Name: _________________________________
Period: ________________
Diagnosing Illnesses: Blood Cell Counts
Introduction:
One tool that doctors use for diagnosing illnesses is blood cell counts. This can be done easily with a
compound light microscope and a sample of the patient’s blood. A slide of the patient’s blood is
prepared, and red and white blood cells are counted to determine the relative abundances of each. For
this activity, you will be using prepared slides of human blood for your blood cell counts.
Ideally, all of a patient’s blood cells could be counted to provide precise information about relative
abundance. However, this is not possible, and even counting all of the cells in a single sample is
impractical. Fortunately, counting the cells from just a few fields of view can provide reasonably
accurate information.
White blood cells (WBCs) are fairly easy to count because there are generally far fewer WBCs than
there are red blood cells (RBCs). The number of RBCs per field of view is usually estimated through
one of several common methods. Two methods are described here. It is up to you to choose which
you use. Whichever method you use however, there are some very important considerations to keep in
mind.
1. First, the field of view must be randomly selected. This is accomplished by moving the slide
without looking through the eyepiece, and then checking to make sure that the field of view meets
the following other requirements. If it does not, the slide must be moved again to a different random
location. It is important to randomly select your field of view because individual preference may cause
some people to choose fields of view with more or fewer WBCs than is actually representative of the
sample as a whole.
2. The blood cells must be uniformly distributed throughout the field of view. This means that any
given region of the field of view has roughly the same number of blood cells as any other region of the
same size. Below are examples of uniform and non-uniform fields of view.
3. The field of view must be away from the edges of the cover slip on the slide. Sometimes, white
blood cells will be more or less likely to be found around the edges of the cover slip. This will distort
the accuracy of your WBC to RBC ratios.
Quadrant Method: In this method, the field of view is imagined as being divided into four equal
portions (quadrants). If your microscope has a pointer, it may be helpful to use the pointer as one
boundary of one quadrant. One of the quadrants is then selected, and all of the cells in that quadrant
are counted. (Count only cells that are at least halfway in the quadrant.) This number is then
multiplied by four to estimate how many total cells are in the entire field of view. When counting
white blood cells, it is important to count all of the WBCs in the entire field of view. This can then be
subtracted from the total number of cells to determine the number of RBCs.
Diameter Method: This method uses blood cells as the unit of measure for the diameter of the field of
view. An imaginary line is drawn through the center of the field of view, and any blood cell that it
passes through is counted (again, it may help to use one edge of the pointer as part of the line). This
number represents the diameter of the field of view in blood cells. (Divide it in half to get the radius.)
The area of the field of view in blood cells can be calculated using the formula for the area of a circle
(A=πr2). Examples are given below.
The accuracy of this method can be improved by calculating the number of cells for each field of view
twice, using perpendicular diameters (double diameter method). The two calculations can then be
averaged.
Procedure:
1. Obtain a prepared slide of human blood cells and using one of the methods described above,
determine the numbers of red and white blood cells for at least three fields of view. Use the same
counting method for all three field of view.
FOV #1: Total # of Cells=______
# of RBCs=______
# of WBCs=______
FOV #2: Total # of Cells=______
# of RBCs=______
# of WBCs=______
FOV #3: Total # of Cells=______
# of RBCs=______
# of WBCs=______
Counting Method Used:
Quadrant Method
Single Diameter Method
Double Diameter Method
2. Use the above data to determine the ratio of RBCs to WBCs for your sample.
Total RBCs __________  Total WBCs __________ = __________
3. Collect ratio data from your classmates on at least five other blood samples and average the results.
4. Look closely at the white blood cells on your slide. Search until you find at least three WBCs that
are distinctly different in appearance. Draw the three cells in the boxes below.
It should be noted that in most modern medical facilities, cells are counted using a flow cytometer. A
flow cytometer uses a computer to rapidly count cells as they pass one by one through a narrow tube.
Alternatively, blood samples can be centrifuged to separate them into individual components (RBCs,
WBCs, platelets, and plasma), and the masses or volumes of each used to calculate the numbers of
cells.
Questions and Analysis:
1. A healthy human has approximately 5 million red blood cells per μL of blood, and approximately
10,000 white blood cells per μL of blood. What is the RBC to WBC ratio for a healthy human?
2. How do the results for your blood sample compare to the expected results for a healthy human?
Explain any discrepancies.
3. Compare your data to the data collected from your classmates. How do your results compare to the
averaged results?
4. What could explain any differences between individual results and average results?
5. Why is it important to always count the white blood cells for the entire field of view rather than
estimating the number through either the quadrant method or the diameter method?
6. Why do you think different white blood cells can be so different in appearance?
7. Are red blood cells as varied in appearance? Why or why not?
8. Use the internet or another resource to identify and label each of the white blood cells that you
drew above.
9. How do you think additional information for diagnosing an illness could be gained by looking only
at the white blood cells?