Blood HealthJun 3, 2026
There is no single BUN number that flips a switch from "safe" to "dangerous." But across multiple large studies of hospitalized and critically ill patients, a consistent pattern emerges: once BUN climbs into roughly the high 20s mg/dL and above, the risk of dying, both short-term and long-term, rises meaningfully.
That said, a BUN of 28 mg/dL in a dehydrated but otherwise healthy person is a very different situation than the same number in someone in the ICU with heart failure. The number matters, but so does everything around it.
Blood HealthMay 28, 2026
A high RDW-SD doesn't point to one specific problem. It points to almost all of them. In a study of more than 3 million adults, higher RDW-SD predicted mortality and multiple adverse health events, performing at least as strongly as the more commonly referenced RDW. Risk of death, heart attack, stroke, cancer, hospitalization, and long-term care placement all climbed steadily as RDW and RDW-SD values rose through higher percentiles.
That breadth is exactly what makes this marker so useful and so easy to misunderstand. RDW-SD (red blood cell distribution width, standard deviation) measures how much your red blood cells vary in size. When the number is high, it means your body is producing red blood cells of inconsistent sizes, a signal that something is stressing the system. The catch: it almost never tells you what.
