Two qualified analysts get systematically different results. How would you investigate?

Why

ICH Q2(R2) expects intermediate precision to cover variation between days, analysts and equipment. A consistent analyst difference means the method has a step that people perform differently and that validation did not capture or control. The procedure is probably the cause more often than the person.

Typical culprits are a sonication or shaking time described as "until dissolved", pipetting technique with viscous diluents, filter choice and discarded volume, timing between preparation and injection for an unstable solution, and manual integration of a shoulder peak. Start by confirming the difference is real with a designed comparison. Each analyst prepares replicates from the same homogeneous lot, on the same day, with the same reagent lots and instrument.

If the bias persists, swap one factor at a time: have each analyst inject the other's solutions to separate preparation from measurement, and have both integrate the same raw files to test integration. Observing each analyst do the preparation side by side often finds the difference in minutes. The result usually feeds back into the method, with a tighter instruction and possibly a robustness study of that parameter under ICH Q14.

It also raises the question of past results, since one analyst's data may be biased, and a risk assessment is needed for released lots.