The peripheral blood smear in sideroblastic anemia typically shows a dimorphic picture: two populations of red cells, one small and pale and one normal-looking. It often also shows Pappenheimer bodies (iron-containing granules) and sometimes basophilic stippling. The defining cell, the ring sideroblast, is not usually seen in peripheral blood; it is found in a bone marrow sample stained for iron. The blood smear raises the suspicion, and the marrow confirms it.
This distinction trips up many students and even some reports, so this article explains what each test shows, why, and how the findings fit together. For a wider overview, see my article on sideroblastic anemia causes, diagnosis, and management.
What Is Sideroblastic Anemia?
Sideroblastic anemia is a group of blood disorders in which developing red cells cannot use iron properly to make heme, the iron-containing part of hemoglobin. Heme is assembled inside the mitochondria of immature red cells in the bone marrow.
When heme production fails, iron still enters the cell but has nowhere to go. It piles up in the mitochondria, which sit around the nucleus. Many of these damaged precursors die before maturing, a process called ineffective erythropoiesis, so fewer healthy red blood cells reach the circulation.
The body senses anemia and absorbs more iron from the gut, so patients can develop iron overload even while anemic.
Causes: Inherited and Acquired
The causes are covered in depth in my article exploring the causes and mechanisms of sideroblastic anemia. In brief, they fall into two groups.
- Inherited: the most common form is X-linked, caused by mutations in ALAS2, the enzyme that starts heme synthesis. Other rarer forms involve genes such as SLC25A38 and ABCB7.
- Acquired, clonal: myelodysplastic syndrome with ring sideroblasts, a marrow disorder most often seen in older adults and frequently associated with SF3B1 mutations.
- Acquired, reversible: heavy alcohol use, certain drugs (notably isoniazid, and also chloramphenicol and linezolid), copper deficiency (including from excess zinc intake), and lead poisoning.
What the Blood Smear Shows
A peripheral smear is a thin film of blood stained and examined under the microscope. In sideroblastic anemia, the red blood cell findings tell a characteristic story.
| Smear finding | What it looks like | What it means |
|---|---|---|
| Dimorphic red cells | A mix of small, pale (hypochromic) cells and normal cells | An abnormal red cell population alongside normal ones; classic for sideroblastic anemia |
| Pappenheimer bodies | Small, dark granules clustered near the cell edge | Iron-containing mitochondrial fragments; cells carrying them are called siderocytes |
| Basophilic stippling | Fine or coarse blue dots throughout the cell | Abnormal ribosome remnants; prominent in lead poisoning |
| Anisocytosis | Marked variation in cell size | Reflected in a high red cell distribution width (RDW) |
| Macrocytes | Large red cells, sometimes with other dysplastic changes | Suggests an acquired, myelodysplastic cause |
Red cell size depends on the cause. Inherited forms are usually microcytic (small cells), while MDS-related forms are often normocytic or macrocytic. Because the two populations average out, the overall mean cell volume can look close to normal, so some cases are first labeled normocytic anemia. The smear and RDW reveal what the average hides.
A smear alone cannot diagnose sideroblastic anemia, but in my practice a dimorphic film with Pappenheimer bodies and high iron levels is a strong prompt to examine the marrow. Other anemias have their own signature films; compare the sickle cell anemia blood smear, where the shape of the cells is the key clue.
The Bone Marrow and Ring Sideroblasts
The diagnosis is made on a bone marrow aspirate stained with Prussian blue (Perls stain), which colors iron blue. A ring sideroblast is a red cell precursor with iron granules arranged in a ring around at least a third of the nucleus, reflecting iron-laden mitochondria.
A few scattered iron granules in precursors are normal; the ring pattern is not. In myelodysplastic syndrome, pathologists use set thresholds for the proportion of ring sideroblasts, alongside genetic results, to classify the disease. The marrow also shows increased red cell precursors and increased storage iron.
Other Laboratory Tests
- Iron studies: serum iron, ferritin, and transferrin saturation are typically raised, the opposite of iron deficiency.
- Full blood count: anemia with a high RDW; white cells and platelets may be low in MDS.
- Vitamin B6, copper, zinc, and lead levels: to find reversible causes.
- Genetic testing: ALAS2 and other genes for suspected inherited forms; myeloid gene panels, including SF3B1, for suspected MDS.
Treatment and Management
Treatment targets the cause and manages the hematological consequences.
- Remove reversible causes: stop alcohol or offending drugs and correct copper deficiency; these forms often improve.
- Pyridoxine (vitamin B6): a trial is worthwhile in inherited X-linked forms, many of which respond at least partly, and in isoniazid-related cases.
- Transfusion: for symptomatic anemia not responding to other measures.
- Iron chelation or phlebotomy: to treat iron overload and protect the heart, liver, and pancreas.
- MDS-directed therapy: treatment options for myelodysplastic syndrome, chosen by a hematologist based on risk.
- Stem cell transplantation: reserved for selected severe cases.
When to see a doctor
See your doctor if you have ongoing tiredness, breathlessness on exertion, or pale skin, particularly if earlier tests showed anemia that did not improve with iron tablets. Anemia that fails to respond to iron, or comes with high ferritin, deserves a hematology review rather than more iron.
Tell your doctor about alcohol intake, medicines such as isoniazid or linezolid, zinc supplements, and any family history of anemia. These details often point straight to the cause. Never start iron supplements on your own if you have been told your iron levels are high.
Key Takeaways
- The sideroblastic anemia blood smear classically shows dimorphic red cells, Pappenheimer bodies, and a high RDW.
- Ring sideroblasts, the defining finding, are identified in the bone marrow with a Prussian blue iron stain.
- Iron, ferritin, and transferrin saturation are usually raised, unlike iron deficiency.
- Causes range from inherited ALAS2 mutations to MDS, alcohol, drugs, and copper deficiency.
- Treatment depends on cause and includes vitamin B6, removing triggers, and managing iron overload.
Frequently Asked Questions
Can ring sideroblasts be seen on a regular blood smear?
Generally no. Ring sideroblasts are immature cells that stay in the bone marrow, and they need an iron stain to be seen. The peripheral smear shows supporting clues, such as dimorphic cells and Pappenheimer bodies.
How is sideroblastic anemia different from iron deficiency anemia?
Both can produce small, pale red cells, but iron levels point in opposite directions. In iron deficiency, ferritin is low; in sideroblastic anemia, iron and ferritin are usually high because iron cannot be used.
What are Pappenheimer bodies?
They are small iron-containing granules in red cells, visible on routine stains and confirmed with an iron stain. They are seen in sideroblastic anemia and also after spleen removal and in some hemolytic anemias.
Is sideroblastic anemia curable?
Reversible acquired forms often resolve once the cause is removed, and some inherited forms respond well to vitamin B6. MDS-related disease is a long-term condition managed by a hematologist.
