What, Exactly, Is A Unit?

The Quizzically-Named Blood "Unit"

Packed red blood cells are measured in the odd units of…”units”. 

There is very little meaning to this term.  Typically, we measure liquids by the King’s pleasure with terms like “quart” and “gallon”.  Or, if in a scientific mood (as we are in the hospital most days) with terms like “mililiter” or “deciliter”.  Of course there are similar words for solids. 

Each term boasts about as much tangible meaning in this world as any in the English language.  Similar to the idea that a dollar is actually a representation of a chunk of gold in some vault somewhere, so too are the words we use to describe matter backed by very specific – material – standards.

But a unit?  Even the guy I called at our hospital blood bank didn’t know what the term meant, exactly.  So, I done did me some research…

The term relates to the fact that blood is only partly liquid.  Really, human blood is nearly 50% solid matter – blood cells, clotting factors, platelets.  Lots of cool stuff, actually.  Blood is so complicated, doctors regard it as a human tissue.  

I also imagine that we describe blood in units because the substance looks so stark and bright and scary;  it just demands mad respect.  One of the most spectacularly beautiful things I ever saw in medical school was microscopic slides of red blood cells.  I used to stare at them for hours (which is why I nearly flunked histology).

A unit of blood typically is about 450 mL, which is about .95 pints.  If you order a unit of some component of blood, say, plasma, you will get the amount of plasma that would be found normally in a unit of blood.  Again, not especially exact.

BLOOD_0125_JRH_19516Here’s the thing:  A human body contains, roughly, 12 units of blood.  12.  That isn’t much.  So, when a person gives blood, they generally only give about 1 unit.  Any more than that and they’d have a pretty tired week.

The other day, we gave one patient 24 units of blood. It took 24 people deciding to take time out of their day, endure an IV, sit around with an outdated shallow magazine about surgically-enhanced humans…and donate part of their living body tissue to help this one person.

I order 2 to 4 units of blood for patients on a semi-regular basis.  I’d say I do it every other time I’m on a medicine week in the hospital.  As a doctor, I look at the patient’s needs; rarely do I actually think about where the supplies that I order come from.

Really, if you’re sitting around wondering what you can give away that genuinely, truly helps humanity.  Give a little blood.  It’s gross, the IV hurts and it takes up a not insignificant amount of your day.  And it’s measured in weird amounts that have no meaning so you won’t really know what it is that you actually donated.  

But you really do help others in dire need when you give blood.  Nobody “just kinda” needs a blood transfusion.  I’m not sure what we’d do if people didn’t donate regularly.  We really do save lives with the stuff.  So, when you get the chance, give it a try.

8 thoughts on “What, Exactly, Is A Unit?

  1. secretwave101

    Really? They shot you down just for living in Haiti?

    Nice. Probably based on LOTS of good data. Not.

    Just keepin’ it pure, man.


  2. Bob

    Hi doc! Sorry for this being off topic but I have an appointment with a urologist in a few days for an enlarged prostate.

    I know the doc will probably need to do a digital rectal exam. Should I take a laxative or something else similar to what you’d take before a colonoscopy in order to make sure the rectal cavity is empty?

    This is rather embarassing and I don’t want to make this any more unpleasant for myself or the doctor than it has to be.

    Sorry to have to ask this and thanks for any advice you could provide.


  3. Bob

    Sorry. A couple more questions: If I should use a laxative which one should I use, and how long before the appointment should I use it? Thanks.


  4. Tara

    Many people tell me that if they visit Haiti for even one week they are then not allowed to give blood in the USA for one whole calendar year. I was wondering if you knew what the rationale was? Could it be Malaria? Like it could lie dormant or something? I truly want to know if there is any reason other than the usual “we are afraid of Haiti” thing that the State Dept has stirred up with all of its travel warnings.


    1. secretwave101

      Yes, it’s a malaria thing. What I didn’t know was that there was a stipulation on donors who have spent even a short time in countries where the disease is endemic.

      I’m also not sure I’m convinced, like you, that the rule is entirely scientific or medical. A year is a long time. I’ll do some hunting around and see what I can turn up. In a bit, I’ll also put together a run-down on malaria dormancy, etc. You probably already know about that stuff, but it could be interesting for others.


      1. Tricia

        I work for a company that does blood banking. I work on the testing side, but I know that we do exactly zero tests for malaria because it’s an insanely manual process and expensive. Especially in the US where malaria isn’t common like other countries. I’ll tell you, you could get heart surgery really cheap in India, but there’s a good chance you could take home some malaria with you because they don’t test for malaria either even though they are in an endemic area…I’d rather the blood banking industry be safe rather than sorry.


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