INTRODUCTION

During the first half of the twentieth century, the complete blood count (CBC), one of the most commonly ordered laboratory tests, was performed using exclusively manual techniques:

• Blood cell counts (red cells, white cells, platelets) were performed using appropriately diluted blood samples and a ruled counting chamber (hemocytometer).
• Hemoglobin concentration was analyzed colorimetrically by the cyanomethemoglobin method.
• The hematocrit (packed cell volume) was measured by high speed centrifugation of a column of blood, either in a specially designed tube (the Wintrobe tube) (picture 1), or in sealed microcapillary tubes (ie, the "spun" hematocrit, often obtained by fingerstick blood collection) (picture 2).
• The white blood cell differential was obtained by examining and enumerating by class (eg, granulocytes, lymphocytes, monocytes) 100 to 200 individual white blood cells on a suitably stained blood smear.

In 1932, Wintrobe developed a set of calculated indices that estimated erythrocyte size and hemoglobin content based on the red blood cell count (RBC), hemoglobin concentration (HGB), and hematocrit (HCT). These indices included:

• Mean corpuscular volume (MCV) — the volume (in femtoliters, fL) of the average circulating red blood cell
• Mean corpuscular hemoglobin (MCH) — the hemoglobin content (in picograms) of the average circulating red blood cell
• Mean corpuscular hemoglobin concentration (MCHC) — the hemoglobin concentration within circulating red blood cells (grams of hemoglobin per 100 mL of packed red blood cells)

These three indices were calculated manually, as follows: