One of the unavoidable consequences of CML is enduring various tests and procedures. These are a necessary evil because they pinpoint your diagnosis and tell your hematologist the stage of your disease and how well you’re responding to treatment.
Here is a summary of some of the tests.
Complete blood count (CBC)
Procedure: Obtaining a sample of blood.
Why it’s done: The blood is analysed to determine the number of different blood cells, such as white blood cells (WBCs), red blood cells (RBCs), and platelets. A high WBC count (above 10 billion per litre of blood) indicates leukemia (the normal value is about 4.5 to 7 billion WBCs per litre). Note that the units are per litre – but you won’t have to provide a litre of blood. Only a small sample is taken. A “differential” is also done to determine the relative proportions of different types of WBCs (neutrophils, eosinophils, basophils, lymphocytes and monocytes).
When it’s done: At the time of diagnosis. Additional blood tests are done every three months (for cytogenetics and PCR, see below).
Bone marrow testing
Procedure: A needle is inserted into a bone (usually the pelvis) and a small sample (less than a teaspoon) of bone marrow is removed.
Why it’s done: The marrow is where blood cells are formed and a sample is needed to identify abnormalities in these blood-forming (or hematopoietic) cells. In CML, the blood is the problem but the bone marrow is the source of the problem.
When it’s done: At the time of diagnosis. Another bone marrow sample may be obtained at 6 months, and every 6-12 months thereafter.
Several tests can be performed on the bone marrow sample (all from one sample). Your doctor looks at the size and appearance of the marrow cells. Cytogenetics examines the genetics of cells to look for abnormalities. The liquid part (called the aspirate) is used to estimate the proportion of immature cells (called blasts), which indicates the phase of the disease process. A lower proportion of blasts (less than 15%) indicates chronic-phase CML. This proportion is higher in accelerated-phase CML (10-19% or 15-29%, depending on the criteria used), and higher still (more than 20-30% depending on the criteria) in blast-crisis CML.
Once you start treatment, your cytogenetic response may be determined using cytogenetic testing of the marrow or blood sample.
A more rapid technique that evaluates more cells (so it’s more sensitive) is FISH (for fluorescence in situ hybridization). FISH identifies abnormalities by tagging segments of DNA with different colours so that leukemic cells (containing the Philadelphia chromosome) can be more easily counted.
The most precise method of testing bone marrow or blood samples is PCR (polymerase chain reaction). This allows technicians to take a small sample of DNA and make more copies of it. (PCR is used by police labs for DNA testing of samples obtained at crime scenes.) PCR can detect one leukemia cell in 10,000 cells and so it’s a highly sensitive way of detecting leukemia down to the level of the molecule. A limitation is that it can’t detect other genetic abnormalities that may be present in CML, so PCR is used in conjunction with other tests.