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.

Background: FLAER-based flow cytometry assay is considered the gold standard for diagnosis of paroxysmal nocturnal hemoglobinuria (PNH). CD157 is a recently reported marker for GPI-anchored protein found both on neutrophils and monocytes. This study highlights the robustness of FLAER and CD157 combination to identify PNH clones in a high sensitivity assay. Though rare, the data shown highlight the presence of CD157 negativity in few cases re-emphasizing the importance of FLAER for PNH diagnosis.

Methods: A single 5-color tube-FLAER Alexa488/ CD157PE/ CD15PECy5/ CD64PE-Cy7 & CD45APCH7-was used for a high sensitivity PNH assay.

Results: Of 364 cases, 59(16.2%) cases had PNH clone in both granulocytes and monocytes. PNH clone sizes ranged from 0.02% to 96.6% in granulocytes and 0.07% to 96.3% in monocytes based on their FLAER-negative, CD157-negative phenotype. Twenty-two of the 59 PNH cases (37.3%) had WBC clone size of <1%. In addition, there were 10 cases which showed absence of CD 157 expression on both granulocytes and monocytes but on FLAER staining showed normal staining patterns. Three of these ten cases also showed a PNH clone based on absence of FLAER expression on both granulocytes and monocytes.

Conclusion: FLAER and CD157 is a robust combination for diagnosis of clinical and subclinical PNH. Absence of CD157 expression in normal WBCs, though rare, should be kept in mind and re-emphasizes the importance of FLAER for the high sensitivity PNH assay.