Response curves representing deviations from the trend line. Red diamond, transition 1; blue asterisk, transition 2; green triangle, transition 3. (a) Study I, site 52, MBP-HGF: interference in transition 1 of the analyte. (b) Study IIIb, site 95, MYO-LFT: interference in transition 2 of the analyte, which was also observed in study I, II and IIIa for this laboratory. (c) Study II, site 86, CRP-ESD: endogenous protein level increased the estimated protein concentration at the low end of the concentration range of spiked-in proteins, resulting in flattening of slope. (d) Study IIIa, site 56, LEP-IND: unstable electrospray conditions resulted in a substantial increase in interlaboratory CV to 99%. (e) Study IIIa, site 19, MBP-YLA: no detection of MBP-YLA peptide at any site. (f) Study I, site 86, PSA-IVG: saturation at highest two concentrations. Site codes are identical to those given in and .