Acute myeloid leukaemia (AML) can be divided up (or classified) into various sub-types. This is important as not all types of AML are treated in the same way. Your doctors need to know which type of AML you have to help them plan the most appropriate treatment for you.

Your bone marrow sample will be tested to find out which sub-type of AML you have. Tests that may be done include:

• Examining the leukaemia cells under the microscope - to see what type of blood cell has become abnormal and at what stage of its development.
• Testing leukaemia cells with antibodies to look for specific proteins on their surface. This is called immunocytochemistry, and also helps doctors to identify what type of cell has become abnormal.
• Looking for particular types of changes in the chromosomes of leukaemia cells. This is called cytogenetics. It can help doctors to predict how well the leukaemia may respond to treatment.

Almost all the cells in our body contain chromosomes. Chromosomes are made up of genes, which control the activities of the cell. There are often changes in the structure of the chromosomes in leukaemia cells. A test on the bone marrow sample, called a cytogenetic test, looks for these changes.

Different types of AML are associated with particular genetic changes. So these tests can help doctors to decide on the best treatment and predict how well the leukaemia may respond to it.

In the UK, doctors usually classify AML according to the World Health Organisation (WHO) system and the French American British (FAB) system. The WHO system classifies AML according to the type of cell that has become abnormal and whether:

• there are particular chromosomal changes (cytogenetics)
• there are abnormal changes in more than one type of blood cell
• the leukaemia has developed from a previous blood disorder called myelodysplasia
• the leukaemia is related to previous cancer treatment (treatment-related AML).

This system is important as it is useful for planning treatment and predicting response.

The FAB system looks at the appearance of the leukaemia cells under a microscope (morphology). Each type of AML is named according to the cell type and given a number from M0 to M7.

Doctors may classify AML with the FAB system while waiting for the results of other tests.