Context .- Delta checks have been suggested to increase patient safety by identifying preanalytic and analytic errors, including wrong name mislabeling on the sample tube. Objective .- To implement an effective and practical complete blood cell count (CBC) delta check by optimizing specificity and sensitivity using weighted deltas of multiple parameters. Design .- The mean red blood cell volume (MCV) delta (>3.0 fL) check was retrospectively assessed. The composite CBC delta (CCD) test was formulated using serial same-patient CBC data and random interpatient CBCs. The logical delta check (LDC) ignores CCD failures due to platelet change only. The effect of LDC implementation was evaluated. Results .- The MCV delta check test recognized only 3 of 6 confessed mislabeled specimens in the initial review period, whereas all were identified using the CCD. The LDC flagged 2% (205 of 13 234) of eligible results, one-third as many as the MCV delta check. The CCD and LDC checks revealed 20 presumed or confirmed mislabeling events, only half of which were caught by the MCV delta check. Thirty-four percent of LDC failures not due to transfusion reflected problematic results, including presumed or confirmed wrongly labeled patient samples (36% of flags for real problems). Implementation of the LDC, requiring immediate verbal feedback to the caregivers, was associated with more retracted erroneous results in patients' medical records. Conclusions .- The MCV delta check test was found not to have led to correction of errors in our laboratory due to impractically low specificity and sensitivity. The LDC is a useful tool for identifying preanalytic and analytic specimen problems, including wrong name mislabeling on the sample tube.