Background: Reliable serum creatinine measurements in glomerular filtration rate (GFR) estimation are critical to ongoing global public health efforts to increase the diagnosis and treatment of chronic kidney disease (CKD). We present an overview of the commonly used methods for the determination of serum creatinine, method limitations, and method performance in conjunction with the development of analytical performance criteria. Available resources for standardization of serum creatinine measurement are discussed, and recommendations for measurement improvement are given.

Methods: The National Kidney Disease Education Program (NKDEP) Laboratory Working Group reviewed problems related to serum creatinine measurement for estimating GFR and prepared recommendations to standardize and improve creatinine measurement.

Results: The NKDEP Laboratory Working Group, in collaboration with international professional organizations, has developed a plan that enables standardization and improved accuracy (trueness) of serum creatinine measurements in clinical laboratories worldwide that includes the use of the estimating equation for GFR based on serum creatinine concentration that was developed from the Modification of Diet in Renal Disease (MDRD) study.

Conclusions: The current variability in serum creatinine measurements renders all estimating equations for GFR, including the MDRD Study equation, less accurate in the normal and slightly increased range of serum creatinine concentrations [<133 μmol/L (1.5 mg/dL)], which is the relevant range for detecting CKD [<60 mL · min−1 · (1.73 m2)−1]. Many automated routine methods for serum creatinine measurement meet or exceed the required precision; therefore, reduction of analytical bias in creatinine assays is needed. Standardization of calibration does not correct for analytical interferences (nonspecificity bias). The bias and nonspecificity problems associated with some of the routine methods must be addressed.

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Recommendations for Improving Serum Creatinine Measurement: A Report from the Laboratory Working Group of the National Kidney Disease Education Program

Measuring serum creatinine is cheap and commonly done in daily practice. However, interpretation of serum creatinine results is not always easy. In this review, we will briefly remind the physiological limitations of serum creatinine due notably to its tubular secretion and the influence of muscular mass or protein intake on its concentration. We mainly focus on the analytical limitations of serum creatinine, insisting on important concept such as reference intervals, standardization (and IDMS traceability), analytical interferences, analytical coefficient of variation (CV), biological CV and critical difference. Because the relationship between serum creatinine and glomerular filtration rate is hyperbolic, all these CVs will impact not only the precision of serum creatinine but still more the precision of different creatinine-based equations, especially in low or normal-low creatinine levels (or high or normal-high glomerular filtration rate range).

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Serum Creatinine: Not So Simple!

Background. International recommendations suggest that measurement of serum creatinine should be supplemented with an estimate of glomerular filtration rate (GFR) using the Modification of Diet in Renal Disease (MDRD) study equation. One problem has been the lack of standardization of commercially available creatinine assays resulting in varying estimates of GFR. A revision of the MDRD equation offers traceability to a reference method. This study evaluates the use of isotope dilution mass spectrometry (ID-MS), the compensated Jaffe and enzymatic creatinine methods compared with the Beckman CX3 Jaffe assay used to derive the MDRD equation and investigates their impact on GFR estimation using both the original and ID-MS-traceable MDRD equations. Methods. Serum creatinine was measured in 277 patients by (i) ID-MS, (ii) a Roche enzymatic assay, (iii) a Roche compensated kinetic Jaffe assay and (iv) a Beckman CX3 kinetic Jaffe assay. Estimated GFR was calculated using the MDRD equations. Results. The ID-MS (� 7.5%), Roche enzymatic (� 8.6%) and compensated kinetic Jaffe (� 11.9%) assays were all negatively biased (P < 0.0001) compared with the Beckman CX3 assay, causing predictable, clinically significant, overestimation of GFR when the original MDRD equation is used. This positive bias was reduced (ID-MS 6.7 to 0.4%; enzymatic 8.8 to 3.4%; compensated kinetic Jaffe 13.7 to 7.1%) when GFRs were calculated using the ID-MS-traceable MDRD equation. Conclusions. Compensated assays that account for non-creatinine chromogen interference produce significantly higher estimates of GFR when using the original MDRD equation. Use of the ID-MStraceable MDRD equation ameliorates this effect. There is good agreement between estimated GFR derived from the original MDRD equation using Beckman Astra CX3 data and estimated GFR derived from the new ID-MS-traceable MDRD equation using a local ID-MS creatinine assay. This suggests that the ID-MS-traceable MDRD equation may be reliably used with both ID-MS and true ID-MS-traceable creatinine assays without the requirement for standardization to the MDRD laboratory.

Does the ID-MS traceable MDRD equation work and is it suitable for use with compensated Jaffé and enzymatic creatinine assays?

A multicentric evaluation of IDMS-traceable creatinine enzymatic assays

Specimen materials, target values and commutability for external quality assessment (proficiency testing) schemes.

The results of an external quality-assessment experiment for serum creatinine measurement are described. Fifty-one laboratories performed quintuplicate analyses during three different analytical runs on six lyophilized sera and two frozen human serum pools. Isotope dilution gas chromatography–mass spectrometry (ID GC-MS) target values were assigned to all the materials. Intralaboratory within- and between-run imprecision results were very similar for all the materials tested (CV ≤2.20% and ≤4.70%, respectively). The overall imprecision obtained was high (CV 6.5–20.0%) because of increased interlaboratory–intermethod variability. A significant positive bias (+9.2–+43.7%) was found for all the materials at lower creatinine concentration. By using two human sera at different concentrations, we could calculate the constant and the proportional calibration bias displayed by each peer group. The majority of the lyophilized materials showed a behavior divergent from the frozen pools, indicating matrix-related problems. We propose a new algorithm for calculating matrix bias correction factor instrument–reagent specific for each material.

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Elena De Giorgi

Papers

Creatinine measurement proficiency testing: assignment of matrix-adjusted ID GC-MS target values