
A mildly elevated RDW in someone who feels fine isn't an emergency, but ignoring it could mean missing early warning signs of problems that are easier to address sooner rather than later. Think of it as your body's dashboard light: it's not telling you exactly what's wrong, but it's telling you something deserves attention.