Simplified direct Kjeldahl method suitable as a primaryreference procedure for the determination of total protein inreference materials used in clinical chemistry

Chromý et al. found in 2009, that albumin-calibrated analysis of serum total protein with biuret reagent according to a candidate reference method developed by Doumas working group in 1981, provide results with unacceptable positive bias, caused namely by endogeneous serum bilirubin and lipids. They also found, that a simplest way to rectify this defect, is the use of serum and/or plasma-based protein standards, commutable with patients samples, which are certified by the Kjeldahl method. In our foregoing two articles, (Chromý et al., 2015, and Vinklarkova et al., 2015), we reviewed Kjeldahl methods adopted by laboratory medicine, selected a direct analysis on washed protein precipitates with trichloroacetic acid and verified its preliminary analytical performance parameters. In this article, we describe a simplified Kjeldahl analysis with modified digestion flask, now directly connectable to accordingly adapted Parnas and Wagner distillation unit, and present its re-validation. Simplified Kjeldahl method is suggested as a standard operating procedure for total protein in reference materials used in clinical chemistry