Light obscuration (LO) is useful for an approximate particle range from 1 to 300 μm, depending on the instrument and configuration (see Measurement of Subvisible Particulate Matter in Therapeutic Protein Injections 〈1787〉 and Methods for the Determination of Subvisible Particulate Matter 〈1788〉). Certain solution formulations may not be easily analyzed by LO. The LO method may encounter problems with products that do not have the clarity and viscosity approximating that of water. Further, formulation characteristics such as color and high viscosity, or inherent formulation properties such as shear-induced changes, may generate erroneous LO data. Similarly, products that produce air or gas bubbles when drawn into the LO sensor, such as bicarbonate-buffered formulations, may generate erroneous data. For these product types, the membrane microscope (MM) method may be more appropriate.