A two-point calibration of Jaffe method for serum creatinine combining only one serum standard with creatinine solution matching standard's allow matrix-present interferents present results well comparable with enzymatic determination, providing the standard is attested/linked to the reference measurement procedure.

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.

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Standard Reference Material 927c based on pure bovine albumin is still recommended and used as the primary standard for assays of total protein by colourimetric methods. The albumin calibrator is responsible for a positive bias of approximately 3%-5% in serum total protein assayed by the biuret reaction both in the reference and in current methods. Its substitution by a serum calibrator attested by the Kjeldahl method could solve this drawback. Clin Chem Lab Med 2009;47:91-101.

We studied the optimal composition of EDTA-chelated biuret reagent for the determination of protein in serum. A reagent containing 18 mmol of EDTA, 15 mmol of Cu2+, and 1 mol of NaOH per liter exhibits a very low blank but about the same sensitivity as tartrate-chelated reagents. Maximum color intensity is attained within 25 min at room temperature, and adheres to Beer's law up to 120 g of protein per liter.

We compared various procedures and biuret reagents suggested for determination of protein in lipemic sera. A modified procedure, consisting of the precipitation of serum protein with acetone and dissolution of the pre¬cipitate in a biuret reagent, has been developed. The sample (0.02 ml) is mixed with 0.1 ml of distilled water and 2 ml of acetone. The mixture is shaken, centrifuged, and the precipitate dissolved in 1.5 ml of ethylenediaminete¬traacetate-chelated biuret reagent. This method is suitable for both clear and lipemic sera. Values obtained with the procedure proposed are in good agreement with values by the Kjeldahl method.

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