Investigation: one set of blood bags with the filter from the collection set was returned for evaluation.The leukoreduction filter was tested for flow rate and air leaks.A fast flow rate of 50ml/min was noted, and it was confirmed there were no air leaks.The filter was disassembled to observe the appearance of filter membranes and noticed creases in the filter membranes of the filter.The creases in the filter were not different from those in conforming products and in the filter membranes and no aggregation was observed in any filter membranes.The manufacturing records, test records, and inspection records were reviewed for abnormalities and none were found.The records regarding the particulate removal rates of the filter membranes were reviewed.All membranes conformed to established specification.Shipping testing was performed on the reserve samples from the reported lot number.The reserve samples were also visually examined, and the solution volume and solution composition were tested with no abnormalities noted.All product conformed to the established specification.Root cause: the above-mentioned investigation results revealed that there were no abnormalities in the manufacturing record and the testing and inspection record of the reported lot number.The returned filter also revealed no abnormalities in itself.We noticed that the third through fifth filter membranes were dyed dark with toluidine blue.Therefore, occlusion may have occurred and blood may have been filtered by the filter area which was smaller than usual and the linear speed (flow rate per unit area) increased, and then leukocyte leakage occurred.The instructions for use provide a caution to not squeeze or apply pressure to the filter while it is attached to the bag containing the filtered blood and also to clamp the blood-filled tubing before blood enters the filter in order to avoid leukocyte leakage.In addition, we observed no aggregation in any filter membranes and confirmed that the first and second filter membranes had some areas that had not been used for filtration.It was likely to take a longer time for priming of the filter for some reason; however, we were not able to identify the cause of the issue.We infer that the unused areas of the filter membranes impeded blood flow and it caused to increase the linear speed of the areas where blood flowed.As a result, leukocyte leakage may have occurred.
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