What Is a Ring Sideroblast: Causes and Diagnosis

A ring sideroblast is a young red blood cell precursor in the bone marrow that has accumulated abnormal deposits of iron inside its mitochondria, forming a visible ring around the cell’s nucleus when stained with a special dye. Finding ring sideroblasts on a bone marrow sample is the hallmark of a group of conditions called sideroblastic anemias, which range from rare inherited disorders to acquired blood cancers and even reversible reactions to certain drugs or toxins. The causes and the diagnostic workup involved are more varied than most people expect.

What Makes a Sideroblast “Ringed”

Normal developing red blood cells in the bone marrow do contain small amounts of iron. That iron is there for a reason: it gets incorporated into heme, the molecule that lets red blood cells carry oxygen. A regular sideroblast, which simply means a red blood cell precursor with a few visible iron granules, is completely normal and not a sign of disease.

A ring sideroblast is different. Instead of a handful of scattered granules, iron piles up excessively inside the cell’s mitochondria and arranges itself in a characteristic ring or collar encircling the nucleus. According to World Health Organization criteria, a cell qualifies as a ring sideroblast when it has at least five iron-containing granules covering at least one third of the way around the nucleus.¹ You cannot see this pattern on a routine blood test. It only becomes visible when a pathologist stains a bone marrow aspirate with Perls’ Prussian blue, a dye that turns iron deposits bright blue against the pinkish-red background of the cell.

How the Iron Gets Trapped

Under normal circumstances, iron enters a developing red blood cell, passes into the mitochondria, and gets used to build heme. Ring sideroblasts form when something goes wrong in that process. The iron arrives at the mitochondria but cannot be properly used, so it accumulates there instead of being built into functional hemoglobin.² The result is a cell stuffed with iron it cannot use, which is both a waste of iron and toxic to the mitochondria themselves.

The specific step that breaks down varies by cause. In some cases, the enzyme that begins building heme is faulty. In others, the machinery that assembles related iron-containing molecules called iron-sulfur clusters is disrupted. And in still other conditions, the problem is not the cell’s own enzymes but an outside interference, like a drug that blocks a necessary vitamin or a toxin that shuts down an enzyme. What all these scenarios share is the same downstream outcome: iron floods the mitochondria and goes unused, producing the telltale ring.²

Inherited Causes

Congenital sideroblastic anemias are rare, but several distinct genetic forms have been identified. In all of them, a mutation impairs the way developing red blood cells handle iron in the mitochondria, leading to ring sideroblast formation from early in life.³

The best-known inherited form is X-linked sideroblastic anemia, caused by mutations in the ALAS2 gene. This gene encodes the first enzyme in the heme-building pathway inside red blood cell precursors.⁴ Because it is X-linked, males are typically more severely affected, though female carriers can sometimes develop symptoms later in life. One important feature of this form is that some patients respond to high-dose vitamin B6, which acts as a cofactor for the faulty enzyme and can partially restore its function.

The most common recessive form involves a different gene called SLC25A38, which encodes a transporter that carries glycine into the mitochondria. Glycine is one of the raw ingredients needed to start building heme, so when this transporter is defective, heme production stalls and iron backs up.⁵ Unlike X-linked cases, SLC25A38 mutations cause severe anemia that usually does not respond to vitamin B6, and affected children often become transfusion-dependent early on.⁶

A less common inherited cause is Pearson syndrome, a multisystem mitochondrial disorder caused by large deletions in mitochondrial DNA. Ring sideroblasts appear in the bone marrow alongside other abnormalities, including vacuolization of precursor cells, and the condition affects multiple organs beyond the blood.⁷ Other rare syndromic forms exist as well, involving genes for iron-sulfur cluster assembly or mitochondrial protein transport, but these are individually very uncommon.

Acquired Causes and the SF3B1 Connection

By far the most common reason a pathologist encounters ring sideroblasts in clinical practice is not a hereditary condition but an acquired bone marrow disorder: myelodysplastic syndrome, or MDS. MDS is a group of blood cancers in which the bone marrow produces blood cells that are abnormal in shape and function. A specific subtype, MDS with ring sideroblasts, is defined in part by the presence of ring sideroblasts in the marrow.

