Abstract Deposition of particulates, especially colloids, during membrane filtration causes a decline in permeate flux. The existing modified fouling index (MFI) uses a microfilter membrane as a quic test of feedwater quality. The MFI is based on ca e filtration, and thus a model can be developed for flux decline prediction. However, this MFI is not sensitive to the presence of smaller particles. Therefore, more recently the MFI using ultrafiltration membranes (MFI-UF) was developed. This research investigates various criteria of the MFI-UF test for use as a water quality indicator; stability of the MFI-UF over time, linearity of the index with particulate concentration, and reproducibility (1) of the test (reusability of a UF module) and (2) module manufacture. The pressure dependency of the MFI-UF was also examined. The aforementioned criteria were examined using a polyacrylonitrile module of 13,000 molecular-weight-cut-off for low fouling (tap and process water) and high fouling waters (diluted canal water). The MFI-UF was stable over time and directly related to colloidal concentration. The MFI-UF test was reproducible for one module with repeated testing and reproducible module manufacture was found for 80% of tested modules. Ca e compression occurred at higher applied test pressures and using the compressibility factor determined in this research, accurate pressure correction was found.