Can Manufacturing Challenge QC On Viscosity Measurement?

The current batch of skin cream lotion seems ok to the eye. It has the same feel and appearance as all the other batches produced earlier in the week. Yet QC reports that it’s too thin, the viscosity value is low and the batch needs rework in order to meet the control spec. Manufacturing is suspicious that there is something wrong with the viscosity measurement. How do you go back and challenge QC when they own the instrument that made the reading?

There are two thoughts to consider. Did the person making the viscosity measurement use the correct test method and do they have proper training to run the test? Is the instrument correctly calibrated? The two possible sources of error, if the batch is truly in spec, are the operator and the instrument.