MCV, MCH and RDW Meaning: Small, Large or Mixed Red Cells
MCV, MCH and RDW describe the size and the spread of your red cells, and they sit just under haemoglobin in a CBC. Low MCV means small cells, high MCV means large cells, and a high RDW means the sizes are mixed, which together give a doctor a pattern to follow up, not a diagnosis.
In brief
- MCV: the average size of a red cell.
- MCH and MCHC: how much haemoglobin each cell carries and how packed it is.
- RDW: how uneven the sizes are.
- Reference range: it varies by lab, so use the range printed on your report.
- Insider clue: low MCV with a normal RDW and a high red cell count is worth raising with the doctor.
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What are red cell indices, in plain words?
Picture a basket of tomatoes. Haemoglobin tells you the total weight of tomatoes. MCV tells you whether they are small or large, MCH tells you how much flesh is in each, and RDW tells you whether they are all the same size or a messy mix. The set of numbers is called the red cell indices, and they live in the CBC report.
What do low MCV, high MCV and RDW mean?
| Pattern | Broad hint | Doctor often checks |
|---|---|---|
| Low MCV, high RDW | Iron shortage | Ferritin |
| Low MCV, normal RDW | Inherited trait possible | Hb electrophoresis |
| High MCV | B12 or folate shortage, alcohol, thyroid, some medicines | B12, folate, TSH |
| Normal MCV, high RDW | Early mixed change | Repeat and iron tests |
Low MCH and low MCHC usually travel with low MCV, because small cells hold less haemoglobin. RDW comes as RDW-CV, a percentage, and RDW-SD, a measured width in fL. Labs print one or both, and each has its own range, so compare like with like.
Is it iron shortage or thalassaemia trait?
Both give small red cells, which is why people search “MCV 70 thalassemia or iron”. The difference is often in the spread and the count. In iron shortage the cells vary in size, so RDW is high and the red cell count is usually low. In an inherited trait the cells are uniformly small, RDW is often normal and the red cell count can be normal or even high.
The pattern clue, with a warning
Clinicians compare the spread (RDW) and the red cell count as a rough pointer. Treat this only as a talking point for your doctor, never as a diagnosis, because it can mislead. If your pattern is small cells, normal RDW and a high red cell count, raise it and ask whether Hb electrophoresis makes sense.
Which follow-up test does the doctor usually pick?
- Small cells: ferritin, then iron studies. Read about ferritin versus haemoglobin first.
- Large cells: vitamin B12, folate and thyroid tests.
- Small cells that do not respond to iron: haemoglobin electrophoresis.
What should you do with these numbers?
- Find Hb, MCV and RDW together; one value alone says little.
- Note whether MCV is low, normal or high against your report’s range.
- Write down symptoms, diet and any blood loss such as heavy periods.
- Take the printed report to the doctor and ask which cause they suspect and what test is next.
- Do not start iron or vitamins on your own because of the pattern; the doctor decides, since extra iron can harm people with some inherited conditions.
Indices point, they do not prove. See a doctor promptly for breathlessness at rest, chest pain, fainting, black stools or heavy bleeding. Seek advice before pregnancy if a trait is suspected, since partner testing may matter. Recent transfusions, alcohol use and some medicines can change MCV.
Worked example
Say Hina, 24, has Hb 10.8, MCV 68, RDW-CV 17 and a low red cell count. Small cells with a wide spread and a low count fit iron shortage, so the doctor asks for ferritin and looks for a reason such as heavy periods. Compare Faizan, 26, with Hb 11.9, MCV 64, RDW-CV 13.5 and a red cell count of 6.1. Small cells, an even spread and a high count can make the doctor think of a trait, so electrophoresis is suggested before anyone prescribes iron.
Why a normal Hb does not make the indices irrelevant
Early in iron shortage, haemoglobin can still look fine while MCV drifts down and RDW climbs. The indices often move first, which is why a careful doctor reads them even when Hb is in range. The same is true for B12 shortage: MCV can creep up long before anaemia shows.
Things that can skew the numbers
- Old blood samples, which swell the cells and push MCV up.
- Recent transfusion, which mixes two populations of cells and raises RDW.
- Very high white counts or clumped cells, which can confuse the analyser.
- Pregnancy, which slightly lowers haemoglobin and shifts the indices.
If a result looks out of line with how you feel, a repeat from a reliable lab is reasonable before you chase any cause.
How to prepare for the test
A CBC needs no fasting. Tell the lab about recent blood loss, a transfusion, pregnancy and the medicines you take, and ask for the full report with MCV, MCH, MCHC and both RDW lines. A reliable lab with a recent calibration matters more than the brand of machine.
MCV, MCH and RDW questions
What does low MCV mean?
It means the average red cell is smaller than usual, most often due to iron shortage or an inherited trait such as thalassaemia. A ferritin test and the other indices help sort them.
What causes high MCV?
B12 or folate shortage and some medicines are well-recognised causes; alcohol use, thyroid and liver conditions can also be involved. The doctor chooses tests based on the whole picture.
What does high RDW CV mean?
It means the red cells vary a lot in size. This is common in early iron shortage and in mixed shortages, and the doctor reads it with MCV.
What does low MCH mean in a blood report?
Each red cell carries less haemoglobin than usual, which goes with small cells. It usually appears together with low MCV.
What is the normal MCHC range?
Ranges differ between labs and analysers, so use the range printed on your own report.
Is MCV 70 thalassemia or iron deficiency?
Either is possible, and the CBC alone cannot decide. A normal RDW with a high red cell count may lean towards a trait, while a high RDW may lean towards iron shortage; the doctor confirms with further tests.
What is the difference between RDW SD and RDW CV?
RDW-SD is a direct width of the size spread in fL, and RDW-CV is a percentage of the average size. Each has its own range.
What is MCV in a blood test?
It is the mean corpuscular volume, the average size of your red blood cells, reported in femtolitres.
See more in Lab Reports Decoded.