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Reviews and Summaries

Hemoglobin is NOT Different from Hematocrit... Once and for All!

26/8/2015

 
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It starts in medical school, regularly appears in your medical training, sneaks around nursing schools and is an impetus for discussions in the ICU: The great myths about Hemoglobin (Hb) and Hematocrit (Hct). 

These two haematological lab-parameters are part of our daily life at work and are mostly measured together... as a package. Some clinicians look at haemoglobin levels, others prefer hematocrit levels... but then there is always someone making a great deal of differentiating between the two parameters and making all sort of diagnostic conclusions. 'Hct is better to determine dilution of the patient' or 'Acute blood loss is better determined by Hb than Hct'... and so on.

So here's the question: What actually is the difference between Hb and Hct? Do we need to measure both in clinical practice?


What's the difference?

Hemoglobin levels are mostly measured by automated machines designed to perform different tests in blood. Within the machine, the red blood cells are broken down to get the haemoglobin into a solution. The concentration of haemoglobin is then measured by spectrophotometry using the methemoglobin cyanide method.

Hematocrit levels in contrast are actually calculated by an automated analyzer... It is actually not measured directly! The analyser multiplies the red blood cell count by their mean corpuscular volume.


What is Fact?

There simply is NO difference between Hemoglobin and Hematocrit by means of clinical information!
  • In fact, virtually all haemoglobin in our blood is contained within erythrocytes
  • Therefore, whether the amount of Hb per litre of blood is determined or the blood’s volume occupied by the Hb filled erythrocytes is determined, similar information is gained.
  • Nijboer at al. have brilliantly proven that Hb and Hct correlate in all ranges and all patients and also nicely show this in their figure 1 (see below)
  • The only rare exceptions are macrocytic and polycytemic anaemia in which the Hct is defined by erythrocytes containing a normal mean corpuscular Hb concentration

Conclusion
  • Hemoglobin is NOT different from Hematocrit
  • Both parameters provided identical clinical information


                                                 Once and for all!


Nijboer J et al. J Trauma. 2007;62(5):1310-2.

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Jonathan
27/8/2015 15:07:28

"The only rare exceptions are microcytic and polycystic anaemia" -- I don't know about polycystic anaemia, but microcytic anaemia isn't rare!

CHARLENE WASHINGTON link
3/7/2017 18:02:47

Thank you I work in a lab we occasionally have to cancel and re-order blood work, I had a order for Hemoglobin and Hematocrit I decided to order only the hemoglobin and I begin to doubt myself as if I did the correct thing by ordering just the hemoglobin instead of ordering the two of them together as the doctor sent down to the lab.

Tim @BIJCorg
27/8/2015 15:39:40

Hi Jonathan,

thanks indeed for your feedback! This shows what a simple exchange of one letter can do! It's mAcrocytic anaemia of course... Corrected now!

But hey, thanks for making this post peer reviewed, Tim ;)

Beth Neary
19/10/2016 17:13:05

thank you for this... I am an MD trying to explain this to a community group and this made it clear

Alicia
2/5/2017 15:48:22

Thank you for your post. Clinically, there is no need to measure both in most cases. However, can you confirm that for research purposes (estimating prevalence of anemia) if one clinic is collecting hematocrit and the other hgb, these clinics can't be compared regarding the level of anemia? I'm seeing Quinto, et al 2006 'Relationship between haemoglobin and hematocrit in the definition of anaemia' in Tropical Medicine and International Health, vol II (8). 1295-1302. The posted comments also suggest this is the case, right?. (I'm an MPH student in public health, so I apologize if this is a very novice question to ask....I'm a novice!).

Tim link
10/5/2017 12:50:03

Dear Alicia,

Thanks for your comments on this post. As mentioned in the short summary in most clinical scenarios there is no real difference between hemoglobin and hematocrit. As pointed out above, there are some exceptions like macrocytic and polycytic anemia in which the hematocrit is defined by erythrocytes not containing a normal mean corpuscular hemoglobin concentration.
Another theoretical exception might be a state of relevant hemolysis in which free plasma hemoglobin would occur.
The question is also what sort of information you are looking for. If you're looking into oxygen capacity and oxygen binding capacity you will be better off measuring hemoglobin. In polycytemic states hematocrit might be of greater interest. Finally, comparing two different kind of parameters for research purposes is never a good idea as unintended bias might occur. Hope this helps, best regards!

Paul
24/7/2017 04:43:38

So if they are the same the ratio Hct/Hgb should be constant ~ however mine has changed from 3.5:1 to 3:1 in the last two years. Obviously not the same !

pachchi
10/9/2017 09:13:47

Thanks for the post.!
can you throw some light on how it may vary in newborn especially in a preterm, where there is increased Hct/Hb and it would change as the baby grows.?
Has it any bearing on the different cutoff we use to transfuse these growing preterm

Budding Doctor
6/3/2018 08:36:26

Really nice...Had this doubt for long time... you explained it clearly..

Mark Peters
11/3/2019 15:02:53

A correlation on a scatter plot is not proof of equivalence. For example if the gradient is not 1 then there will be a systematic difference. There is a close correlation between heart rate and respiratory rate but does not mean they contain the same information.
Bland-Altman plot would be more informative surely

Simone
6/4/2019 11:25:15

Dear Timothy

Please inform is this is also the case with blood gas analyser results. Do they use the same techniques to measure as you have stated.
I am an anaesthetist and we regularly look at arterial blood gas sample results and these show the Hb AND the haemoatocrit. Does what you have stated apply here too?


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