What is the difference between laboratory (observed) units and international units?
Laboratory units are the actual values obtained from running an assay with a particular protein in an assay in your lab; i.e., the activity (ED50) you observe on your target cells. International units are consensus values of potency derived from a collaborative NIBSC effort to standardize reported use of proteins. These values are derived from bioassay testing of the same protein by many target cell types/substrains. It is very likely that the laboratory units you observe and the NIBSC values will not correlate 1:1; e.g., it might take 0.1 – 20 U/ml to see 1 U/ml in your experiment. These are bioassay standards describing potency in bioassay only. The mass values assigned to these are not hard values and use of these for immunoassay standardization is of limited value unless assays calibrated by the NIBSC standard use the same capture and detection antibody clones.
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