ABSTRACT
Context: Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal stem cell disorder characterized by complement-mediated hemolysis due to reduced expression of glycosyl phosphatidylinositol-anchored complement deactivating proteins such as CD55 and CD59 on RBC. Flow cytometric analysis of CD55 and CD59 expression by RBC is a reliable tool for the diagnosis of PNH. Aims: Detection and quantification of PNH clone and comparison of the relative role of CD55 and CD59 expression by RBC in the diagnosis of PNH. Materials and Methods: Flow cytometric analysis of RBC was performed in blood samples of 239 patients by direct immunofluorescence using monoclonal anti-CD55 and anti-CD59 antibodies. CD55 and CD59 expressions by RBC were compared in 54 cases in which PNH clones were detected. Results: Out of 54 cases, 85% and 72% revealed CD59 and CD55 negative populations, respectively. Various combinations of type II and III erythrocytes could be identified in all cases having CD59 deficient RBC. In contrast, distinct populations of CD55-deficient RBC were seen in only 33% cases. In the remaining (67%) cases, CD55 negative RBC caused sloping of the ascending limb of the histogram resulting in difficulties in interpretation. Fifteen percent cases had false CD55-deficient RBC and in 23% cases anti-CD55 antibody failed to identify PNH clones which were detected by CD59. Conclusion: CD59 is a better marker for the diagnosis of PNH. Although CD55 negativity supported the diagnosis of PNH in cases with CD59-deficient RBC, its role as an independent diagnostic marker for PNH is questionable due to its lower sensitivity and specificity.