What makes an assay fit for purpose?

Why

Begin with the measurable property, sample matrix and the decision to be made. A discovery screen needs robust ranking and follow-up confirmation, while a release assay or clinical measurement has stricter traceability and validation requirements. Define acceptance criteria for selectivity, precision, accuracy or bias, reportable range, stability and sensitivity where applicable.

Use reference materials and positive, negative and system suitability controls in the actual matrix. For example, a fluorescence assay may reproducibly rank high versus low binders in buffer but fail in serum because of quenching. It might remain useful for early screening yet be inappropriate as a quantitative serum biomarker test.

ICH Q2(R2) and Q14 guide formal analytical procedure validation and development in their regulated scopes. They should not be portrayed as mandatory rules for every exploratory assay. Qualification is a defensible evidence package supporting a bounded decision, not a single metric or attractive standard curve.