Nominal pore size rating describes the ability of the filter to retain the majority of the particles at the rated pore size and larger (60-69%) and cannot be used to compare filters among manufacturers. Glass fiber filters and screen filters are a good example of nominally rated filtration.   

Processing conditions such as operating pressure and concentration of contaminant have a significant effect on the retention efficiency of the nominally-rated filters.

Absolute ratings are used to represents the size of the smallest particle completely retained. Complete retention is within the experimental uncertainty of a standard test method consistent with the intended filter use. Among the test conditions that must be specified are test organism (or particle size), challenge pressure, concentration, and detection method used to identify the contaminant. Most membrane filters are rated as absolute terms.