OD (or optical density) or absorbance values are a direct measure of color intensity by an ELISA reader or spectrophotometer. Color development (therefore ODs) during assay processing may change from run to run (even within the same kit lot) due to room temperature fluctuations, incubation time, pipetting, plate washing and tech-to-tech variability. As long as the Quality Control criteria listed in the package insert are met, the CF (or calibrator correction factor) will accommodate for OD variability when calculating index ratios and interpretive results.