Paroxysmal nocturnal hemoglobinuria (PNH) is an ultra-rare, acquired blood disorder that is characterized by hemolysis, thrombosis, and pancytopenia (anemia, leukopenia, and thrombocytopenia) due to bone marrow failure. PNH is seen equally in both sexes, in all races and ages, and at a rate of approximately 5 to 10 cases per million people.[1] Patients with PNH lack a protein called glycosylphosphatidylinositol (GPI), normally present on the membrane of blood cells.[2] GPI is a bridge protein that binds many proteins on the outside of the cell, including complement-inactivating proteins CD55 and CD59. In patients with PNH, the absence of CD55 and CD59 results in complement-mediated hemolysis, thrombosis, and pancytopenia.

Clinical features of PNH

The clinical features of PNH are shown in Table 1.[3,4] The most common are fatigue (typically out of proportion to the degree of anemia), shortness of breath, and hemoglobinuria (red urine), especially in the first morning sample (Figure 1). Upper abdominal pain due to esophageal spasms is a characteristic finding. Erectile dysfunction is seen in the majority of men with PNH. Arterial or venous thrombosis is seen in 25% to 40% of patients and is the leading cause of death.[5] Thrombosis can occur at common sites (e.g., deep veins of lower limb) as well as uncommon sites (e.g., hepatic vein, dermal veins, cerebral veins).[6] The course of PNH is variable, with the median survival time for an untreated patient being around 10 years.

Newer therapeutic options, such as stem cell transplantation and the anticomplement antibody eculizumab (Soliris), have radically changed the outlook for patients with PNH.[7] Eculizumab produces an immediate and sustained improvement in all symptoms, reducing hemolysis, decreasing risk of thrombosis, and increasing quality of life.[8,9] Because of the availability of effective treatment, patients with unexplained hemolytic anemia, thrombosis, or cytopenia should be tested for PNH.

As well as being useful in patients suspected of having paroxysmal nocturnal hemoglobinuria, PNH testing is useful in patients with aplastic anemia and myelodysplastic syndrome. A positive result may be seen in a subgroup of these patients and is a predictor of good response to immunosuppressive treatment.[10] International consensus guidelines have been developed to define the indications for testing PNH.[5]

PNH testing at Vancouver General Hospital

Testing for paroxysmal nocturnal hemoglobinuria involves using a flow cytometer to identify PNH-positive blood cells, which lack one or more GPI-bound antigens. Tests for the majority of cases in British Columbia where PNH is suspected are conducted at Vancouver General Hospital. PNH testing is also offered by two other centres. Testing is expensive, costing several hundred dollars per patient, and the steady increase in PNH testing in British Columbia since 2001 (Figure 2) represents a significant cost to the health care system.

To further refine the indications used to order tests for this ultra-rare disease, we retrospectively evaluated PNH tests performed at Vancouver General Hospital over a 3.5 year period (July 2010 to December 2013).[11] Of the 935 cases, 101 (10.8%) were found to have a subpopulation of PNH-positive cells. A chart review identified indications in most of the cases evaluated.

Three indications made up 18% of all tests: previously diagnosed PNH, hemolysis, and thrombosis (Table 2). These indications are relatively specific for PNH and should lead to testing. Another indication, aplastic anemia, should also lead to testing because a finding of PNH phenotype cells is likely. However, yet another indication, cytopenia, is very common in the general population and much less specific for PNH. Despite this, cytopenia was the indication for testing in more than 50% of cases (490/935).

We subdivided the cytopenia cases depending on the number of cell lines involved (Table 3). The indications included pancytopenia (all three cell lines), bicytopenia (two cell lines), and unicytopenia (one cell line). The proportion of positive results was highest in cases of pancytopenia (9.9%) and lowest in cases of bicytopenia (1.8%) and unicytopenia (0.4%). We also subdivided the unicytopenia cases (Table 4) and found these were notable for the rarity of positive results: 1/145 in anemia, 0/42 in leukopenia, and 0/39 in thrombocytopenia. These findings indicate that not conducting PNH test in cases of cytopenia involving only one or two cell lines could reduce the testing volume by about one-third.[11]

Recommendations

Physicians should be aware of PNH as an ultra-rare disease that can now be treated effectively if the disorder is included in the differential diagnosis and an early and accurate diagnosis is made. However, because testing for PNH is expensive, physicians should also be aware of the appropriate indications for testing.
We recommend that all patients with proven aplastic anemia or unexplained hemolysis, thrombosis, or persistent pancytopenia be tested for PNH. We recommend that patients with cytopenia involving only one or two cell lines not be tested for PNH.

Competing interests

Dr Dalal has received speaking fees and research grants from Alexion Pharma-ceuticals, the developer of eculizumab (Soliris), a complement inhibitor for treating paroxysmal nocturnal hemoglobinuria. The research summarized in this article was supported by a grant from Alexion Pharmaceuticals.

This article has been peer reviewed.

Dr Dalal is a clinical professor in the Department of Pathology and Laboratory Medicine at the University of British Columbia. He is also a staff hematopathologist and medical director of the flow cytometry laboratory at Vancouver General Hospital. Mr Suyama is a medical student, studying at the University of St. Andrews in Scotland, who conducted research with Dr Dalal in 2015.