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PubMed This is a summary of 17 peer-reviewed journal articles Updated
Hematology

The Biology and Diagnosis of PNH

At a Glance

Paroxysmal Nocturnal Hemoglobinuria (PNH) is an acquired blood disorder diagnosed using high-sensitivity flow cytometry. This test measures your 'clone size,' or the percentage of affected cells. White blood cell clone size provides the most accurate picture of your disease severity and risks.

Understanding the biology of Paroxysmal Nocturnal Hemoglobinuria (PNH) helps clarify why your body is reacting this way. PNH is not a disease you were born with; it is an acquired condition that begins in your bone marrow’s stem cells due to a random mutation in a gene called PIGA [1][2]. Because it is a somatic (acquired) mutation, you cannot pass it to your children.

The Anchor and the Shields

To understand PNH, imagine your red blood cells are ships that need protection from a stormy sea (your immune system).

  • The Anchor (GPI): The PIGA gene is responsible for making a “mounting bracket” or anchor called GPI. This anchor’s only job is to hold protective proteins onto the surface of your blood cells [2][3].
  • The Shields (CD55 and CD59): In a healthy person, the GPI anchor holds two critical shields in place: CD55 and CD59. These shields tell your immune system’s “complement cascade” not to attack your own cells [2][4].

In PNH, the mutation means your stem cells cannot make the GPI anchor. Without the anchor, the shields (CD55 and CD59) have nothing to hold onto and they simply float away [2][5]. Your red blood cells are left “naked” and defenseless. When your immune system’s complement system encounters these unprotected cells, it identifies them as threats and destroys them (hemolysis) [6][7].

How PNH is Diagnosed: Flow Cytometry

The “gold standard” for diagnosing PNH is a test called high-sensitivity flow cytometry [8]. This test uses a laser to look at thousands of individual blood cells to see how many are missing their protective anchors.

Doctors specifically look for a marker called FLAER (Fluorescently Labeled Aerolysin). FLAER is a specialized protein that binds directly to the GPI anchors. If the anchors are missing, the FLAER has nowhere to stick. This makes it a very sensitive and accurate way to find even tiny groups of PNH cells [9][10].

Understanding Your “Clone Size”

When you receive your lab results, you will see a percentage called the clone size. This is the percentage of your blood cells that have the PNH mutation [11].

Your doctor will measure the clone size in two different types of cells:

  1. Red Blood Cells (RBCs): This tells us how many red cells currently lack protection. However, this number can be misleading because these cells are constantly being destroyed by the disease or diluted by blood transfusions [11][12].
  2. White Blood Cells (Granulocytes/Monocytes): This is often considered the “truer” measurement of the disease. Because white blood cells aren’t destroyed by the complement system in the same way red cells are, the white cell clone size gives a more stable picture of how much of your bone marrow is affected [11][13].

Why Clone Size Matters:

  • Large Clones (>50%): Patients with large white cell clones (especially over 50%) are at a much higher risk for dangerous blood clots and significant hemolysis [14][15].
  • Small Clones: A small clone might not cause many symptoms, but it is often a sign that there might be another underlying bone marrow issue, such as aplastic anemia [8][16].
  • Cell Types: Doctors also categorize cells as Type III (cells with no shields at all) or Type II (cells with some shields left). Type III cells are much more likely to be destroyed quickly [12][17].

Common questions in this guide

What does PNH clone size mean on my lab report?
Clone size is the percentage of your blood cells that have the PNH mutation. A larger clone size, especially in white blood cells, usually indicates a higher risk for red blood cell destruction and dangerous blood clots.
Why is flow cytometry used to diagnose PNH?
High-sensitivity flow cytometry is the gold standard test because it uses a laser to look at thousands of individual blood cells. It accurately detects which cells are missing their protective anchors by using a specialized marker called FLAER.
What is the difference between white blood cell and red blood cell clone size?
White blood cell clone size is considered a more stable and accurate measurement of your disease. Red blood cell clone size can be misleading because those cells are constantly being destroyed by the immune system or temporarily diluted if you've had blood transfusions.
Can I pass PNH on to my children?
No, you cannot pass PNH to your children. It is an acquired condition caused by a random mutation in the PIGA gene that happens in your bone marrow stem cells over the course of your lifetime.
What do Type II and Type III cells mean in PNH?
Type III cells have completely lost their protective shields, making them highly vulnerable to rapid destruction by the immune system. Type II cells have partial protection left, meaning they are less likely to be destroyed immediately.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What was the exact percentage of my PNH clone in my white blood cells (granulocytes) versus my red blood cells?
  2. 2.Does my report show 'Type II' (partial deficiency) or 'Type III' (complete deficiency) cells, and how does that affect my risk of hemolysis?
  3. 3.Was high-sensitivity FLAER flow cytometry used for my diagnosis to ensure we have the most accurate measurement?
  4. 4.Is my clone size large enough (typically over 50%) that I should be particularly concerned about the risk of blood clots?
  5. 5.Since my clone size is small, does this suggest I might have an underlying bone marrow condition like aplastic anemia?

Questions For You

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References

References (17)
  1. 1

    Pathogenesis of paroxysmal nocturnal hemoglobinuria.

    Murakami Y

    [Rinsho ketsueki] The Japanese journal of clinical hematology 2016; (57(10)):1900-1907 doi:10.11406/rinketsu.57.1900.

    PMID: 27725586
  2. 2

    The Role of T Lymphocytes in the Pathogenesis of Paroxysmal Nocturnal Hemoglobinuria.

