USP 729 Emulsion Size Testing

The size of the lipid droplets is critical because larger-size fat globules (>5 µm) can be trapped in the lungs and are also an indicator that the emulsion is destabilizing. The USP <729> test requires two analytical techniques: DLS or laser diffraction to measure the mean and standard deviation of the distribution and light obscuration to measure the large tails >5 µm.

Method I: Mean and standard deviation of the distribution by DLS:
•Test the system with PSL standards at 100, 120 & 400 nm
•The coefficient of variation (COV) must be < 10% of the reference values
•Measure the sample, report the intensity-weighted mean, which must be less than 500 nm

Method I is best measured using the Nicomp 380 DLS system

Method II: Large-diameter droplet tail (>5 µm) by a single particle optical sizing (SPOS):
•Perform a dilution study to achieve a COV <10% between samples
•Test the sizing and counting accuracy at 5 µm & 10 µm
•The size and count should be within 10% of the certified vales
•Set the lower size limit at 1.8 µm and upper limit at 50 µm
•Vary the measurement time so that there is a factor of two difference > 5 µm between two runs
•The volume-weighted result > 5 µm (PFAT5)  must be < 0.05%

Method II is best measured using the AccuSizer AD system*

*See reference: Driscoll, D., et. al., Physicochemical assessments of parenteral lipid emulsions: light obscuration versus laser diffraction, International Journal of Pharmaceutics 219 (2001) 21–37