Pre-leukemia is an older, informal term for a group of blood disorders now called myelodysplastic syndromes, or MDS. In these conditions, the bone marrow produces blood cells that are abnormal or don’t mature properly, leading to low blood counts that can cause fatigue, infections, and easy bleeding. About 15% of people with MDS eventually develop acute myeloid leukemia (AML), which is why the name “pre-leukemia” stuck in common use, but MDS is considered a distinct diagnosis with its own spectrum of severity.
How MDS Differs From Leukemia
The key distinction is what’s happening inside the bone marrow. In MDS, the bone marrow still tries to produce blood cells, but many of those cells come out defective. They may look abnormal under a microscope, carry DNA mutations, or die before they fully mature. The result is a shortage of healthy red blood cells, white blood cells, or platelets circulating in your blood.
Leukemia, specifically AML, crosses a different threshold. Doctors draw the line at the percentage of immature cells called blasts in the bone marrow. When blasts make up less than 20% of marrow cells, the condition is classified as MDS. At 20% or above, it’s reclassified as AML. That 20% cutoff is the formal boundary between the two diagnoses.
Among people initially diagnosed with lower-risk MDS, roughly 9% progress directly to AML, while another 6.5% move through a higher-risk MDS stage first and then convert. For those who do progress, the median time from diagnosis to leukemia is about 29 months.
The Spectrum Before MDS
MDS itself doesn’t appear out of nowhere. Researchers now recognize a progression that begins with something called clonal hematopoiesis, where a small population of stem cells in the bone marrow acquires genetic mutations and starts outcompeting normal cells. At this stage, blood counts are still normal and there are no symptoms. It’s an incidental finding, sometimes discovered through genetic testing done for other reasons, and it becomes increasingly common with age.
If those mutations lead to persistently low blood counts (lasting four months or more) but the bone marrow doesn’t yet show enough abnormalities to qualify as MDS, the condition is called clonal cytopenia of undetermined significance, or CCUS. Think of it as a middle step: the blood counts are dropping, there’s a genetic explanation, but fewer than 10% of cells in any given lineage look abnormal and blasts aren’t elevated. CCUS is considered a precursor to MDS by both major classification systems used worldwide.
Symptoms to Recognize
Many people with MDS have no symptoms at first. The condition is often caught on a routine blood test that reveals unexpectedly low counts. When symptoms do appear, they reflect whichever type of blood cell is most affected:
- Low red blood cells (anemia): fatigue, weakness, shortness of breath, pale skin
- Low white blood cells (neutropenia): frequent or severe infections that take longer to clear
- Low platelets (thrombocytopenia): easy bruising, prolonged bleeding from cuts, tiny red spots on the skin called petechiae
Some people have low counts in just one type of blood cell, while others have all three affected simultaneously. The severity varies widely. A person with mildly low hemoglobin might feel slightly more tired than usual, while someone with severely depleted platelets could experience dangerous bleeding.
What Causes It
Most cases of MDS have no identifiable cause. They arise from acquired DNA mutations in bone marrow stem cells that accumulate over a lifetime, which is why the condition primarily affects people over 60.
The clearest known risk factor is prior cancer treatment. Chemotherapy and radiation therapy can damage bone marrow stem cells in ways that surface years later as MDS. This is sometimes called “therapy-related MDS” and tends to behave more aggressively than cases that arise on their own. Prolonged exposure to certain industrial chemicals, particularly benzene, also raises risk. In rare cases, especially when MDS appears before age 50, an inherited genetic predisposition may be involved.
How It’s Diagnosed
A standard blood test can raise suspicion, but confirming MDS requires a bone marrow biopsy. During this procedure, a small sample of bone marrow is drawn from the hip bone and examined under a microscope. Doctors evaluate at least 500 cells, looking for abnormal shapes (dysplasia), counting the percentage of blasts, checking for ring sideroblasts (red blood cell precursors with iron deposits), and assessing overall marrow cellularity.
Beyond the microscope, several layers of genetic testing help pin down the diagnosis and predict how the disease will behave. Chromosome analysis (karyotyping) looks for large-scale changes like missing or extra chromosomes. If that comes back normal, more sensitive techniques such as chromosomal microarray or FISH testing can detect smaller deletions. Molecular genetic testing scans for mutations in specific genes associated with MDS, including SF3B1 and TP53, along with dozens of others that influence prognosis and treatment choices. Flow cytometry, which uses lasers to characterize individual cells, may also be used to identify abnormal cell populations.
Before all of this, doctors typically rule out simpler explanations for low blood counts: vitamin B12 or folate deficiency, iron problems, thyroid disorders, and kidney dysfunction. MDS is a diagnosis that requires excluding these more common causes first.
Risk Scoring and What It Means
Once MDS is confirmed, doctors assign a risk score that guides treatment decisions. The most current system, called IPSS-M, integrates blood counts (hemoglobin and platelet levels), the percentage of blasts in the bone marrow, chromosome abnormalities, and mutations across 31 different genes. The result places each patient into one of six risk categories, from very low to very high.
This matters because MDS behaves very differently depending on where someone falls. A person in the very low risk group may live for many years with only mild anemia that needs occasional management. Someone in the very high risk group faces a much greater chance of progression to AML and needs more aggressive treatment promptly.
Treatment Options
Treatment for MDS ranges from watchful monitoring to stem cell transplant, depending on risk level and overall health.
Lower-Risk MDS
For people with lower-risk disease, the primary goal is managing symptoms, particularly anemia. Options include drugs that stimulate red blood cell production when levels of the body’s natural signal for red cell production are low. For patients with specific genetic features, newer targeted therapies have become preferred options. Some patients need regular blood transfusions to keep counts at safe levels, along with treatments to prevent iron overload from repeated transfusions.
Higher-Risk MDS
Higher-risk MDS calls for treatments aimed at slowing progression and, when possible, achieving remission. The backbone of therapy is a class of drugs that work by reactivating genes the cancer has silenced, given as injections on a regular schedule. These are increasingly combined with a drug that triggers abnormal cells to self-destruct, though this combination can cause significant drops in blood counts that require careful monitoring. For patients whose MDS carries a specific mutation in the IDH1 gene, targeted inhibitors of that mutation are also available.
Stem Cell Transplant
The only treatment that can potentially cure MDS is an allogeneic stem cell transplant, where a donor’s healthy stem cells replace the patient’s diseased bone marrow. This option is most commonly considered for patients under 65 whose disease has gone into remission after initial treatment. The transplant process involves intensive conditioning to eliminate remaining abnormal cells before the new stem cells are infused. Older patients or those with other health conditions may receive a less intensive preparatory regimen to reduce the risk of serious side effects. For a substantial portion of eligible patients, transplant can be curative.