Representative protein mass spectra in the range of 2000 to 4000 m/z. Cases 1 and 2 were patients receiving hemodialysis and cases 3 and 4 were volunteers with normal renal function. Peaks at 2192, 2789, and 2851 m/z were observed in case 1 but not in cases 2 and 4. In case 3, the peak at 2789 m/z was slightly increased. r indicates Pearson correlation coefficient.