Routine microscopic exam: Any samples taken (blood, bone marrow, or cerebrospinal fluid) are examined under a microscope by a pathologist (doctor specializing in diagnosis of disease by laboratory tests) and are often also reviewed by the patient's hematologist/oncologist (doctor specializing in medical treatment of cancer and blood diseases).
Based on the bone marrow cells’ size, shape, and granules, doctors can classify them into specific types. A key element of this cell classification is whether the cells appear mature (resembling normal cells of circulating blood) or immature (lacking features of normal circulating blood cells). The most immature cells are called blasts.
The percentage of cells in the bone marrow that are blasts is particularly important. Having at least 20% blasts in the marrow is generally required for a diagnosis of acute myeloid leukemia. IAMLcan also be diagnosed if the blasts have a chromosome change that occurs only in a specific type of AML, even though the blast percentage doesn’t reach 20%. In order for a patient to be considered to be in remission after treatment, the blast percentage must be no higher than 5%.
Sometimes this examination does not provide a definite answer, and other lab tests are needed.
Cytochemistry: Cytochemistry studies involve placing cells from the sample on glass microscope slides, then exposing them to chemical stains (dyes) that are attracted to or react with only some types of leukemia cells. These stains cause color changes that can be seen only under a microscope. For example, one stain distinguishes AML from acute lymphocytic leukemia (ALL). The stain causes the granules of most AML cells to appear as black spots under the microscope, but it does not cause ALL cells to change colors.
Flow cytometry: This technique is often used to examine the cells from bone marrow and blood samples. It is very accurate in determining the exact type of leukemia.
Leukemia cells can be distinguished by the kinds of molecules on their surface. A sample of cells is treated with special antibodies which stick to the cells only if certain molecules are present on their surfaces. The cells are then passed in front of a laser beam. If the sample contains cells that now have antibodies attached to them, the laser will cause them to give off light, which is measured and analyzed by a computer. Groups of cells can be separated and counted by these methods.
Immunocytochemistry: During this test, as in flow cytometry, cells from the bone marrow aspiration or biopsy sample are treated with special antibodies that react only to certain molecules. But instead of using a laser and computer for analysis, the sample is treated so that certain types of cells change color. The color change can be seen only under a microscope. Like flow cytometry, it is helpful in distinguishing different types of leukemia from one another and from other diseases.
Cytogenetics: These tests involve looking at a cell’s chromosomes under a microscope. Normal human cells contain 46 chromosomes, pieces of DNA that control cell growth and metabolism.
In certain types of leukemia, 2 chromosomes may exchange some of their DNA, so that part of one chromosome becomes attached to part of a different chromosome. This change, called a translocation, can usually be seen under a microscope. Other changes in chromosomes, such as inversions, deletions, or additions, are also possible. Recognizing these changes helps to identify certain types of AML and is important in determining the outlook for the patient.
The testing usually takes about 3 weeks, because the leukemic cells must grow in laboratory dishes for a couple of weeks before their chromosomes are ready to be viewed under the microscope. The results of cytogenetic testing are written in a shorthand form that describes which chromosome changes are present.
A translocation, written as t(1;2), for example, means a part of chromosome 1 is now located on chromosome 2 and vice versa.
An inversion, written as inv(16), for example, means that part of the chromosome 16 is upside down and is now in reverse order but is still attached to the chromosome it originated from.
A deletion, written as del(7) or -7, for example, indicates part of chromosome 7 has been lost.
An addition, +8, for example, means that all or part of chromosome 8 has been duplicated, and too many copies of it are found within the cell.
Molecular genetic studies: Special tests of leukemia cell DNA can also find most translocations that are visible under a microscope in cytogenetic tests, as well as some translocations too small to be seen with usual cytogenetic testing under a microscope.
This sophisticated testing, called FISH (fluorescent in situ hybridization), is helpful in classifying leukemia because many subtypes of AML have distinctive translocations. Information about these translocations may be useful in predicting how the patient will respond to treatment.
These tests may also be used after treatment to find small numbers of leukemia cells that can be missed under a microscope.