What is the major cause of a shift in assay results?

• A. Calibration error
• B. Random error
• C. Deterioration of reagent
• D. Pipetting error

Answer: (A)

A shift in assay results reflects a systematic bias affecting measurements in the same direction, and calibration sets the instrument or assay response to known standards. If the calibration is off, all results move consistently—high or low—creating that uniform shift you observe. Random error, by contrast, adds scatter without a fixed direction. Deterioration of reagent can cause drift over time but doesn’t typically produce an immediate, uniform shift across many measurements, and pipetting error tends to introduce imprecision (random variation) rather than a consistent bias. So calibration error best explains the shift.