Calculating Red Cell Indices And Their Role In Sample Integrity

The red cell indices provide information concerning the size and Hgb content of red cells by providing the MCV, MCH, and MCHC. The MCV is one of the most stable parameters in the CBC, with little variability over a period of time: less than 1%.13 For this reason, the MCV plays an extremely valuable role in monitoring the pre-analytic and analytic qualities of the sample. The MCV is either directly read by the instrumentation method, or it is a calculated value. If it is calculated, the formula is as follows:

MCV = (hematocrit/red cell count) X 100

The normal value is between 80 and 100 fL and implies a red cell that has a size of 6 to 8 pm. Legitimate explanations for a shift in MCV include the presence of cold agglutinins (red cells coated with cold antibody, causing a false increase in size), transfusion therapy (newly transfused cells are larger), and reticulocytosis (presence of polychromatophilic macrocytes). Specimen or preanalytic factors that may account for a shifting MCV include the following14,15:

1. Contamination by drawing through the intravenous lines or in-dwelling catheters

2. Specimens from hyperglycemic patients

3. Patients on some chemotherapy drugs or zidovudine (AZT) therapy

Any shift in MCV that cannot be explained as a result of the circumstances listed should prompt the laboratory to investigate a possible sample mismatch or misidentification. As a delta check parameter, the MCV has a high value when determining sample integrity. See Table 2.5 for causes of MCV shifts.

The MCH and MCHC provide information concerning red cell hemoglobinization. The MCH can be calculated by the following formula:

MCH = (hemoglobin/red cell count) X 100

The normal value is 27 to 31 pg, which implies that the average weight of Hgb in a given amount of red cells is in the appropriate range. The MCHC content can be calculated using the following formula (expressed in percentage):

MCHC = (hemoglobin/hematocrit) X 10

The normal value is 32% to 36%, which implies that the amount of Hgb per red cell is in the appropriate concentration.

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