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# Units for common medical lab values

Video transcript

In the last video, we talked
about normal blood values and the ranges as well. But we didn't
really get to units. So I thought we would
talk about that now. I think the best way to think
about units and the information they tell you is to break it
into two basic categories. One is amount, and
the other is function. And we'll talk about
each of them in turn. So what I mean by
amount is, let's say that you want to convey
to someone information about the amount of something. What are the different
ways you could do it? One way to do it is you could
say, well, what about the mass, let's figure out the
mass of whatever it is you're interested
in, and the volume. And you could
convey information-- you could say, well, it's
maybe five grams in a liter. Or, in the case
of hemoglobin, you could say it's 15
grams in a deciliter, and a deciliter just
means 1/10 of a liter. So you can actually convey
information that way. And it's actually
done a few times, you can see, on this
list of lab values. So you can see that we
do that for hemoglobin. And we also do it down here
for blood urea nitrogen, the fasting glucose,
we do it for calcium, we do it for phosphate, a
few things, total bilirubin. Direct bilirubin. So this strategy of
conveying information by simply figuring out
the mass over a volume is used a few times. But here's something that
I think a lot of people sometimes miss in lab
values, and that's the fact that you can actually compare
things to one another. So I could, for example,
I could look at this list and say, well, hemoglobin, I
have 15 grams in a deciliter, but I only have 15
milligrams of blood urea nitrogen in a deciliter. So in one deciliter,
the same volume, I have a thousand
times more hemoglobin than I do blood urea
nitrogen by mass. So that's actually
quite interesting because that tells you that
it seems on the surface, at least, that
hemoglobin by mass then, obviously, is much,
much more common in the blood than
blood urea nitrogen. So that's one way to talk
about the amount of something. Now another way of talking
about the amount of something, let's switch colors,
would be just to count up, just to actually literally
just go and count up the number of
something that you find in a certain volume of blood. And we do that too sometimes. We actually will count,
for white blood cells, we actually count up 5,000
cells in one mililiter. Or, for platelets, you might
say there are 227,000-- and I'll put cells in
quotes because there are cell fragments
--in a microliter. And even looking at those two,
microliter versus milliliter, you can see that
there are way more platelets in the same volume
than there are white blood cells. They may be smaller, but
there are many more of them. So this is actually the second
way to talk about amount. Now if you look
through this list, do you see any other examples
of number of something in a volume? And if you're
thinking no, then I'm going to point something out
to you that might surprise you. Think about these--
milliequivalents per liter, all of those ions are
milliequivalents per liter. And so, thinking about what
milliequivalent means-- maybe we should use an example here
to make it a little bit more concrete, maybe this would
be a good example --sodium. So I have here a
sodium-- and remember sodium is a cation,
it's one charge. So I have 141
milliequivalents of it. And that's actually
the same as saying that I have 0.141 equivalents,
because we know milli- is just a thousandth, so that's
the same thing as that. And we know that, because sodium
has just one positive charge, that, in terms of equivalence,
one mole of sodium equals one equivalent of sodium. Therefore, if I have 0.141
equivalents of sodium, I have exactly the same
number, 0.141, moles of sodium. And you know that moles-- this
is just Avogadro's number, this is just 6.02 times
10 to the 23rd, just a big, big number. So if I actually multiplied
this times this-- actually the whole thing, obviously
--and I actually just did it right here
with my calculator, you can actually find that
this 0.141 times 6.02 times 10 to the 23rd is about 8.5
times 10 to the 22 sodium ions. So if I was to
literally count them up, it would obviously
take forever to count this huge number of ions up. But if I was to
do it, all that is is just a number over, in
this case, what do we have, a liter, that's a volume. So in one liter, that's how
many sodium ions I have. So, again, it's just a
number over a volume. So it's really no different. But I think the
term milliequivalent I think can be a little
bit surprising for folks to see that. And so we don't think of
it always as a number. Now the third way to think
about amount would be a percent. And one example would be
right here, all these numbers. So these are all percents. And here these percents
are of a number. So these are all percents of
that white blood cell number. So that would be a percent
of number over volume. So that's saying, for
example, for bands, 3% times 5,000 per milliliter. So that would be example one. And another way to
think about percent would be like this
one right here. You think about, what
does hematocrit mean? 45%. What is that percent of? That's a percent of
total blood volume. So if I took the total blood
volume, whatever that is, I just take some blood out of
my arm and it's 10 milliliters. Then 45% of it, or
4.5 milliliters, would actually be
red blood cells. And the rest, the
rest of that fluid, would be mostly water, of
course, but in that water it could be proteins and
immunoglobulins and all sorts of things that help
us stay healthy. So that percent represents
just how much of my total blood volume is taken up
by red blood cells. That's what that number is. Now we've talked about
amounts in three ways. But I've also mentioned
function at the bottom, I talked about function
in the beginning of this. So now imagine that
you have a situation-- let's say you have
a little enzyme, I'm going to draw it
here, it's a little y. And let's say
enzyme y has a job. And this enzyme's job is to
take little molecules like this, five of them, and add
little red things to them. So it's supposed to add
little red things to them. And you let this go
on for five minutes. You basically just keep
an eye on enzyme y. And you see how many
does it convert. And you find out that, in
the end of five minutes, it actually was able to
convert four of them. So you get four converted
in five minutes. So far so good. That's fine. Four were converted. Now you have another person. They have enzyme y as well. And they have more of it. So they have more enzyme
y than the first person. And this person
has the same task. Or you basically
give them a bunch of these little purple
molecules, whatever, they are, and you say, well how many can
they converse in five minutes? And you might think
well, obviously, that there's more of the
yellow enzyme y protein. So if they have more of
the enzyme y protein, they can obviously do
more than person one. This is person one up here. Let's say this is a person
one, and this is person two. So person two, after five
minutes-- let's say you time it --they actually convert three. So they were able to convert
three, and five minutes are up. So a little surprising, But now you think back
on this and say, OK, well if I was, let's
say, to just figure out the amount of an
enzyme-- let's say your job, just as in part
one, was to figure out the amount of an enzyme
--and I did it by mass, I use the mass over
volume of the enzyme. Then person two would look
like they have more, over here. Or if I did the
number of enzyme, then again person two looks
like they have more. Or if I did some sort of
percent, I don't know, percent of all enzymes,
again, person two looks like they have
more because they have just more of
the enzyme around. But if you look at
function, if you actually look at the amount
of work that's being done by these enzymes,
or the outcome of this enzyme, then, clearly, person
one actually wins out. They have more of the
work done than person two. So if you're looking
at actual activity-- and that's what this
is when I say function, you're looking at
enzyme activity, or sometimes it's even
hormone activity --then that would be a different story. So actually let me make a
little bit of space here. So function really alludes
to-- let me give you a nice, there we go --so this alludes
to hormone or enzyme activity. And, in this case,
person one would actually have a higher number because
they have more enzyme activity in this case. And so all three of these,
these at the bottom, are examples of
looking at activity. So whenever you
see IU or U, that refers to international
units or units. So it's a way of standardizing
across the entire globe, or sometimes if it's
through a set of labs, just in one country how
we look at the function of a certain enzyme. And so anytime
you see IU over L, it's not really telling you
about the amount of an enzyme, although of course
you would expect that there would be
some correlation. But it really tells you
more about the function of an enzyme.