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Abstract
To select a Kjeldahl procedure suitable for the determination of total protein in reference materials used in laboratory medicine, we reviewed in our previous article Kjeldahl methods adopted by clinical chemistry and found an indirect two-step analysis by total Kjeldahl nitrogen corrected for its nonprotein nitrogen and a direct analysis made on isolated protein precipitates. In this article, we compare both procedures on various reference materials. An indirect Kjeldahl method gave falsely lower results than a direct analysis. Preliminary performance parameters qualify the direct Kjeldahl analysis as a suitable primary reference procedure for the certification of total protein in reference laboratories.
Acknowledgments
The authors would like to express thanks to our glass blower, Mr. Jiří Nečas, who manufactured all special glassware, and to Ing. Pavel Krásenský, who manufactured aluminium digestion blocks, for their valuable work.
Abbreviations
| CV | = | coefficient of variation |
| IMP | = | intermediate measurement imprecision |
| KC | = | Kjeldahl catalyst |
| KM | = | Kjeldahl method |
| LOD | = | lower limit-of-detection |
| LOQ | = | lower limit-of-quantification |
| MR | = | measurement repeatability |
| NPN | = | nonprotein nitrogen |
| POC | = | point-of-clearing |
| RL | = | reference laboratory |
| RM | = | reference material |
| S | = | serum |
| SD | = | standard deviation |
| SOP | = | standard operating procedure |
| T | = | titrant titet |
| TCA | = | trichloroacetic acid |
| TF | = | titration flask |
| TKN | = | total Kjeldahl nitrogen |
| TP | = | total protein |
| U | = | urine |
| uc | = | combined uncertainty |
| W, w/v | = | weight, weight per volume |