A breakthrough in understanding this subtype came with the discovery that most of these patients carry a mutation in a gene called SF3B1, which is involved in RNA processing rather than iron metabolism directly. One landmark study found SF3B1 mutations in about 65% of MDS patients whose bone marrows showed ring sideroblasts, while a follow-up study in a larger group of patients with the ring sideroblast subtypes detected the mutation in about 81% of cases.⁸⁹ The connection between an RNA-splicing gene and iron-loaded mitochondria is not immediately obvious, but it appears that the mutation alters how several downstream genes are processed, ultimately disrupting mitochondrial iron handling in red blood cell precursors.

The SF3B1 mutation has become so central to this diagnosis that updated classification systems now lean heavily on molecular testing rather than relying solely on microscopic counting of ring sideroblasts. This matters for prognosis, because MDS patients with the SF3B1 mutation tend to have a more favorable outlook than those with other MDS subtypes, with a low risk of transforming into acute leukemia.¹⁰

Reversible and Secondary Causes

Not every ring sideroblast points to cancer or a genetic disorder. Several acquired, reversible conditions can produce them, and recognizing these is important because the treatment is entirely different: remove the offending agent and the ring sideroblasts disappear.

Certain medications are well-known triggers. Isoniazid, a cornerstone drug for tuberculosis treatment, can cause sideroblastic anemia by interfering with vitamin B6 availability, which the heme-building enzyme ALAS2 depends on.¹¹ Other drugs occasionally implicated include chloramphenicol and linezolid (both antibiotics), as well as heavy alcohol use, which impairs multiple steps of heme synthesis and folate metabolism.

Lead poisoning is another classic cause. Lead inhibits several enzymes in the heme pathway, which can result in ring sideroblast formation alongside other signs of lead toxicity. Copper deficiency, whether from malnutrition, gastric bypass surgery, or excessive zinc supplementation, can also produce ring sideroblasts. In one illustrative case report, a patient developed sideroblastic anemia and low white blood cell counts from zinc-induced copper deficiency, initially mimicking a more serious blood disorder before the true cause was identified.¹² These secondary causes underscore why a thorough clinical history is essential before assuming ring sideroblasts mean MDS or an inherited condition.

How Ring Sideroblasts Are Detected

Ring sideroblasts cannot be found on a routine complete blood count. They require a bone marrow aspiration, in which a small sample of marrow is drawn from the hip bone with a needle, smeared onto a glass slide, and stained with Perls’ Prussian blue. Under the microscope, iron deposits turn blue, and the pathologist looks for cells where blue granules form a ring pattern around the nucleus.¹³

This staining can be done manually or through an automated protocol. Validation work has shown that automated iron staining performs comparably to the traditional manual method, which is useful for labs processing large volumes of samples.¹⁴ Either way, counting ring sideroblasts still requires a trained human eye, because distinguishing pathologic rings from normal sideroblasts with scattered granules takes experience.

Before a bone marrow biopsy is ordered, clues on a standard blood smear can raise suspicion. Patients with sideroblastic anemia often have a dimorphic red blood cell population, meaning the blood contains a mixture of normal-sized cells and smaller, paler ones. Pappenheimer bodies, which are tiny iron-containing inclusions visible in mature red blood cells on a regular Wright-Giemsa stain, can also hint at the diagnosis.¹ But these findings are suggestive, not definitive. The bone marrow stain remains the gold standard.

Why the Diagnostic Threshold Recently Shifted

For years, the standard rule was that ring sideroblasts had to make up at least 15% of all developing red blood cells in the marrow for a diagnosis of MDS with ring sideroblasts. That threshold was straightforward and easy to apply. The problem was that it missed people.

A retrospective study found that among patients with confirmed SF3B1 mutations, only about 74% met the 15% ring sideroblast cutoff by morphology alone. Roughly a quarter of SF3B1-positive patients had ring sideroblast counts below that threshold, including about 15% with counts between 5% and 14%, and about 10% with counts below 5%.¹⁵ In other words, relying solely on microscope counting left a meaningful proportion of patients undiagnosed.

The 2022 updates from both the WHO and the International Consensus Classification addressed this gap by shifting toward a more molecularly defined approach. Under the revised criteria, a patient with an SF3B1 mutation can be diagnosed with this MDS subtype even if ring sideroblasts fall below the old 15% mark, as long as the overall clinical picture fits. This change reflects the reality that the genetic mutation is the more reliable marker, since ring sideroblast percentages can vary depending on sampling, staining quality, and where the pathologist counts.

