Acute lymphoblastic leukemia (ALL) in children — what every parent needs to know

The complete blood count is a simple blood test that measures all three types of blood cells. In a child with ALL, the CBC typically shows one or more of the following: haemoglobin below the normal range for the child's age, causing the pallor and fatigue parents notice; platelet count that is too low, which explains easy bruising and petechiae; and a white blood cell count that is abnormal — either elevated, depressed, or composed of abnormal cell types. The pattern of these three abnormalities together, especially when the child also has symptoms, is what prompts the next step. An abnormal CBC alone does not confirm leukemia — many other conditions can cause similar results — but it does mean further evaluation must happen without delay.

A blood film (or blood smear) takes a drop of the same blood sample and spreads it thinly on a glass slide. A specialist — usually a haematologist or pathologist — examines the slide under a microscope to look at each cell individually. In some children with ALL, abnormal blast cells are visible on the film: immature, large cells that should be inside the bone marrow, not circulating in the blood. The appearance and staining pattern of these cells helps the specialist estimate the type of leukemia. In other children, the blood film may look close to normal even when the bone marrow is heavily involved, which is why the blood film result alone is never enough to confirm or rule out the diagnosis.

The bone marrow biopsy is the test that confirms or rules out ALL with certainty. A small sample of marrow — about the size of a pea — is taken from the back of the hip bone (the posterior iliac crest) using a fine needle. The procedure is done under sedation or general anaesthesia so your child does not feel pain during it. Afterwards, your child may feel some soreness at the site for a day or two; this is normal and managed with simple pain relief. The marrow sample is then sent to specialist laboratories for several analyses simultaneously, which is why results take a few days. Your child does not need to be admitted to hospital overnight for this procedure in most cases.

Immunophenotyping uses a laboratory technique called flow cytometry to identify proteins on the surface of the blast cells taken from the bone marrow sample. These surface proteins act like a barcode: they tell the laboratory whether the blast cells are derived from B-lymphocyte or T-lymphocyte precursors, and how immature (or 'arrested' in development) the cells are. This result determines whether your child has B-cell ALL or T-cell ALL, which directly determines the treatment protocol that will be used. Immunophenotyping results are typically available within two to three days of the bone marrow sample being taken.

Cytogenetics examines the chromosomes inside the leukemia cells to look for gains, losses, or rearrangements. Certain chromosomal changes are associated with a very good treatment response (such as high hyperdiploidy, where the cells have extra chromosomes); others indicate that a more intensive treatment approach will be needed. Molecular testing goes one step further and looks for specific gene fusions or mutations, including the Philadelphia chromosome (BCR-ABL1), KMT2A rearrangements in infants, and other markers. Together, these results are used alongside the child's age and white cell count at diagnosis to assign a risk group — standard, high, or very high — which determines the intensity and duration of chemotherapy your child will receive.

The brain and spinal cord are bathed in a fluid called cerebrospinal fluid (CSF). Leukemia cells can sometimes travel to the central nervous system and hide in this fluid, which is why a lumbar puncture — also called a spinal tap — is performed as part of the initial workup and also at the start of treatment. The procedure involves collecting a small sample of CSF from the lower part of the spine through a fine needle, again under sedation. The sample is examined under a microscope to look for blast cells. The result determines whether your child needs CNS-directed treatment as part of their protocol. This same procedure is later used to deliver some of the chemotherapy directly into the CSF — the most effective way to treat and prevent CNS involvement in ALL.