Red Cell Indices (MCV, MCH, MCHC)

What MCV, MCH and MCHC are

Red cell indices are three averages calculated from the red blood cell panel, which describe the typical red cell in the sample. According to Sarma's chapter in Clinical Methods (1990), they were introduced by Wintrobe in 1929 to define the size and hemoglobin content of red blood cells.

• MCV (mean corpuscular volume): average size of the red cell, in femtoliters (fL).
• MCH (mean corpuscular hemoglobin): average mass of hemoglobin per red cell, in picograms (pg).
• MCHC (mean corpuscular hemoglobin concentration): average concentration of hemoglobin inside the red cells, in g/dL.

Automated analyzers report these indices in every complete blood count. Calculating them by hand remains useful for checking reports, studying and verifying the consistency between hemoglobin, hematocrit and red cell count. In microcytic anemia, the MCV is the basis of derived indices, such as the Mentzer index, and the iron assessment includes transferrin saturation.

How to use the calculator: inputs and results

The calculator asks for three values from the same complete blood count, all greater than zero:

• Hematocrit, in %. Example: 45.
• Hemoglobin, in g/dL. Example: 15.
• Red Blood Cell Count, in millions/µL. Example: 5.0.

Zero, negative or wrong-unit values produce an error message. The tool returns three numbers, with one decimal place:

1. MCV, in fL;
2. MCH, in pg;
3. MCHC, in g/dL.

There is no classification on screen: the tool shows the indices and reminds you that the reference range is the laboratory's. With the example values in the fields, the result is MCV 90.0 fL, MCH 30.0 pg and MCHC 33.3 g/dL.

The red blood cell count must be entered in millions per microliter, for example 4.5; if it comes in cells per microliter (4,500,000), the tool warns you and does not calculate. In reports that use ×10¹²/L, the number is the same: 4.5 × 10¹²/L equals 4.5 million/µL.

Red cell index formulas and where the factors come from

The three formulas use hematocrit (Hct) in %, hemoglobin (Hb) in g/dL and red blood cells (RBC) in millions/µL. They are:

MCV (fL) = Hct ÷ RBC × 10

MCH (pg) = Hb ÷ RBC × 10

MCHC (g/dL) = Hb ÷ Hct × 100

The factor 10 in the MCV comes from the units. A hematocrit of 45% corresponds to 0.45 L of red cells per liter of blood, and 5 million/µL equals 5 × 10¹² cells per liter; 0.45 ÷ (5 × 10¹²) = 90 × 10⁻¹⁵ L, or 90 fL. In the MCH, 15 g/dL is 150 g/L; 150 ÷ (5 × 10¹²) = 30 × 10⁻¹² g, or 30 pg.

The MCHC can also be obtained as MCH ÷ MCV × 100, because it is the hemoglobin divided by the volume occupied by the red cells. If the hematocrit is expressed as a fraction (0.45), the MCHC formula becomes Hb ÷ Hct, without the factor 100; the calculator asks for the hematocrit in %.

Worked example with cross-check

The following example is didactic and was checked in the tool itself. Consider a hematocrit of 36%, hemoglobin of 11.5 g/dL and 4.5 million red blood cells/µL.

1. MCV: 36 ÷ 4.5 × 10 = 80.0 fL.
2. MCH: 11.5 ÷ 4.5 × 10 = 25.56, displayed as 25.6 pg.
3. MCHC: 11.5 ÷ 36 × 100 = 31.94, displayed as 31.9 g/dL.

Cross-check: MCH ÷ MCV × 100 = 25.56 ÷ 80.0 × 100 = 31.9 g/dL, the same MCHC. When this check does not match, there is a typing or unit error in one of the three values.

For comparison, Sarma cites as normal values in adults an MCV of 87 ± 7 fL, an MCH of 29 ± 2 pg and an MCHC of 34 ± 2 g/dL. In the example, the MCV is at the lower limit of this range, and the MCH and MCHC are below it. The range that applies to a report, however, is that of the laboratory that issued it, defined according to method, age and sex.

How to understand MCV, MCH and MCHC

The MCV indicates whether the red cells are, on average, small, normal or large, and the MCH and MCHC indicate whether they carry less hemoglobin than usual. In terms of nomenclature:

• Microcytosis: MCV below the laboratory's range. In microcytic anemia, iron deficiency and thalassemia trait are the most discussed causes, as shown by the meta-analysis by Hoffmann and colleagues (2015) on discriminant indices.
• Macrocytosis: MCV above the range. It has varied causes, and the workup depends on the clinical context.
• Hypochromia: MCH and MCHC below the range, usually together with microcytosis.
• Elevated MCHC: should first prompt a check for sample artifacts, described in the errors section.

The indices are averages and hide the variation between cells. For this reason, Bessman and colleagues (American Journal of Clinical Pathology, 1983) proposed classifying anemias by combining the MCV with the RDW, which measures the variation in red cell size. Sarma cites a normal RDW value of 13 ± 1.5%. Interpretation of the whole picture, together with the clinical findings, is the responsibility of the professional in charge.

Common errors, interferences and checklist

Most errors in the manual calculation of the indices come from a mixed-up unit. Points to check:

• Red blood cells in cells/µL (4,500,000) instead of millions/µL (4.5): in a manual calculation, the indices would come out millions of times smaller; the calculator rejects counts above 20.
• Hematocrit as a fraction (0.36) instead of % (36): the MCV would come out one hundred times smaller; the calculator rejects hematocrit below 1.
• Hemoglobin in g/L (115) instead of g/dL (11.5): MCH and MCHC would come out ten times larger; the calculator rejects values above 30.
• Data from different blood counts: the three values must come from the same sample.

There are also interferences from the sample itself. Bessman and colleagues observed that red cell volume histograms identify fragmentation or agglutination of the cells and dimorphic populations, situations in which the averages lose meaning. An MCHC inconsistent with the rest of the blood count is a warning sign for this type of problem.

Checklist: hematocrit in %; hemoglobin in g/dL; red blood cells in millions/µL; cross-check MCH ÷ MCV × 100 equal to the MCHC; result compared with the laboratory's own range.

Calculated indices, analyzer indices and other parameters

In automated analyzers, some indices are measured directly and some are calculated, which explains small differences from the manual calculation. According to Sarma, with the availability of electronic counters the indices came to be measured automatically in all blood counts.

Starting from these indices, other tools take the investigation further. The Mentzer index helps screen microcytosis between iron deficiency and thalassemia trait. Transferrin saturation assesses available iron. The reticulocyte production index shows whether the bone marrow is responding to the anemia.

Frequently asked questions about the red cell indices MCV, MCH and MCHC

References

1. Sarma PR. Red cell indices. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 152. View
2. Bessman JD, Gilmer PR Jr, Gardner FH. Improved classification of anemias by MCV and RDW. Am J Clin Pathol. 1983;80(3):322-6. doi:10.1093/ajcp/80.3.322. View
3. Hoffmann JJ, Urrechaga E, Aguirre U. Discriminant indices for distinguishing thalassemia and iron deficiency in patients with microcytic anemia: a meta-analysis. Clin Chem Lab Med. 2015;53(12):1883-94. doi:10.1515/cclm-2015-0179. View