Antilogs of the mean differences were calculated to assess the mean proportional biases of the TCEP method with respect to the TBP method.

The mean proportional bias (antilog of the mean difference calculated from log-transformed data) was largest for tCys and tGSH determinations: 65.

The mean proportional biases between the TBP and TCEP determinations of tCys, tCys-Gly, and tGSH (Table 2) therefore originated in the differences between the aminothiol peak areas in the clinical plasma samples (see above) rather than from the differences between calibration slopes.

When GC-MS vs FPIA was compared with GC-MS vs HPLC, it became apparent that the above-mentioned ranges of percentages for the two comparisons were virtually the same size for fasting tHcy concentrations, but that HPLC showed a larger negative mean proportional bias (the antilog of the mean difference calculated from log-transformed data) than FPIA.

The mean proportional bias of HPLC vs GC-MS was very similar to that observed for fasting tHcy concentrations.