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Units for common medical lab values

Figure out how to interpret the units in common medical labs including the CBC, Chem 10, and LFTs. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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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.