Bio-Rad Apoptosis Assays - Manual (Page 33)

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Possible Solutions
If samples contain little or no analyte,
negative values observed may
be due to statistical variation. If
assay drift is suspected, retest the
samples by positioning them next
to the standards. If contamination
of standards is suspected, check
the standard replicate value and
be careful when adding samples to
the wells. Matrix effects could also
produce negative sample values.
Bio-Plex Manager
™ software
automatically subtracts the blank (B)
MFI value from all other assay wells.
While this has no impact on observed
concentrations of samples within the
assay working range, it may result
in a negative MFI value if the blank’s
MFI value is greater than either the
standard or the sample value. If this is
undesirable, then reformat the blank
wells as sample (X) or control (C) in
the protocol or results file.
Possible Causes
Impact of Sample Matrix
Negative MFI values in samples