MWCO

This range is small enough to retain major blood proteins like albumin, which would otherwise foul chromatography columns or mass spectrometer inlets, while still being large enough to let small-molecule drugs and metal ions diffuse freely out into the dialysate for analysis. A much tighter cut-off would slow diffusion of the analytes of interest.

Yes, if a drug is bound to plasma proteins or exists as a large metabolite complex, dialysis at a standard MWCO may retain more of it in the protein fraction than expected, reducing the amount reaching the dialysate for detection. Analysts account for this by choosing extraction methods suited to the target's protein-binding behavior.

They are related but not identical. MWCO is a nominal rating based on the molecular weight of solutes the membrane is expected to retain under standard conditions, while actual pore size distribution and solute shape both influence real-world retention, so two membranes with the same MWCO can behave slightly differently.