The measurement of a fibrinogen α C-chain 5.9 kDa fragment (FIC 5.9) using MALDI-TOF MS and a stable isotope-labeled peptide standard dilution

We previously identified a 5.9 kDa peptide fragment of fibrinogen α C-chain (FIC 5.9) as a novel biomarker candidate for heavy drinking. In an effort to improve FIC 5.9 measurement for potential use in clinical diagnostics, we combined the ClinProt System and a stable isotope-labeled peptide standard dilution as a simple and reproducible system for measuring FIC 5.9. We analyzed 104 serum samples that were obtained from patients with alcohol dependency, from patients with chronic hepatitis C, and from healthy volunteers. Serum FIC 5.9 levels were measured using the ClinProt system with and without a stable isotope-labeled synthetic FIC 5.9 as an internal standard. The within-day and between-day CVs were significantly smaller with stable isotope dilution mass spectrometry (SID-MS) than with conventional MALDI-TOF MS. Of the two different MALDI-TOF MS platforms, we obtained concordant results with SID-MS. Furthermore, only SID-MS detected a small but significant difference between the serum FIC 5.9 levels in the chronic hepatitis C group and the controls. MALDI-TOF MS with a stable isotope-labeled peptide spike can determine serum FIC 5.9 levels more precisely than conventional MS. This will make inter-laboratory FIC 5.9 comparisons possible.