Postscript

**Timeline of technological discoveries related to complete blood count.** Abbreviations: Hb, hemoglobin; Hct, hematocrit; RBC, red blood cell; MCV, mean cell volume; MCH, mean corpuscular hemoglobin; MCHC, mean corpuscular hemoglobin concentration.

Initially, the Hct was designed to provide a more rapid test for RBC count (which was much more time-consuming):

What is the problem with the assumption that Hct = surrogate for RBC count?

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Linear correlation between Hct and RBC count depends on MCV (Hct = MCV x RBC count)!

What can you say for certain about other CBC values in patients A and B?

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MCV must be higher in patient A. In fact MCV was 106 fL in A, 62 fL in B.

Now let’s return to the manual method of centrifuging blood. You can only imagine the challenge of counting off the number of revolutions per minute all the while keeping close track of time. At the turn of the 20th century, house officers, not lab technicians were responsible for performing their own Hct measurements. Around this time, we begin to see mention of electric versions of the centrifuge:

James Ewing, Clinical Pathology of the Blood, 1901.

**Electric Hematocrit**. James Ewing, Clinical Pathology of the Blood, 1901.

Once it was realized that the Hct is not an acceptable surrogate for the red blood cell count (since it depended on the MCV), the Hct was adopted as a useful as a way to measure RBC volume (Hct/RBC count). Only later – well into the 20th century – was the Hct used as a surrogate for red cell mass and hemoglobin.

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