If the QC accuracies are consistent for all measured analytes, but differ significantly from the expected values (i.e. all metabolites for any particular QC sample are at 50% accuracy), the following may have occurred:

• Pipetting issue: More or less than 10 µL of the QC sample was transferred to the plate. An air bubble in the pipette tip, pipette tip missed the center of the filter paper, or excess liquid was attached to the outside of the tip. It could also be that the internal standard mix was not transferred correctly (not applicable to the MxP® Quant 500 kit).
• Reconstitution issue: The QC vials were not resuspended in the correct volume. For instance, if 200 µL were used instead of 100 µL, the accuracies would be around 50%.
• Mixed QC samples: The QC samples were not pipetted in the correct order.
• Calibration standard issue: Check both the FIA and LC parts of the kit. If the problem only occurs in the LC part of the kit, it is likely related to a problem with the calibration curves or calibration standards might have been mixed up during pipetting onto the kit plate (e.g. Cal3 to Cal4 well and vice versa).