To investigate EDTA-plasma/serum (P/S) differences, we collected paired samples from 25 volunteers and measured total cholesterol (TC), triglyceride (TG) and high-density-lipoprotein cholesterol (HDLC), using the Cobas FARA, Ektachem 700, DuPont Dimension, and Baxter Paramax Analyzers. The mean (SD) P/S ratios for TC, HDLC, and TG concentrations were, respectively: 0.980 (0.0171), 1.063 (0.0704), and 0.961 (0.363) for Paramax; 0.976 (0.0189), 1.034 (0.1091), and 0.950 (0.557) for Dimension; 1.003 (0.0221), 1.059 (0.0304), and 0.988 (0.0179) for Ektachem; and 0.993 (0.0162), 1.063 (0.0830), and 1.013 (0.0410) for Cobas. We conclude that P/S ratios vary by analytical methods, and that HDLC ratios tend to be larger in magnitude and in the opposite direction from TC and TG. Both effects lead to significant biases in computed disease risk.