A trend in QC results is most likely caused by:

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A trend in QC results reflects a gradual, directional drift in control values over consecutive runs. This pattern points to a progressive change in the measurement system, most commonly because the reagent is deteriorating. As a reagent loses potency or concentration with time, each subsequent assay tends to yield slightly different results, producing a steady rise or fall in control values on a quality-control chart.

Miscalibration of the instrument usually causes a bias that appears as a shift or step change rather than a slow, consistent trend across many runs, unless calibration drifts steadily—which is less typical. Improper dilution of standards can introduce bias as well, but it tends to produce a more uniform error rather than a continuous, time-ordered drift. Electronic noise, on the other hand, leads to random scatter without a directional pattern.

So, the most likely cause of a persistent trend is deterioration of the reagent, creating a gradual change in assay performance over time.