Hematology-1

Hematology

Hematology, the study of blood! Blood is considered the largest organ in the body and the normal range for total blood in the body is about 4-6 liters. When a tube of blood is centrifuged to separate it, blood is comprised of about 55% plasma, 44% red blood cells (RBCs), and 1% white blood cells (WBCs).

 

For the clinical laboratory scientist it is critical to know hematology normal values, and that’s where we’ll start.

RBCs: 5-6 x 10^6/uL
WBCs: 4-11 x 10^3/uL
Hemoglobin (Hgb): 12-18 g/dL
Hematocrit (Hct): 37-52 %
MCV (Mean corpuscular value): 80-100 fL (femtoliters)
MCH (Mean corpuscular hemoglobin): 27-31 pg (picograms)
MCHC (Mean corpuscular hemoglobin concentration): 32-36 %
Platelets: 150-450 x 10^3/uL

Study Tip
There are a number of ways to remember these values, one trick for RBCs, hemoglobin, and hematocrit is the rule of 3. If you take an average RBC count of 5 and multiply it by 3, you’ll get 15. Then multiplying 15 by 3 will give you 45. You end up with the numbers 5-15-45 which roughly correspond to the normal values (minus units) for RBCs, hemoglobin, and hematocrit!

Perspective Note
In a tiny drop of blood, there are on average about 5 million RBCs! This speaks to the sophistication and complexity of the human body. There are also about 7,000 WBCs in this same drop. Since we said earlier the human body has about 5 liters of blood, this means there are about 25 trillion RBCs and 35 billion WBCs circulating in your body right now! Wow!

Ok, so why do these values matter?

Hemoglobin, the main component of RBCs, has a primary function of transporting oxygen to tissue and facilitating carbon dioxide excretion. (The hemoglobin lab value will be used in calculating MCH and MCHC, which we will touch on shortly).

Hematocrit is the percentage of blood volume that is RBCs. If you reference back to our spun down blood tube, you will notice the RBCs at the bottom, the percentage of RBCs compared to plasma and WBCs is essentially the hematocrit (the percentage of blood that is RBCs!) Hematocrit is important because it tells you the amount of RBCs that can potentially transport oxygen. It can detect conditions such as anemia (low hematocrit lab value) and polycythemia (high hematocrit lab value).

MCV is used to measure the size of RBCs. It is calculated by the equation:

Since the Hct tells you the percentage of RBCs and the RBC count tells you the amount of RBCs, this calculation will give you an average size of RBCs. The normal range for this calculation is 80-100 fL (femtoliters). Results falling below this range will be considered microcytic (literally small cells), in range will be normocytic (normal), and above range will be macrocytic (large).

Study Tip
Hematology is the subject where it pays off the most to understand root word meanings. The larger your medical vocabulary, the easier time you will have test taking. Often times test questions will hinge on one uncommon term where if you don’t know what it means, it’s going to be tough to answer the question.

Ok back to lab values. MCH is used to measure the average amount of hemoglobin per RBC (not accounting for size). It is calculated by the equation:

The normal range for this calculation is 27-31 pg (picograms).

MCHC is used to measure the concentration of hemoglobin per RBC. It is calculated by the equation:

The normal range for this calculation is 32-36%. Results falling below this range will be considered hypochromic (low color), in range will be normochromic (normal color), and above range will be hyperchromic (high color). Hemoglobin is what gives RBCs their red color.

What is the difference between MCH and MCHC?

This can be a confusing concept. MCH will tell you the amount of hemoglobin per RBC. MCHC tells you the concentration of hemoglobin per RBC, in other words, what percentage of the RBC is hemoglobin.

To draw an analogy let’s imagine we have 100 crème donuts. Mmmmm donuts. In this analogy the MCH would tell you how much crème, in weight, is in each donut; whereas the MCHC would tell you what percentage of each donut is crème. MCH does not account for size, MCHC does.

Another way to help understand MCHC is to use some algebra and substitute Hct for (RBCs x MCV)/10 in the MCHC calculation. The (RBCs x MCV)/10 came from reworking the MCV calculation above. That would leave us with: MCHC = Hgb/(RBC x MCV) x 1000. This example helped me to gain a very clear understanding because MCH is Hgb/RBC x 10 and MCHC is Hgb/(RBC x MCV) x 1000. We know MCV is representative of RBC size, so there you have it! The algebra can be seen below.

Nerdy Note
The example above is how many labs actually calculate MCHC. On automated Coulter analyzers, MCV is derived from a size distribution curve, and that MCV value along with the RBC and Hgb values are used to calculate MCHC. Hct is also calculated using RBC and MCV values, and MCH is calculated using RBC and Hgb values.

Platelets have a normal reference range of 150-450 x 10^3/uL or more simply put 150,000 – 450,000 platelets/uL. Platelet function is covered in detail in the coagulation section.