Protein-free filtrate: (PFF) is a clear, colorless liquid formed by ultrafiltration of human blood serum. It contains everything in serum other than proteins greater than 10,000 Da, the molecular weight cutoff for the filter. PFF differs from serum ultrafiltrate in that the latter is prepared without addition of anticoagulants, which are sometimes present in PFF.

PFF is the starting material for many immunoassays, as the absence of proteins helps avoid assay interference. For example, PFF is frequently used as the matrix for measuring steroids and drugs with immunoassays.