With incomplete synthesis of heme, iron will not be inserted into the porphyrin ring. Instead, iron will appear in erythroid precursors as a perinuclear ring of green-blue granules when stained with Prussian blue stain (Perl's stain). These are called ringed sideroblasts and can be seen in the image on the right. In normal individuals, there will be 20-50% bone marrow erythroid precursors with fewer than 5 siderotic granules found in the cytoplasm.26
In 2008, three types of sideroblasts were identified by the International Working Group on Morphology of Myelodysplastic Syndrome, listed in Table 8. However, the identification of more than 15% of ring sideroblasts in the bone marrow in conjunction with genetic studies and clinical symptoms has provided a more specific tool for diagnosis.26
Table 8. Working Group on Morphology of Myelodysplastic Syndrome.| Type | Siderotic granules |
| 1 | Up to 5 siderotic granules |
| 2 | More than 5 siderotic granules with no perinuclear distribution |
| 3 | 5 or more siderotic granules in a perinuclear position, covering at least 1/3 of the nuclear circumference |
26. Mufti GJ, Bennett JM, Goasguen J, Bain BJ, Baumann I, Brunning R, Cazzola M, Fenaux P, Germing U, Hellström-Lindberg E, Jinnai I, Manabe A, Matsuda A, Niemeyer CM, Sanz G, Tomonaga M, Vallespi T, Yoshimi A; International Working Group on Morphology of Myelodysplastic Syndrome. "Diagnosis and classification of myelodysplastic syndrome: International Working Group on Morphology of myelodysplastic syndrome (IWGM-MDS) consensus proposals for the definition and enumeration of myeloblasts and ring sideroblasts." Haematologica, 2008 Nov;93(11):1712-7, https://pubmed.ncbi.nlm.nih.gov/18838480/.27. Carll, T., Venkataraman, G. "Ringed sideroblasts (iron stain)." American Society of Hematology, 23 March 2018, https://imagebank.hematology.org/collection/61365.