Iron Overload as a Complication

One of the most clinically significant consequences of having ring sideroblasts, particularly in acquired MDS, is systemic iron overload. This happens through two routes working simultaneously. First, many patients with MDS and ring sideroblasts need regular red blood cell transfusions to manage their anemia, and each unit of transfused blood delivers a substantial dose of iron that the body has no efficient way to excrete. Second, the bone marrow itself contributes to iron loading through a mechanism involving hormonal signals.

In MDS with ring sideroblasts, the bone marrow churns out large numbers of red blood cell precursors that never mature properly, a process called ineffective erythropoiesis. These defective precursors release abnormally high levels of a hormone called erythroferrone, which suppresses a liver hormone called hepcidin. Hepcidin normally acts as the body’s iron gatekeeper, limiting how much iron gets absorbed from the gut. When hepcidin is suppressed, the gut absorbs more iron than it should, piling onto the iron already coming in from transfusions.¹⁶

Research has shown that in patients with SF3B1-mutated MDS specifically, a variant form of erythroferrone is produced that retains its ability to shut down hepcidin, amplifying this effect. Plasma erythroferrone levels are higher in SF3B1-mutated patients than in other MDS subtypes, which helps explain why iron overload is a particularly prominent problem in this group.¹⁷ Over time, excess iron deposits in the liver, heart, and endocrine organs, causing damage that compounds the problems already created by the anemia itself.

Treatment Approaches

Treatment for sideroblastic anemia depends entirely on the underlying cause. There is no single therapy that works across all forms, which is why accurate diagnosis matters so much.

For the inherited X-linked form caused by ALAS2 mutations, the first-line treatment is high-dose pyridoxine (vitamin B6). Some patients respond well, with their hemoglobin levels rising enough to avoid transfusions. Others have mutations that do not respond to B6, and these patients may need chronic transfusions along with iron chelation therapy to prevent iron overload. For the recessive form caused by SLC25A38 mutations, vitamin B6 is generally ineffective, and supportive care with transfusions and chelation has been the mainstay.¹⁸

For acquired MDS with ring sideroblasts, a newer treatment called luspatercept has become an important option. Luspatercept is an erythropoietic maturation agent, meaning it helps late-stage red blood cell precursors finish their development rather than dying in the marrow. It is approved for both MDS with ring sideroblasts and beta-thalassemia. Case reports have also documented its successful use in congenital sideroblastic anemia patients who stopped responding to vitamin B6 and chelation, where it helped achieve transfusion independence.¹⁹

For reversible causes, the treatment is removing the trigger. A patient who develops ring sideroblasts from isoniazid can be supplemented with vitamin B6 or switched to an alternative tuberculosis drug. Someone with copper deficiency from excessive zinc intake needs zinc restriction and copper repletion. Alcohol-related sideroblastic changes typically reverse with cessation of drinking and nutritional support.

Outlook for MDS With Ring Sideroblasts

Among the various subtypes of MDS, those characterized by ring sideroblasts and SF3B1 mutations carry one of the more favorable prognoses. Most patients fall into lower-risk categories on the standard prognostic scoring systems used by hematologists, and the risk of the disease transforming into acute myeloid leukemia is low.²⁰ That said, “favorable for MDS” is still a relative statement. These patients live with chronic anemia, often need ongoing transfusions, and face the cumulative burden of iron overload.

A related condition, MDS/MPN with ring sideroblasts and thrombocytosis, has features of both MDS and a myeloproliferative neoplasm, including high platelet counts. Its prognosis sits between that of standard MDS with ring sideroblasts and essential thrombocythemia, a generally indolent blood disorder.¹⁰ Both subtypes benefit from the increasingly personalized approach that molecular testing enables, since knowing the mutational profile helps clinicians tailor monitoring intensity and treatment decisions to each patient’s actual risk rather than relying on morphology alone.

When Ring Sideroblasts Do Not Mean What They Seem

One underappreciated pitfall in clinical practice is that ring sideroblasts are not exclusive to MDS. They show up in a range of non-clonal, secondary conditions including lead exposure, drug side effects, copper deficiency, zinc excess, and hereditary sideroblastic anemias of various genetic origins. Mistaking one of these for MDS can lead to unnecessary bone marrow surveillance, inappropriate prognostic counseling, or missed treatment of a straightforwardly correctable problem. A careful clinical history, attention to medication and supplement use, and measurement of copper and lead levels can all help differentiate reversible causes from a true bone marrow malignancy. The stakes of getting this distinction right are high, since the management pathways diverge sharply depending on which category the patient falls into.