The present invention relates to an erythropoietin-containing solution preparation comprising a poloxamer and having a pH of 6.5 to 7.5. The present invention also relates to a method for quantifying a protein in a trace amount, the method comprising the following steps: 1) binding a protein sample to a high-intensity fluorescent dye； 2) separating a desired analyte from the obtained sample by appropriate separation means； and 3) quantifying the desired analyte and converting the amount of the protein from the amount of the analyte.
