Absolute (molar) quantification of proteins provides the analytical rationale for system-level modelling of diverse molecular mechanisms. FastCAT method employs multiple short (<50 kDa) stable-isotope labeled chimeric proteins (CPs) composed of concatenated quantotypic (Q-)peptides representing the quantified proteins. Each CP also comprises scrambled sequences of reference (R-)peptides that relate its abundance to a single protein standard (BSA). FastCAT not only alleviates the need in purifying CP or using SDS-PAGE, but also improves the accuracy, precision and dynamic range of the absolute quantifications by grouping Q-peptides according to the expected abundance of target proteins. We benchmarked FastCAT against the reference method of MS Western and tested it in the direct molar quantifications of neurological markers in human cerebrospinal fluid at the low ng/mL level.