A High RDW with a Normal MCV
RDW measures how much your red cells vary in size, and it moves before the average size does. A raised RDW with a normal MCV is often the earliest visible sign that a deficiency has started, caught at a point where the rest of the count still looks fine. Often it is an iron store that has started to fall, and a ferritin is the investigation; just as often it is the nonspecific mark of a long-term condition, alcohol or age.
The pattern on your report
- RDW High · mild Key
- MCV Normal Key
- Hemoglobin Normal Key
Printed as: Hemoglobin in g/Lor g/dLMCV in fLRDW in %— RDW-CV is a percentage; RDW-SD is reported in femtoliters and is a different measurement with a different range. Reports rarely say which they mean.
Why the numbers look like this
The marrow turns out cells continuously, and each one carries the conditions it was built under. Run short of iron and the cells built this month come out smaller than the ones built three months ago, but the older ones are still in circulation. The average sits between them and can stay inside its range for a long time, while the spread between them widens immediately.
So RDW is a measure of change over time, read from a single tube of blood. That is what makes it early. It is also what makes it non-specific: a wide spread says something has shifted, not what.
There is a second situation it catches. Iron deficiency shrinks cells and B12 deficiency enlarges them, so someone with both can produce a completely ordinary MCV. The two errors cancel in the average but not in the spread, and the RDW is the only number on the count that notices.
Not being flagged is not the same as normal
Most laboratories report RDW-CV, a percentage with an upper limit around 14 to 15%. Some report RDW-SD instead, in femtoliters, with an entirely different range near 39 to 46. The two are not interchangeable and a result compared against the wrong one will look wrong. Check which your report gives before reading anything into the number.
What else on the report can hide this
Read it beside the MCV, because the pair is more informative than either alone. Wide spread with small cells is the iron deficiency signature; wide spread with large cells suggests emerging B12 or folate deficiency, or a marrow recovering; wide spread with a normal average is where a mixed deficiency hides. Thalassemia trait belongs to the opposite pattern: it produces uniformly small cells and a narrow spread, so a wide RDW argues against the trait and toward iron deficiency. A reticulocyte count also raises the RDW on its own, because young cells are larger, so a marrow responding to bleeding or hemolysis widens the spread with no deficiency at all.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Early iron, B12 or folate deficiency
Stores falling while cell size has not yet moved. A ferritin is the test, and it is often already low by the time the RDW widens; B12 and folate behave the same way from the other direction.
- Very common
Chronic illness, inflammation, alcohol or older age
The most common reason a mildly raised RDW turns up on an otherwise normal count, and the least specific. It tracks inflammation, kidney and heart disease, alcohol intake and age, which is why a small rise in someone with a long-term condition usually needs no chase of its own.
- Very common
Recovery from any deficiency
The weeks after starting iron, B12 or folate, when a new population of properly built cells joins the old one. Expected, temporary, and a sign treatment is working.
- Common
Recent blood loss or a transfusion
Transfused cells differ in size from your own, and a marrow replacing lost cells releases larger young ones. Both widen the spread temporarily.
- Uncommon
A mixed deficiency
Iron plus B12 or folate together. The MCV averages out to normal while the film shows two distinct populations of cells.
- Uncommon
Hemolysis
Cells destroyed at different ages, plus a rush of reticulocytes. Look at LDH, bilirubin and haptoglobin.
- Uncommon
Chronic liver disease
Membrane changes produce cells of uneven size. Usually with other liver abnormalities and often a low platelet count.
- Rare
Myelodysplastic syndrome
Considered when the RDW stays wide with normal iron, B12 and folate, and one of the cell counts begins to drift down.
What is usually checked next
- Ferritin Covers the commonest cause, and it can be genuinely low while every other number on the count still reads normal.
- B12 and folate Finds the other half of a mixed deficiency, which is precisely the case a normal MCV conceals.
- Blood film Shows two populations of cells directly, where the analyzer only reports their average.
- Your previous blood counts An RDW that has widened over two or three years means something changed; one that has always been slightly wide usually does not.
When to seek care sooner
- Same day Yellowing of the eyes with dark urine
- Soon A widening RDW with a hemoglobin that is now falling
- Soon A wide RDW with a falling white cell or platelet count
- Mention it A persistently wide RDW after iron, B12, folate and liver tests all come back normal
Questions worth bringing to your appointment
- Is my report giving RDW-CV or RDW-SD?
- Has my ferritin been checked, given the RDW is up before the MCV has moved?
- Could I have more than one deficiency at once, with them cancelling out in the MCV?
- How has my RDW changed compared with earlier blood counts?