Why does cell confluence matter for interpreting a signaling assay?

Why

In adherent cultures, a near-confluent monolayer may activate contact-dependent pathways and slow proliferation, while sparse cells can have different integrin and growth-factor responses. If a compound is applied to sparse treated wells and dense vehicle wells, differences in phospho-protein signals may reflect density rather than the compound. Standardize seeding, time since plating, medium and stimulation interval, and measure or image confluence when collecting lysates.

Normalize to viable cell number or total protein only when that normalization fits the biological question. It cannot automatically correct density-dependent changes in pathway state. Some primary cells differentiate at high density, so matching cell count may not be equivalent to matching state.

Document confluence ranges and repeat the critical contrast at at least one second seeding density. The main objective is to make cellular context comparable before attributing signaling differences to an intervention.