Delay in the measurement of eosin-5′-maleimide (EMA) binding does not affect the test result for the diagnosis of hereditary spherocytosis

Background: The eosin-5'-maleimide (EMA) binding test is a flow cytometric test widely used to detect hereditary spherocytosis (HS). EMA binds to plasma membrane proteins of red blood cells (RBCs), mainly to band 3 protein. The mean fluorescence of EMA-stained RBCs in HS patients is lower when compared with control RBCs due to the decreased amount of target proteins. EMA dye in aqueous solution is sensitive to light and high temperature. Its fluorescence can decrease when exposed to light or ambient temperatures higher than 4 degrees C. The aim of the study was to evaluate the stability of fluorescence readings of EMA-labeled RBCs over a period of 24 h. Methods: The EMA test was performed in peripheral blood from 35 patients with microcytic anemia (five with HS, and 30 without HS). Peripheral blood samples were stained immediately after blood collection and analyzed using a flow cytometer at three time points: 0, after 1 and 24 h of storage at 4 degrees C in the darkness. The results are presented as the percentage of normal control RBCs fluorescence. Flow cytometric studies were performed with Cytomics FC500 (Beckman Coulter, USA). Results: In HS patients the mean result of the test reached 66.72% +/- 9.26% of normal controls, and in non-HS patients the EMA result was 99.48% +/- 5.03% of normal control cells. The results of patients with HS were 66.72% +/- 9.26%, 66.90% +/- 10.24% and 67.86% +/- 11.31% at 0 h, and after 1 and 24 h of storage, respectively. The results obtained from non-HS patients at time 0, after 1 and 24 h of storage reached 99.48% +/- 5.03%, 99.49% +/- 5.34% and 99.78% +/- 6.13%, respectively. There was no difference between the results from each time point in samples from patients with or without HS. Conclusions: Results of the EMA binding test do not depend on storage time of stained samples when stored at 4 degrees C up to 24 h after staining.