Question 3 of 6 Intermediate Scenario
Why does an OOS result not disappear after a passing retest?
Short answer
An OOS result is documented evidence that must be explained. Retesting can provide useful information but does not erase the first result or prove it was invalid.
Why
Consider a potency result below the specification followed by a passing result from a newly prepared aliquot. The first test may reflect analyst preparation, sample heterogeneity, true batch variability or instrument issues. Review objective contemporaneous evidence and the approved method before accepting any assignable laboratory cause.
Retest number and plan should be scientifically justified rather than chosen after seeing values. If the original result remains unexplained, evaluate product impact and production records through a fuller investigation. Document the complete sequence including all failing and passing data so the quality unit can make an informed decision.
A retest can support a hypothesis but cannot alone demonstrate that a genuine quality defect never existed.
Common mistakes
- Calling an OOS value an outlier only because later tests passed
The interviewer might ask next
What evidence would convincingly show that the initial assay preparation was invalid?