The mean cell hemoglobin or MCH is a calculated red blood cell (RBC) index. It is calculated from the hemoglobin concentration and RBC count so it represents the amount of hemoglobin per RBC. It is thus affected by hemoglobin synthesis and volume (a smaller RBC will have less hemoglobin and vice versa).
The formula used to calculate MCH is:
MCH (pg) = Hemoglobin concentration (g/dL) ÷ RBC count (mill/µL)
The MCH does not always provide helpful information and usually tracks with changes in the mean cell volume or MCV, in that animals with a high MCV (e.g. regenerative anemia with numerous immature RBC) will have a high MCH. The exception to this is a high MCV due to hypernatremia or in vitro RBC swelling with storage, in which the MCH decreases along with the mean cell hemoglobin concentration or MCHC. Usually an animal with a low MCV (e.g. iron deficiency, portosystemic shunts) will have a low MCH and a low MCV (if severe enough). Again, artifactual changes in MCV (shrinkage with excess EDTA or hyponatremia), the MCH tracks with the MCHC, both of which increase, whereas the MCV goes down.
Factors that affect the hemoglobin but not the RBC count, such as in vitro (artifactual) hemolysis or intravascular hemolysis, will result in a hemoglobin being disproportionately higher than the RBC count (because lysed RBCs are not counted, but their hemoglobin is still measured by the machine) and a false increase in MCH and MCHC, whereas the MCV will not be affected because it is a directly measured result. Similarly, lipemia (moderate to severe) will falsely increase the hemoglobin (and MCH and MCHC) but not the MCV.
The cellular hemoglobin is a result that is provided from optical measurement (based on light scatter) of intact RBCs by some hematology analyzers, like the ADVIA. This is the “optical equivalent” of the MCH and is used to calculate the CHCM (using the MCV), which is the “optical equivalent” of the MCHC and the cellular or calculated hemoglobin (using the RBC count), which is the “optical equivalent” of the hemoglobin that is measured spectrophotometrically by absorbance at a specific wavelength after the machine lyses all the RBCs. Why are we telling you about this? Because the CH, CHCM and cellular hemoglobin are more accurate than the MCH, MCHC and hemoglobin in the setting of a false increase in hemoglobin, e.g. moderate to severe lipemia. The CH and CHCM are also more accurate than the MCH and MCHC with in vitro or in vivo intravascular hemolysis due to the reasons mentioned above. We provide these additional optical results on our hemograms under the circumstances mentioned above, i.e. when we think the MCH and MCHC (and hemoglobin) are falsely increased for some reason. Note that the CH and CHCM are affected similarly to the MCH and MCHC with RBC swelling with storage or hyperosmolality (all decrease, while MCV increases, particularly in dogs) and cell shrinkage with EDTA or hyponatremia (all increase while MCV decreases).