The Kjeldahl Method as a Primary Reference Procedure for Total Protein in Certified Reference Materials Used in Clinical Chemistry. I. A Review of Kjeldahl Methods Adopted by Laboratory Medicine

Abstract

We found previously that albumin-calibrated total protein in certified reference materials causes unacceptable positive bias in analysis of human sera. The simplest way to cure this defect is the use of human-based serum/plasma standards calibrated by the Kjeldahl method. Such standards, commutative with serum samples, will compensate for bias caused by lipids and bilirubin in most human sera. To find a suitable primary reference procedure for total protein in reference materials, we reviewed Kjeldahl methods adopted by laboratory medicine. We found two methods recommended for total protein in human samples: an indirect analysis based on total Kjeldahl nitrogen corrected for its nonprotein nitrogen and a direct analysis made on isolated protein precipitates. The methods found will be assessed in a subsequent article.

Keywords:

• calibration
• clinical chemistry
• Kjeldahl methods
• reference materials
• total protein

Abbreviations

KM	=	Kjeldahl method
LM	=	laboratory medicine
NPN	=	nonprotein nitrogen
RL	=	reference laboratory
RM	=	reference material
SOP	=	standard operating procedure
TCA	=	trichloroacetic acid
TKN	=	total Kjeldahl nitrogen
TP	=	total protein