Encapsulation efficiency (EE) estimates the encapsulated fraction of measured RNA in an mRNA lipid nanoparticle (LNP) preparation, whereas particle occupancy measures the fraction of particles containing detectable RNA. Neither metric describes the full distribution of RNA among particles. Particle-resolved studies show that high bulk EE can coexist with RNA-negative particles and broad copy-number distributions, but reported empty-particle fractions vary with composition, preparation, and analytical method. This Minireview examines fluorescence spectroscopy, particle-resolved microscopy and RNA counting, mass spectrometry, and population-separation approaches, distinguishing direct RNA-sensitive measurements from inferred loading. It considers manufacturing determinants and evidence linking payload distributions to product performance. Process changes can alter loading without parallel changes in mean particle size. mRNA-expression, multi-cargo gene-editing, and adjacent siRNA-knockdown studies suggest possible functional consequences, although the independent effect of binary occupancy on potency remains unresolved. Historical lipid-mediated delivery experiments caution against assuming that eliminating RNA-negative particles always improves activity. Occupancy is therefore best considered a context-dependent extended-characterization metric for development and comparability. Product-specific evidence is needed to determine whether occupancy is a critical quality attribute. Its use in routine testing would also require a fit-for-purpose validated analytical procedure and an appropriate control strategy.
