Red Cell Indices Interpreter
Red Cell Indices Interpreter
Classify an anaemia from the MCV, MCHC and RDW, and see the differential each pattern opens up — and where the pattern stops and the confirmatory test has to start.
Red cell indices
MCV + MCHC → patternMCV 72 fL, MCHC 29.5 g/dL, RDW-CV raised above 14.5%
How the pattern is read
MCHC <32 g/dL hypochromic · 32–36 g/dL normochromic · >36 g/dL raised
RDW-CV >14.5% — cell size is variable
- MCV
- an average, and averages hide mixtures. A population of small cells and a population of large ones average to a normal MCV, which is why the RDW is read alongside it rather than after it
- MCHC
- haemoglobin per unit of packed cell volume. It is the most robust of the three indices and moves late, so hypochromia confirms a picture rather than detecting it early. A value above 36 g/dL is more often artefact than spherocytosis
- RDW
- the spread of cell sizes. It is what separates conditions sharing an MCV — iron deficiency widens it, thalassaemia trait usually does not — and it is often the first index to move
- what this cannot do
- name a cause. Iron deficiency and thalassaemia trait give the same microcytic hypochromic picture, which is precisely why the discriminant indices and then ferritin and electrophoresis exist
Worked example
MCV 72 fL, MCHC 29.5 g/dL, RDW-CV raised above 14.5%
MCV 72 fL is below 80 → microcytic
MCHC 29.5 g/dL is below 32 → hypochromic
RDW raised → the cells vary in size, which fits a supply that has failed progressively
→ iron deficiency heads this differential, and that is where the interpretation stops. Send ferritin, read against CRP
Change the RDW to normal and the same MCV and MCHC read differently: thalassaemia trait moves up the list, because a constitutional defect affects every cell equally. Neither answer is a diagnosis
The six combinations and what each opens up
| MCV | MCHC | Pattern | Usual differential | Next test |
|---|---|---|---|---|
| <80 | <32 | Microcytic hypochromic | Iron deficiency, thalassaemia trait, anaemia of chronic disease, sideroblastic anaemia, lead | Ferritin with CRP; electrophoresis or HPLC if ferritin is normal |
| <80 | 32–36 | Microcytic normochromic | Early iron deficiency, thalassaemia trait, anaemia of chronic disease | Ferritin with CRP; red cell count and the discriminant indices |
| 80–100 | 32–36 | Normocytic normochromic | Blood loss, haemolysis, renal anaemia, anaemia of inflammation, marrow failure, endocrine disease | Reticulocyte count first — it splits underproduction from loss or destruction |
| 80–100 | <32 | Normocytic hypochromic | Early iron deficiency, or a mixed deficiency averaging to a normal MCV | Film, RDW, ferritin and B12 together |
| >100 | 32–36 | Macrocytic | B12 or folate deficiency, alcohol, liver disease, hypothyroidism, myelodysplasia, drugs, reticulocytosis | B12, folate, reticulocytes, TFTs, LFTs, drug chart, film |
| 80–100 | >36 | Raised MCHC | Hereditary spherocytosis — but artefact is commoner: lipaemia, cold agglutinins, high white count, in vitro haemolysis | Film and a repeat sample before anything else |
Why the RDW is read with the MCV, not after it
| MCV | RDW | What that combination suggests |
|---|---|---|
| Low | Raised | Iron deficiency — a failing supply produces cells of progressively differing size |
| Low | Normal | Thalassaemia trait — a constitutional defect affects every cell equally, so they are uniformly small |
| Normal | Raised | A mixed or evolving picture the MCV alone would miss entirely — combined deficiency, partial treatment, recent transfusion |
| High | Raised | B12 or folate deficiency, brisk reticulocytosis, myelodysplasia |
| High | Normal | Alcohol, liver disease, hypothyroidism, a marrow-suppressing drug — uniform effects across the population |
A classification that narrows the tests, and stops there
Classifying an anaemia by red cell indices is the oldest useful thing in haematology and it survives because it works: it turns an undifferentiated low haemoglobin into a short list of tests worth sending. Three numbers do the work. The MCV is the average volume of a red cell, and it splits anaemias into microcytic below 80 fL, normocytic between 80 and 100, and macrocytic above 100. The MCHC is the haemoglobin packed into that volume, and below about 32 g/dL the cells are hypochromic. The RDW is how much the cell sizes vary.
