Paroxysmal Nocturnal Hemoglobinuria (PNH)


Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hematologic condition that is caused by somatic mutations in the phosphatidylinositol glycan class A (PIGA) gene of hematopoietic stem cells.1 Hematopoietic cells with PIGA gene mutations are unable to produce glycosylphosphatidylinositol (GPI) anchors, which attach cell surface proteins to the plasma membrane. The disease is characterized by hemolysis, risk for thrombosis, and impaired bone marrow function.2

Flow Cytometry

Flow cytometry is regarded as the most reliable method for diagnosing PNH. This highly specific and sensitive test can evaluate a number of GPI-anchored proteins, particularly CD55 and CD59. A decreased to absent expression of CD55 and CD59 on red cells is a signal of PNH. Test results express GPI anchor proteins as type 1, 2, or 3, ranging from normal expression in type 1, partially deficient expression in type 2, and completely absent expression in type 3. All flow cytometry test should study at least 2 GPI-linked proteins to ensure no false-negative results have occurred.3

Typically, patients should undergo a complete assessment for hemolysis, silent thrombosis, and end-organ damage when flow cytometry testing reveals a granulocyte clone of greater than 20%. D-dimer and brain natriuretic peptide testing, liver function panel, iron panel, bone marrow aspiration or biopsy, and cytogenetic studies are also valuable in such cases. Bone marrow aspiration and biopsy help to detect underlying bone marrow abnormalities for the further characterization of PNH.3

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Peripheral Blood Smear 

Patients with PNH have varying degrees of chronic hemolysis along with episodes of more severe hemolysis. Peripheral blood smears often provide a more detailed histology of the blood cell abnormalities involved. Red blood cells are morphologically normal. The only red blood cell finding is polychromasia along with an increased reticulocyte count. In some cases, decreased neutrophil counts, decreased platelet counts, or both are noted. Neutrophil alkaline phosphatase activity is characteristically reduced or absent.4 Examination of the peripheral blood smear may also show spherocytosis (spherical red blood cells), although this is a nonspecific finding.2

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Bone Marrow Aspiration and Biopsy

A patient’s bone marrow is examined to diagnose and classify PNH and to exclude other bone marrow failure syndromes.5 Bone marrow testing can help confirm a diagnosis of PNH and identifies effective or ineffective functioning of the bone marrow.6 The main purpose of this test is to check if PNH is present in the setting of aplastic anemia (AA) or myelodysplastic syndrome (MDS). Erythroid hyperplasia, decreased myelopoiesis, and decreased megakaryocyte levels are the histological features generally identified in the bone marrow of individuals with PNH.2 Histological examination of a bone marrow trephine biopsy specimen can show erythroid hyperplasia or generalized hypoplasia, depending on the type of PNH.4,5

Various aspects of bone marrow histology in patients with a PNH clone were studied in a large series. Normal cellularity or hypercellularity was noted in the bone marrow biopsy slides of approximately 65% of patients with classic PNH. Erythroid hyperplasia was a common finding, myelopoiesis was reduced in 86%, and megakaryopoiesis was decreased in 46% of cases. Of the patients who had PNH and bone marrow disease associated with aplastic anemia, 95% had hypocellular bone marrows, 85% had reduced myelopoiesis, and all had decreased megakaryopoiesis.5

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References

  1. Lima M. Laboratory studies for paroxysmal nocturnal hemoglobinuria, with emphasis on flow cytometry. Pract Lab Med. 2020;20:e00158. doi:10.1016/j.plabm.2020.e00158
  2. Erem AS, Asakrah S. Paroxysmal nocturnal hemoglobinuria (PNH). PathologyOutlines.com. Accessed November 22, 2022.
  3. Shah N, Bhatt H. Paroxysmal nocturnal hemoglobinuria. StatPearls [Internet]. Updated August 1, 2022.
  4. Bain BJ, Clark DM, Wilkins BS. Bone marrow pathology. Hoboken, NJ: John Wiley & Sons; 2019:585 (Chap 9: Miscellaneous Disorders). 
  5. van Bijnen STA, Ffrench M, Kruijt N. et al. Bone marrow histology in patients with a paroxysmal nocturnal hemoglobinuria clone correlated with clinical parameters. J Hematopathol 2013;6:71-82. doi:10.1007/s12308-013-0179-7

Paroxysmal nocturnal hemoglobinuria. Diagnosis. AA • MDS International Foundation. Accessed November 22, 2022.

Reviewed by Hasan Avcu, MD, on 11/23/2022.

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