Red Cell Distribution Width

Red cell distribution width measures how much variation there is in the size of your red blood cells. Rather than telling you the average size, as MCV does, RDW reveals whether your cells are all roughly uniform or whether there is a wide spread of sizes circulating together. Consistency in red blood cell size suggests your bone marrow has been producing cells in a steady, orderly way. A wider variation can sometimes be an early signal that something has shifted, even before other markers move outside their normal ranges.

Picture a box of oranges at the supermarket. Ideally, they are all roughly the same size, showing they were grown under consistent conditions. But if the box contains a mix of very small and very large oranges, it suggests the growing conditions varied. Your red blood cells work in a similar way. When your body is well supplied with the nutrients it needs, the cells it produces are uniform.

When supplies fluctuate (perhaps after a change in diet, supplements or absorption), you may see a mix of older cells made under different conditions alongside newer ones, widening the distribution. Common influences on RDW include iron, B12, or folate status, recent blood loss, and chronic inflammatory conditions. It is also one of the earlier markers to shift when a nutritional deficiency is developing, which makes it a useful early-warning signal. Because RDW is most meaningful when read alongside your other red cell indices, your GP is the best person to interpret it in context and advise whether any further investigation would be worthwhile.