Patients do seem to get a little confused about “blood tests”. Your ‘usual’
blood tests do not test for “everything”. The reason for this is simple. There
are so many tests that can be done, that testing would go on for weeks if you
wanted “everything” checked. For example, the Australian Royal College of
Pathologist’s Manual of Use and Interpretation of Pathology Tests that sits on
my desk, lists 150 pages of tests that can be carried out. These include such
items as a Reptilase Time, something I have never requested in 40 years of
practice, or a red cell Galactokinase - ditto.
No, when we send you off for a blood test, we have to try and
be reasonably specific, and sometimes even have to give the pathologists a clue
as to where we are heading, and be guided by them as to some specific testing.
However, many times we are really just casting a ‘wide net’
to see what abnormalities we can turn up to use as a pointer towards the
definitive diagnosis. One of the commonest is the “Complete Blood Count”,
usually called a CBC, since we medico’s love acronyms, but remember this testing
is in reality very far from “complete”.
The CBC does provide important information about the kinds
and numbers of cells in the blood: red blood cells, white blood cells, and
platelets. The CBC can help us evaluate symptoms such as weakness, fatigue, or
bruising and even directly diagnose conditions such as anemia, infection, and
many other disorders.
The CBC test usually includes the white blood cell (WBC)
count as these cells protect the body against infection. If an infection
develops, white blood cells attack and destroy the bacteria, virus, or other
organism causing it. White blood cells are bigger than red blood cells and
normally fewer in number. When a person has a bacterial infection, the number of
white cells can increase dramatically. There are five major kinds of white blood
cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The
numbers of each one of these types of white blood cells give important
information about the immune system. An increase or decrease in the numbers of
the different types of white blood cells can help identify infection, an
allergic or toxic reaction to certain medications or chemicals, and many
conditions (such as leukemia).
The red blood cell (RBC) count is also part of the CBC. Red
blood cells carry oxygen from the lungs to the rest of the body. They also help
carry carbon dioxide back to the lungs so it can be exhaled. The red blood cell
count shows the number of red blood cells in a sample of blood. If the RBC count
is low, the body may not be getting the oxygen it needs. If the count is too
high (a condition called polycythemia), there is a risk that the red blood cells
will clump together and block blood vessels (thrombosis).
Another part is the Hematocrit (HCT). This test measures the
amount of space (volume) red blood cells occupy in the blood. The value is given
as a percentage of red blood cells in a volume of blood. For example, a
hematocrit of 38 means that 38 percent of the blood’s volume is composed of red
cells.
Hemoglobin (Hb). Hemoglobin is the substance in a red blood
cell that carries the oxygen. The hemoglobin level is a good indication of the
blood’s ability to carry oxygen throughout the body.
There is also the Platelet (thrombocyte) count, which is an
important part of the CBC. Platelets are the smallest type of blood cell and
play a major role in blood clotting. If there are too few platelets,
uncontrolled bleeding may be a problem, such as occurs in Dengue Hemorrhagic
Fever.
So even though the CBC does test for many factors, there are
still another 149 pages of tests that can be done! If you want to know your
blood group, or your HIV status, you have to ask for this separately! These are
not the ‘standard’ tests, which are designed to assist us towards a diagnosis.
So now you know!
