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

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

Answer: (A)

A progressive drift in assay results over time is a systematic change in the measurement, not random scatter. The most common reason for this kind of trend is deterioration of the reagents—their activity or signal gradually changes as they age, so each subsequent measurement shifts in the same direction. Calibration error would generally cause a fixed bias across results rather than a time-dependent drift unless calibration itself is changing during the run. Pipetting error tends to produce random variation from one measurement to the next, not a steady trend, and random error by itself cannot explain a consistent increase or decrease over many measurements. So, reagent deterioration best accounts for a trend in assay results.