Youth Cancer

Blood Test

Blood tests help diagnose cancer and can sometimes be the first indicators of disease. They’re used to track the progression of disease, and to help doctors make decisions about treatment.

Unfortunately with cancer, blood tests will become a regular event in your life, so it's important that you understand what they're for.

  • They help diagnose cancer and can sometimes be the first indicators of disease.
  • They’re used to track the progression of disease, and to help doctors make decisions about treatment.

How are they done?

  • The blood is usually taken from a vein, generally in the arm at the elbow.
  • To take the blood easily, you’ll need to have a tight band put around your arm called a tourniquet, then a small needle is inserted into the vein. It might sting a bit when the needle first goes in.
  • Attached to the needle is either a small blood bottle about the size of an adult's finger, which draws the blood off under a low pressure, or a syringe, which has a plunger and is pulled back to get the blood.

Once they have the sample, the needle is removed and the tourniquet is taken off.

Many people don’t like to see needles or worry about seeing blood being taken and can feel faint. You will be sitting down when you have it done, but if you do feel faint let the person who is taking the blood know.

Blood counts

Doctors will often order a complete blood count (CBC), and track blood counts closely throughout treatment. Why?

Well, three primary types of blood cells are formed in your bone marrow:

  • red cells
  • white cells
  • platelets

Certain kinds of cancer affect the normal production of these cells. Some of the treatments you will receive, especially chemotherapy, will also affect your body's ability to produce blood cells.

Here are some of the most important 'counts' tracked during treatment:

Red Blood Cells

Your red cells are primarily responsible for carrying oxygen throughout your body.


Haemoglobin (Hgb or Hb)

Red cells contain haemoglobin, the molecule that carries oxygen and carbon dioxide in your blood throughout your body.

If your haemoglobin is low, you’ll be anaemic with symptoms like pale skin, shortness of breath, and fatigue. You may have low haemoglobin at diagnosis as well as during treatment because of your bone marrow's inability to produce new red cells.


Haematocrit (Hct)

Haematocrit refers to the volume of red blood cells in your system. (This is also called packed cell volume - PCV). Your haematocrit count is the ratio of red cells to plasma (the liquid part of your blood).

It is expressed as a percentage. As an example, if your haematocrit is 30 it means that 30% of the blood that was drawn is red cells; the remaining 70% is plasma.

When you’re on chemotherapy, your marrow's ability to make new red cells is decreased, so your haematocrit may go down. Because there is less oxygen in your body, you will feel tired and have little energy. If your haematocrit drops below about 18%, you may need a red blood transfusion. Your medical team will help you out with this kind of information.

White Blood Cells (WBCs)

Your white blood cells are responsible for fighting infection. Because cancer treatment affects your body's ability to produce white cells, your risk of infection becomes higher. There are a number of different WBCs in your blood.


White Blood Cell Differential (DIFF)

The differential (or diff) refers to the distribution of different kinds of white cells in your blood. Each type of white cell will be listed as a percentage of the total. Neutrophils are the most important-infection fighting white cells.

On your lab reports, the "differential" or "diff" will show the percentage of each type of white cell, all together equalling 100%.

As an example:

  • Segmented neutrophils (or segs) 49%
  • Band neutrophils (or bands) 10%
  • Basophils 1%
  • Eosinophils 1%
  • Lymphocytes 29%
  • Monocytes 10%

Absolute Neutrophil Count (ANC)

This count is a measure of your body's ability to fight infection. It is the percent of neutrophils (segs + bands) multiplied by the total white blood count (WBC). The magic number is an ANC of 1,000 or more. When your ANC is less than 1,000, you are at a high risk for infection.


Platelets (Plt)

Platelets are the cells in your blood necessary to stop bleeding. Because chemotherapy affects your bone marrow's ability to produce platelets, you will probably need platelet transfusions during your treatment.

Signs of low platelet count include:

  • Bruising with no obvious cause
  • Any unusual bleeding (nosebleeds, bleeding gums, prolonged bleeding from a cut)
  • Petechiae (small red or purple spots on the skin)

Blood Cultures

If your ANC (Absolute Neutrophil Count) is less than 1,000 and you develop a fever of more than 38.0 C, the risk of serious bacterial infection is high. You will probably be hospitalised and started on IV (intravenous line) antibiotics.

To find out exactly what antibiotics to use, blood cultures might be drawn from your central line and sent to the lab. There, they can find out what type of bacteria grows and choose the best antibiotic to kill it.

If there’s ever a risk of bacterial infection, extreme caution must be taken. Even if you feel perfectly fine, this precaution is still necessary. Spending a few days in the hospital - just in case - is much better than taking a chance with a potentially life-threatening