u/ruthie
A useful control should rule out a specific alternative explanation.
Suppose an immunoblot shows no target band but has a clear loading-control band at a different molecular weight. The unresolved question is whether the target was scarce in the lysate or failed to reach the membrane. Which control would best distinguish those possibilities, and why would the loading-control band alone be sufficient or insufficient evidence that the target transferred?
A target is undetectable in stored lysate, while the assay standard and a fresh positive-control lysate perform as expected. Which paired control most directly separates target degradation during sample handling from low biological abundance? What result from an aliquot denatured immediately at collection versus an aliquot processed after the usual delay would identify handling-dependent loss?
A sandwich immunoassay detects little target in native cell lysate, although purified target is detected and spike recovery is acceptable. Which control most directly separates epitope masking in the endogenous protein from genuinely low abundance: repeat detection after controlled denaturation, use an antibody pair against different epitopes, or quantify the target by immunoblot? What result would specifically support epitope masking?
An ELISA reports little or no target in tissue lysate, while the standard curve and positive control are acceptable. Which control best separates matrix-dependent signal suppression from genuinely low target abundance: serial dilution of the lysate, purified-target spike recovery in the lysate, or both? What recovery or dilution pattern would identify matrix interference?
A soluble-protein assay gives no target signal, although total protein concentration is within range. Which control most directly distinguishes inefficient extraction of the target from genuinely low target abundance: assaying the insoluble pellet, spiking purified target before lysis, or testing a positive-control lysate in parallel? What pattern would identify extraction failure?
A target band is absent after immunoblotting, while the loading control is present. Which control would best test whether the target protein failed to transfer rather than being genuinely scarce: staining the membrane for total protein, staining the post-transfer gel, or including a positive lysate known to contain the target? What result would distinguish these explanations most directly?
