# Hemoglobin and hematocrit question

Discussion in 'Medical Students - MD' started by Concubine, May 3, 2012.

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1. ### ConcubinePDE5 inhibited 5+ Year Member

Nov 30, 2006
Can anyone think of any reason, other than a lab error, that a hemoglobin might be elevated while the hematocrit is decreased, or vice versa?

3. ### DrSnipsHospitalist 5+ Year Member

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Apr 18, 2012
Physician
There are probably plenty of examples. Here's one: Iron deficiency anemia prevents adequate production of hemoglobin, but still allows production of defective red blood cells which contribute to hematocrit. Combine this with someone who has increased red cell number or volume (pregnant, smoker, etc.) and you may see decreased hemoglobin with increased hematocrit.

4. ### normtheniner 7+ Year Member

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Doesn't hemorrhagic fevers cause problems like this? Like Dengue Fever?

5. ### MorsetlisI wish I were a dentist 7+ Year Member

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In a portable lab, spun crit is easier to measure than Hgb.

Last edited: Mar 6, 2016
6. ### wholeheartedly SDN Administrator 7+ Year Member

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There are definitely plenty of reasons they can vary independently of each other. That's why you need the other RBC indices (MCV, MCH, and MCHC) to help you evaluate reasons for this. When you're looking at the ratio of those two you're talking about MCHC, hgb*100/hct.

You probably most of this already but to start at the beginning...
Hct is a measure of how much space in the blood is composed of red cells as opposed to plasma or white cells. It's calculated on most analyzers based on the number of RBCs and volume of red cells (MCV). So if number of cells decreases or cell sizes decreases, ie they take up less space, the hct decreases.

Hgb is going to be function of production and loss of hgb molecules. If you have the same amount produced but you're kicking out larger or smaller cells you'll get a discrepancy. That's why anemias are often described in terms of both cell size (normocytic, macrocytic, microcytic) and pallor (normochromic, hypochromic, hyperchromic).

7. ### OveractiveBrainBanned 2+ Year Member

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Hemoglobin is calculated based on the hematocrit. They are the same thing. There are "crit" people and there are "hemoglobin" people. Crit people think in 3s, globin people in 1s. In general, 1 hemoglobin = 3 Hematocrits. The calculation is not exactly 1:3, but is close enough such that within normal variation it doesn't matter which you use, because they are the same number, they are the same thing, just one is measured and the other calculated.

8. ### PMPMD4G MD 10+ Year Member

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Exactly. The lab doesn't do spun crits anymore.

9. ### OveractiveBrainBanned 2+ Year Member

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Delteet'd

Last edited: May 6, 2012
10. ### JackShephard MD

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Oct 17, 2011
New York
I'm just asking here, but Wiki appears to differentiate between the two terms. Semantics? Science?

I'm just a medical student though, so what do I know.

11. ### OveractiveBrainBanned 2+ Year Member

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Delteet'd Sleep deprived and retarded. Ive removed that post

12. ### wholeheartedly SDN Administrator 7+ Year Member

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Beyond the Wall
No, you're right JackShephard, they are definitely different. I'm quite familiar with how they are determined. Because they are often used interchangeably, people miss the difference.

Here's an excerpt from Williams Hematology 8 ed. Ch2 ( on access med if you want to read it) on how these are determined in today's labs. I'm over simplifying it, but most hematology analyzers use a combination of methods like flow cytometry, light scatter, electrical impedance to determine CBC parameters.

"In electronic instruments, the hematocrit (Hct; proportional volume of blood occupied by erythrocytes) is calculated from the product of direct measurements of the erythrocyte count and the MCV (Hct [L/100 L] = RBC [x 10&#8211;6/L] x MCV [fl]/10)" MCV is usually determined by light scatter.

Hgb doesn't factor into hct determination at all in automated instruments.

From same source:
"Hemoglobin is intensely colored, and this property has been used in methods for estimating its concentration in blood. Erythrocytes contain a mixture of hemoglobin, oxyhemoglobin, carboxyhemoglobin, methemoglobin, and minor amounts of other forms of hemoglobin. To determine hemoglobin concentration in the blood, red cells are lysed and hemoglobin variants are converted to the stable compound cyanmethemoglobin for quantification by absorption at 540 nm.12 All forms of hemoglobin are readily converted to cyanmethemoglobin except sulfhemoglobin, which is rarely present in significant amounts. In automated blood cell counters, hemoglobin is usually measured by a modified cyanmethemoglobin or an alternate lauryl sulphate method."

Hgb and Hct don't represent the same thing. Usually they are pretty consistent with each other.

When the ratio of Hgb/Hct isn't roughly 1:3, like the OP asked about, the indices can be used to give some insight as to what the problem is, though there are better ways. It can indicate lab error or actual pathology. They tell you how the cells should look in a blood smear.

Mcv = Hct (%) / RBC (x10^12/L) * 10 = x fL and is the average volume of each individual red cell and is used to classify cells as either normocytic, microcytic, marcrocytic.

MCH = Hgb (g/dL) / RBC (x10^12/L) * 10 = x pg and is the average mass of Hgb in each red cell

MCHC = Hgb (g/dL) / Hct (%) * 100 = x g/dL this indicates the average concentration of hgb/volume and indicates the overall pallor of the cell as normochromic, hypochromic, or hyperchromic (hyperchromic should rarely be used)

examples microcytic/hypochromic - iron deficiency anemia, lead poisoning, thalassemias
normocytic/normochromic - anemia of chronic disease
macrocytic/ normochromic- chemo, folate or b12 deficiency, liver diease

Last edited: May 6, 2012
13. ### wholeheartedly SDN Administrator 7+ Year Member

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..... yeah, sleep deprivation sucks. i work nights, frequently have that problem.

14. ### CBG23 10+ Year Member

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Jun 27, 2007
I don't know how Hb is assayed in the blood, but assuming it can't differentiate between free and intracellular Hb, one would guess that acute massive RBC lysis would cause a normal Hb with decreased crit

15. ### SoDakMLSGood luck on the Path! 5+ Year Member

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Mar 6, 2011
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If anyone is really that interested, here is a link to the manufacturer's site for the analyzer for CBCs that my former clinical lab used:
http://www.medical.siemens.com/weba...e_-111~a_pageId~e_78667~a_storeId~e_10001.htm

It uses light scatter to determine the hemoglobin concentration of the cells (as RBCs are essentially balloons filled with Hgb) after chemical treatment makes them spherical and lyses the WBCs. Specimens with significant hemolysis typically have major variations in the RBC indices (MCV, MCH, MCHC, etc.) due to all the RBC fragments present. The computer software with these analyzers can tell when the indices are wonky and flags the results so the tech should look at the specimen quality.