Childhood leukemia is diagnosed through a series of steps that typically begins with blood tests and ends with a bone marrow examination. No single test confirms the diagnosis on its own. Instead, doctors piece together findings from physical exams, blood work, bone marrow samples, genetic analysis, and spinal fluid testing to identify the exact type of leukemia and how aggressive it is. The entire process can move quickly, often within days of the first abnormal blood test.
Signs That Prompt Testing
Leukemia doesn’t always announce itself with one obvious symptom. A large systematic review of how children present at diagnosis found that five features appeared in more than half of cases: an enlarged liver (64%), an enlarged spleen (61%), paleness (54%), fever (53%), and bruising (52%). Beyond those, roughly 40 to 50 percent of children also had recurrent infections, fatigue, limb pain, swollen lymph nodes, tiny red spots on the skin called petechiae, or unusual bleeding.
None of these symptoms alone points specifically to leukemia. A child with a fever and fatigue could have any number of common illnesses. What raises suspicion is the combination: a child who is pale, bruising easily, running fevers, and has a swollen abdomen or lymph nodes. Doctors examining a child with unexplained illness are trained to feel the abdomen for liver and spleen enlargement, check for swollen lymph nodes, and look carefully at the skin for bruising or petechiae. When multiple signs cluster together, blood work is the next step.
Blood Tests: The First Real Clue
A complete blood count is usually the first test that signals something is wrong. In leukemia, the bone marrow is flooded with abnormal white blood cells (called blasts) that crowd out normal blood-cell production. This shows up in blood work in several ways: the white blood cell count may be very high or very low, red blood cell counts drop (explaining the paleness and fatigue), and platelet counts fall (explaining the bruising and bleeding).
A blood smear, where a drop of blood is examined under a microscope, can reveal blast cells circulating in the bloodstream. In healthy blood, blasts stay in the bone marrow and don’t appear in circulation. Finding them in a blood sample is a strong signal that something is seriously wrong, though it’s not enough for a definitive diagnosis. That requires looking directly at the bone marrow.
Bone Marrow Examination
A bone marrow aspiration is the test that confirms the diagnosis. A needle is inserted into the back of the hip bone to draw out a small sample of marrow, usually while the child is sedated. Pathologists then examine the sample under a microscope to count the percentage of blast cells. Normal bone marrow contains 5 percent blasts or fewer. For a diagnosis of acute myeloid leukemia (AML), the standard threshold is at least 20 percent blasts in the marrow or blood, though recent classification updates from the World Health Organization now allow an AML diagnosis below that threshold if certain defining genetic abnormalities are present.
For acute lymphoblastic leukemia (ALL), which accounts for about 75 to 80 percent of childhood leukemia cases, the marrow is typically packed with blasts well above the 20 percent mark by the time symptoms appear. The bone marrow sample also provides material for the additional specialized tests that determine the exact subtype.
Identifying the Leukemia Type
Knowing that a child has leukemia isn’t enough. Treatment depends heavily on whether the cancer involves the lymphoid or myeloid cell line, and which specific subtype it is. A test called flow cytometry (also known as immunophenotyping) runs the bone marrow sample through a machine that identifies proteins on the surface of the leukemia cells.
Different leukemia types carry different surface proteins. B-cell ALL, the most common childhood subtype, typically shows markers called CD10, CD19, CD20, and CD79a. T-cell ALL lights up for CD3, CD5, and CD7 instead. AML cells carry a distinct set: CD13, CD33, and CD117. This test is fast, often returning results within a day or two, and it gives doctors the information they need to start planning treatment while waiting for the slower genetic results.
Genetic Testing for Risk Stratification
Genetic analysis of the leukemia cells is one of the most important parts of the diagnostic workup because it determines how the cancer will be treated and predicts how well a child is likely to respond. Two main techniques are used. Standard chromosome analysis (called karyotyping) looks at the overall number and structure of chromosomes. A more targeted method called fluorescence in situ hybridization (FISH) hunts for specific chromosome rearrangements that standard analysis can miss.
In childhood ALL, genetic abnormalities can be identified in 75 to 80 percent of cases using these two approaches together. Some findings are favorable. Hyperdiploidy, where leukemia cells have more than the normal 46 chromosomes, and the ETV6/RUNX1 gene fusion are both associated with better outcomes. Other findings signal higher risk: fewer chromosomes than normal (hypodiploidy), the BCR/ABL1 fusion (also known as the Philadelphia chromosome), and rearrangements of the KMT2A gene. In one study, the most frequently detected abnormality was a gain of the RUNX1 gene, found in about 21 percent of cases, followed by the ETV6/RUNX1 fusion at roughly 8 percent.
For AML, the 2022 WHO classification now separates cases into two broad groups: AML with defining genetic abnormalities and AML defined by how the cells look under a microscope. This distinction matters because certain genetic changes are so characteristic that they confirm the diagnosis even when blast counts are below the traditional 20 percent threshold.
Chest X-ray and Imaging
A chest X-ray is routinely performed as part of the initial evaluation. The primary reason is to check for a mediastinal mass, a clump of leukemia cells in the chest area between the lungs. This is especially relevant in T-cell ALL. In a study of nearly 1,000 children newly diagnosed with ALL, about 11 percent had a mediastinal mass, and roughly 5 percent had compression or deviation of the windpipe. Other findings included fluid around the lungs (11%) and signs of an enlarged heart (7%).
Detecting a mediastinal mass early is critical because it can compress the airway. Since bone marrow aspiration often requires sedation, doctors need to know about any chest mass beforehand to avoid dangerous complications during the procedure. Additional imaging, such as ultrasound of the abdomen, may be used to assess the size of the liver and spleen.
Spinal Fluid Analysis
A lumbar puncture (spinal tap) is performed to check whether leukemia cells have spread to the central nervous system. A small amount of cerebrospinal fluid is collected and examined under a microscope. The results are categorized into three levels: CNS1 means no leukemia cells are found, CNS2 means a small number of blasts are present with fewer than five white blood cells per cubic millimeter, and CNS3 means five or more white blood cells per cubic millimeter with visible blast cells, or cranial nerve involvement. CNS3 status calls for more intensive treatment directed at the brain and spinal cord.
Conventional microscopy is the standard method, but it has limitations, particularly when cell counts are low. More sensitive molecular techniques can detect leukemia involvement in the spinal fluid of roughly 45 percent of pediatric ALL patients who test negative by standard methods. Whether this low-level molecular detection changes treatment decisions depends on the overall treatment protocol being used.
How Quickly It All Happens
Once leukemia is suspected, the diagnostic process moves fast. Blood tests return within hours. Bone marrow aspiration, chest X-ray, and lumbar puncture are often performed on the same day or within 24 to 48 hours of referral to a pediatric oncologist. Flow cytometry results typically come back within one to two days. Genetic testing takes longer, sometimes one to three weeks for full results, but treatment usually begins before those results are finalized based on the initial bone marrow and flow cytometry findings.
The bottleneck is more often the time before referral than the testing itself. Surveys have found that over a third of young cancer patients visited a primary care doctor five or more times before being sent to a specialist. Because leukemia symptoms overlap with common childhood illnesses, the diagnosis can be delayed when individual symptoms are treated in isolation rather than recognized as a pattern. A child with persistent, unexplained fatigue, fevers, bruising, or bone pain who isn’t improving with standard treatment warrants blood work sooner rather than later.