Bio-Rad Human MMP and TIMP Assays - Manual (Page 36)

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Possible Solutions
Pipet carefully when adding standards, samples,
detection antibodies, and streptavidin-PE, especially
when using a multichannel pipet. Use a calibrated
pipet. Change pipet tip after every volume transfer.
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.
Check if any interfering components, additives,
or gel from separators were introduced into the
samples. Avoid using hemolyzed and heavily
lipemic samples. Remove visible particulate in
samples by centrifugation. Avoid multiple freeze-
thaw cycles of samples.
Possible Causes
Poor Recovery
Improper pipetting
technique
Impact of Sample Matrix
Negative MFI values in
samples or standards
Poor precision in
serum and
plasma sample
measurements