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Variability of the IFN-γ ELISPOT assay over the study period and determination of cutoffs. Shown are the numbers of spot-forming cells per well after stimulation of quality control (QC) PBMC with the CEF peptide pool from more than 40 time points. Quality control PBMC were recovered from the frozen state and rested overnight before use. For determination of cutoffs, a total of 136 quality controls during the 9-month interval were used.
