What Is a CBC Test and Why Did Your Doctor Order It?

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Topics include: Treatment and Understanding

What is a CBC and why did your doctor order it?  In a segment from our recent webinar, Patient Power Founder, Andrew Schorr, poses this question to panelists Dr. William Wierda, Dr. Susan Leclair and Lynn Ingram.  The experts discuss and explain in detail the CBC test, the parameters with which your doctor is concerned, and how the test is performed in the lab 

Sponsored by AbbVie. Produced by Patient Power in partnership with AACC's Lab Tests Online and The University of Texas MD Anderson Cancer Center.

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Please remember the opinions expressed on Patient Power are not necessarily the views of our sponsors, contributors, partners or Patient Power. Our discussions are not a substitute for seeking medical advice or care from your own doctor. That’s how you’ll get care that’s most appropriate for you. 

Andrew Schorr:

So let's start with the basics, and I want to start with you, Bill.  And here's my question:  Why does a physician order a blood test?  What do you want to check out just as a baseline?  The patient walks in the door, why would you say I need to go down to the lab?  How come? 

Dr. Wierda:

So it partly, it mostly depends on what the diagnosis is.  So what the blood test gives you, there are several things that you can order in terms of blood tests.  You can get a complete blood count, or CBC, and you can get the chemistries as well as a number of other tests.  So what the physician orders will depend on what the patient's diagnosis is. 

In terms of what I look for and what I look at, and just paying particular attention to the CBC, what I'm interested in knowing depends a lot on what the diagnosis for the patient is, but the parameters that are most important for us, for most patients, are the white blood cell count.  

That gives us how many of the white blood cells, and then the next parameter would be the differential, which tells us how many of each type of white blood cell is there.  And then the next parameter that I think is important that we look at across the diagnoses is what the hemoglobin is, and then the platelet counts.  So those parameters are the most important parameters. 

When you get a CBC you get a number of different results.  Most of them we don't look at.  The ones that we really look at and pay attention to are the white blood count, as I mentioned, the differential, the hemoglobin and the platelet counts.  The other ones we do look at occasionally because they can be important for certain diagnoses, but for the most part those are the most important things that we pay attention to.

Andrew Schorr:

Okay.  We're going to get more into when you look at certain things, but I want to get, dig in to some of these things that show up on the blood test.  I will mention also for our audience that of course we're tested at the outset. 

Walk in door, hi, Dr. Wierda.  You're going down to the lab.  Here's the lab order.  But it's also when we are being followed.  For me with CLL it was four-and-a-half years before I ever had any treatment, but depending upon the diagnosis and your situation when you're in treatment—and I know I get blood tests all the time when I was at the FCR treatment for CLL years ago, not just because I was in a clinical trial but because they wanted to monitor me carefully.

And then maybe when you've completed treatment you're being monitored, but maybe you're going to need treatment again.  And do you?  So putting all that in context, Susan Leclair, so Dr. Wierda mentioned a couple things.  He said a couple things I don't know if we always understand.  So he said differential.  So could you help us understand what differential means and use different components of it and why they may be important?

Dr. Leclair:

All right.  And I'm going to start by first apologizing for whatever coughing and scratching you're hearing.  It's a cold, and I can't seem to get rid of it.  

The white blood cells come in five major categories.  Any one of them can be increased or decreased depending upon the situation.  So the logical question from Dr. Wierda to me is it's nice that you got a white count, what kind of cells are they?  Are they mature?  Are they immature?  Do they look functional or not?  

There are two ways to do a differential.  The first one is an automated differential.  The multi?channel instruments we have do those quite well.  They count a very, very large number of cells, so they would be able to define cells that have granules as opposed to cells that do not.  Cells that have a large nucleus against those that do not.  Those are the kind of characteristics that an automated differential is going to give you. 

Then there is something called a traditional or manual differential, and that is a much narrower number of cells.  In an automated differential the machine might count a 1,000, 2,000, 50,000 cells.  I'm not going to do that.  I'm going to sit at a microscope and count anywhere between 100 and 500, more often than not 100, cells, and I'm going to be able to use whatever experience and nuance that I might have to say a little more about the quality of the cells that maybe an instrument might miss or might question.  

An instrument might flag something, saying I don't know if it's X or Y, and then it's back to the human to be able to determine is it X or Y.  So what you'll get when you get a differential is you'll get a count from the machine, it's usually got either ABS or a pound sign after it, and that's the machine evaluation.  And then you'll get something with different—with a percentage sign after it, and that's the manual or individualized differential.  Both give you important information.

Please remember the opinions expressed on Patient Power are not necessarily the views of our sponsors, contributors, partners or Patient Power. Our discussions are not a substitute for seeking medical advice or care from your own doctor. That’s how you’ll get care that’s most appropriate for you. 

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Page last updated on July 17, 2017