    Li C, Dong X, Wang H, Shao Z

    Frontiers in immunology 2021; (12()):777649 doi:10.3389/fimmu.2021.777649.

    PMID: 35003092
  3. 3

    Paroxysmal nocturnal hemoglobinuria.

    Čermák J

    Vnitrni lekarstvi 2018; (64(5)):508-513.

    PMID: 30193519
  4. 4

    Congenital Defects in the Expression of the Glycosylphosphatidylinositol-Anchored Complement Regulatory Proteins CD59 and Decay-Accelerating Factor.

    Kinoshita T

    Seminars in hematology 2018; (55(3)):136-140 doi:10.1053/j.seminhematol.2018.04.004.

    PMID: 30032750
  5. 5

    Limb-clasping, cognitive deficit and increased vulnerability to kainic acid-induced seizures in neuronal glycosylphosphatidylinositol deficiency mouse models.

    Kandasamy LC, Tsukamoto M, Banov V, et al.

    Human molecular genetics 2021; (30(9)):758-770 doi:10.1093/hmg/ddab052.

    PMID: 33607654
  6. 6

    Navigating the Complement Pathway to Optimize PNH Treatment with Pegcetacoplan and Other Currently Approved Complement Inhibitors.

    Hillmen P, Horneff R, Yeh M, et al.

    International journal of molecular sciences 2024; (25(17)) doi:10.3390/ijms25179477.

    PMID: 39273426
  7. 7

    Reduced red blood cell surface level of Factor H as a mechanism underlying paroxysmal nocturnal hemoglobinuria.

    Zhang L, Chen JY, Kerr C, et al.

    Leukemia 2021; (35(4)):1176-1187 doi:10.1038/s41375-020-1008-5.

    PMID: 32814838
  8. 8

    Comparison of High Sensitivity and Conventional Flow Cytometry for Diagnosing Overt Paroxysmal Nocturnal Hemoglobinuria and Detecting Minor Paroxysmal Nocturnal Hemoglobinuria Clones.

    Park SH, Jeong J, Lee SH, et al.

    Annals of laboratory medicine 2019; (39(2)):150-157 doi:10.3343/alm.2019.39.2.150.

    PMID: 30430777
  9. 9

    Diagnosis of paroxysmal nocturnal hemoglobinuria with flowcytometry panels including CD157: Data from the real world.

    Zhang Y, Ding J, Gu H, et al.

    Cytometry. Part B, Clinical cytometry 2020; (98(2)):193-202 doi:10.1002/cyto.b.21847.

    PMID: 31571381
  10. 10

    Utility of FLAER and CD157 in a five-color single-tube high sensitivity assay, for diagnosis of Paroxysmal Nocturnal Hemoglobinuria (PNH)-A standalone flow cytometry laboratory experience.

    Seth N, Mahajan V, Kedia S, et al.

    International journal of laboratory hematology 2021; (43(2)):259-265 doi:10.1111/ijlh.13366.

    PMID: 33058446
  11. 11

    Abdominal pain in combination with an unexplained hemolytic anemia are crucial signs to test for paroxysmal nocturnal hemoglobinuria: A case report.

    Elias NS, Riedl J, Stouten K, et al.

    Clinical case reports 2019; (7(1)):175-179 doi:10.1002/ccr3.1771.

    PMID: 30656036
  12. 12

    CD71 improves delineation of PNH type III, PNH type II, and normal immature RBCS in patients with paroxysmal nocturnal hemoglobinuria.

    Sutherland DR, Richards SJ, Ortiz F, et al.

    Cytometry. Part B, Clinical cytometry 2020; (98(2)):179-192 doi:10.1002/cyto.b.21853.

    PMID: 31705743
  13. 13

    Bone Marrow as a Source of Cells for Paroxysmal Nocturnal Hemoglobinuria Detection.

    Dulau-Florea AE, Young NS, Maric I, et al.

    American journal of clinical pathology 2018; (150(3)):273-282 doi:10.1093/ajcp/aqy053.

    PMID: 29982419
  14. 14

    Thromboembolic Events in Indian Patients with Paroxysmal Nocturnal Hemoglobinuria: A Single Centre Experience.

    Datta G, Verma SP, Raghuveer P, et al.

    Indian journal of hematology & blood transfusion : an official journal of Indian Society of Hematology and Blood Transfusion 2026; (42(4)):1270-1276 doi:10.1007/s12288-025-02141-5.

    PMID: 42333143
  15. 15

    Risk factors for thromboembolic events in patients with paroxysmal nocturnal hemoglobinuria (PNH): a nested case-control study in the International PNH Registry.

    Höchsmann B, Peffault de Latour R, Hill A, et al.

    Annals of hematology 2023; (102(11)):2979-2988 doi:10.1007/s00277-023-05402-3.

    PMID: 37668788
  16. 16

    Somatic mutations and clonal expansions in paroxysmal nocturnal hemoglobinuria.

    Hosokawa K, Nakao S

    Seminars in hematology 2022; (59(3)):143-149 doi:10.1053/j.seminhematol.2022.08.004.

    PMID: 36115691
  17. 17

    Immunophenotypic assessment of PNH clones in major and minor cell lineages in the peripheral blood of patients with paroxysmal nocturnal hemoglobinuria.

    Richards SJ, Dickinson AJ, Newton DJ, Hillmen P

    Cytometry. Part B, Clinical cytometry 2022; (102(6)):487-497 doi:10.1002/cyto.b.22094.

    PMID: 36134740

This page provides educational information about the biology and diagnosis of PNH. It is not intended to replace professional medical advice, diagnosis, or test interpretation from your hematologist.

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