The MCV is a mean, and that is its weakness. A population of small cells mixed with a population of large ones averages to a perfectly normal MCV, so combined iron and B12 deficiency — a common combination in the elderly and after bariatric surgery — can hide behind a normocytic result. The RDW is what catches it: the mean is unremarkable and the spread is not. That is why the two are read together rather than in sequence, and why a normal MCV with a wide RDW is worth a film and a full set of haematinics rather than reassurance.
The MCHC behaves differently from the other two. It is the most robust of the indices and the last to move, so hypochromia confirms a picture that is already established rather than detecting an early one. In the other direction it is chiefly useful as an error detector: a value above about 36 g/dL is more often lipaemia, cold agglutinins, a very high white cell count or in vitro haemolysis than it is hereditary spherocytosis, and the right response is a look at the film and a fresh sample.
What none of this does is make a diagnosis, and the microcytic hypochromic pattern is the clearest demonstration of why. Iron deficiency and beta-thalassaemia trait produce the same three indices, and they need opposite responses — one is treated with iron and an investigation for the source of loss, the other is a lifelong carrier state with implications for the patient's relatives and for any pregnancy. The discriminant indices exist for exactly this ambiguity, and they only prioritise who is sent for electrophoresis. Ferritin, read against CRP, and haemoglobin electrophoresis or HPLC are what settle it. The pattern tells you which tests to send; it does not tell you what the patient has.
Frequently asked questions
What do MCV and MCHC together tell you?
They classify the anaemia rather than diagnose it. The MCV splits it into microcytic, normocytic or macrocytic, and the MCHC says whether the cells are hypochromic. Each combination has a characteristic differential and a characteristic next test, but several different diseases share each pattern.
Is an MCV of exactly 80 fL microcytic?
No — 80 to 100 fL is conventionally an inclusive interval, so an MCV of exactly 80.0 or exactly 100.0 reads as normocytic, and that is how this page classifies it. A result sitting on the boundary should be read with the RDW, the previous result and the clinical picture rather than on the number alone.
Can the MCV be normal in a patient with two deficiencies?
Yes, and it commonly is. The MCV is an average, so small iron-deficient cells and large B12-deficient cells average to a normal figure. The RDW usually gives it away by being wide, and the blood film shows two distinct populations, which is why both are worth asking for.
Does a microcytic hypochromic picture mean iron deficiency?
No. Iron deficiency is the commonest cause, but thalassaemia trait, anaemia of chronic disease, sideroblastic anaemia and lead exposure all produce it, and iron deficiency and thalassaemia trait frequently coexist. Ferritin read against CRP, and haemoglobin electrophoresis if ferritin is normal, are what separate them.
What does a high MCHC mean?
Usually an artefact. Lipaemia, cold agglutinins, a very high white cell count, in vitro haemolysis and an underfilled tube all raise it spuriously, and those are commoner than the real cause, hereditary spherocytosis. Look at the film and repeat on a fresh sample before investigating a membrane disorder.
Related calculators
References
- Tefferi A, Hanson CA, Inwards DJ. How to interpret and pursue an abnormal complete blood cell count in adults. Mayo Clin Proc. 2005;80(7):923–936.
- Bain BJ. Blood Cells: A Practical Guide. 5th ed. Wiley-Blackwell; 2015.
- Hoffbrand AV, Steensma DP. Hoffbrand's Essential Haematology. 8th ed. Wiley-Blackwell; 2019 — hypochromic and macrocytic anaemias.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.