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U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Improving the understanding, detection, and management of kidney disease.

AACC Annual Meeting – Chicago, IL – July 22, 2009

National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
National Kidney Disease Education Program (NKDEP) Laboratory Working Group Meeting

Participants: Greg Miller, Edward Ashwood, Lorin Bachmann, David Bunk, Christa Colbbaert, John Eckfeldt, James Fleming, Matthew Gnezda, Neil Greenberg, Glen Hortin, Yoshihisa Itoh, Chandra Jain, Graham Jones, Anthony Killeen, David Lacher, John Lieske, Leigh Ann Milburn, Andrew Narva, Eileen Newman, Mauro Panteghini, Max Robinowitz, Mary Robinson, Heinz Schimmel,George Schwartz,Lesley Stevens,Reba Wright, Ingrid Zegers

Summary of Action Items:

Meeting Minutes

Part 1: Serum/plasma/blood creatinine, eGFR, and cystatin C

1. CKD-EPI Equation for eGFR, Lesley Stevens, Tufts University

Lesley Stevens provided a review of a new eGFR equation that developed in the CKD-EPI Study and was published in the Annals of Internal Medicine in May, 2009.

Background: This equation was developed from the NIH-funded CKD-EPI study and is intended for use in general clinical practice. The MDRD study had 1,628 participants with predominantly non-diabetic CKD and included age, sex, and race as surrogates for non-GFR determinants. The equation has reasonable accuracy in the CKD population, but there is a systematic underestimation of measured GFR at higher levels and there is imprecision throughout the GFR range. Estimates > 60 mL/min/1.73m2 are not reported and there is a potential for false positive diagnosis of CKD in patients with measured GFR < 60 mL/min/1.73m2.

CKD-EPI Study: The goal of the CKD-EPI study was to address deficiencies of MDRD equation. CKD-EPI used data that were already collected in clinical studies and includes individuals with and without kidney disease in a wide range of measured GFR. Study populations were used if they had more than 250 participants, measured GFR, quality control data, and ability to calibrate creatinine to IDMS traceability. There were 10 studies, 2/3 for development of the equation and 1/3 for internal validation, and a separate set of 16 studies was used for external validation. Multiple models using new transformations of the original 4 variables included in the MDRD equation and additional variables (diabetes, transplant, weight) were included in the analyses during development. The equation was validated in 16 studies using a pre-specified algorithm. It should be noted that all data sets included a reasonable proportion of participants without kidney disease as shown by the mean GFR, which was 68 mL/min/1.73m2 versus 40 mL/min/1.73m2 in the MDRD Study. The final equation has the same 4 variables as the MDRD equation (creatinine, age, race, and sex), but they are used differently. Specifically, the creatinine variable is the spline of the log of creatinine with sex-specific knots versus the log in the MDRD equation, and the age variable is linear versus log.

Results: The CKD-EPI equation had a substantial reduction in bias measured and estimated GFR as compared to the MDRD equation. When these equations are applied to the NHANES Study population, the CKD-EPI equation gave a higher median eGFR and classified about 20% of the population at a higher stage of renal failure and 0.6% at a lower stage than the MDRD equation. In summary, the new equation will have less false positives in the 30-59 range, be able to detect mild decline in the 60-89 range, and be able to determine eGFR in the normal range >90. From the laboratory point of view, the equation is similar to the MDRD and could be implemented easily. The implication from the clinician point of view is the value of being able to report a more accurate value in the range near 60.

Next steps: Implementation will take some time. The renal community will need to use online calculators to evaluate the new equation for prognosis and improved care. The equation will need to be pilot-tested in a few centers. NKDEP could have a task force to develop preliminary education materials.

Discussion:

2. Serum Creatinine Specificity Study—Progress Report, Neil Greenberg, Ortho-Clinical Diagnostics, Inc.

Background: Calibration traceability to IDMS reference methodology does not change the limitations of creatinine results caused by the influence of interfering substances. The NKDEP LWG and IFCC WG-GFRA plan to develop recommendations on specificity requirements and performance characteristics for creatinine measurement procedures. However, there is very little comparative data looking at current creatinine methods, and there are still unresolved concerns with various drugs, endogenous substances, ketoacidosis, hyerbilirubinemia, hemoglobin, protein, and albumin. The protein effect seen especially with Jaffe procedures is an example of this; low protein/albumin in pediatric and hospitalized patients results in too large of a correction and thus an under-estimation of kidney damage.

NKDEP/IFCC Creatinine Specificity Study: This study was designed to evaluate method specificity using unaltered patient samples from several pathological groups (e. g., diabetes, CKD, liver disease, high and low protein syndromes). Also, there were supplemental samples spiked for certain volatile interferents (e. g., ascorbate, acetoacetate). These samples were assayed by eight different commercial methods (including four enzymatic and four Jaffe) and were performed by the manufacturer in their own facilities. An ID-LC MS/MS reference method developed by Neil Dalton's laboratory at Evilina Childrens Hospital in London was used to determine "truth."

Results: Preliminary data show that the reference method obtains results very close to the certificate values for SRM 967 material, calibration bias may still be significant with some routine methods, and significant bilirubin (4+ icteric samples) interference occurs with both (some) Jaffe and (some) enzymatic methods. Data analyses are still in progress and a final report is anticipated to be available in the next six to nine months.

Discussion:

3. CKD and Drug Dosing Educational Statement for Providers, Lesley Stevens, Chair, NKDEP Committee to Develop Educational Advisory on Drug Dosing in CKD

Lesley Stevens presented information from a draft document for recommendations on drug dosing. (A draft of this document was provided prior to the meeting.) The draft discusses the following 6 categories: estimation of kidney function for medication dosage prescriptions in adults, NKDEP's recommended approach to drug dosing, impact of IDMS-standardized creatinine values, the MDRD Study equation, the Cockcroft-Gault equation, and limitations of any serum creatinine-based estimates. The basic question is: Can the eGFR based on standardized creatinine be used to determine drug dosing? To date, pharmacokinetic (PK) study results and FDA labels are dependent on the specific serum creatinine method used resulting in inconsistent translation of the labels to practice. In the future, because of standardization of creatinine methods, the PK study results and FDA labels can be independent of the serum creatinine methods. However, there is a problem with the vast number of drugs that have already been approved. It is possible that the FDA or drug manufacturers will choose to re-label based on studies performed using standardized creatinine methods, but not all drugs can be re-labeled. We need data on the impact of creatinine calibration for each estimating equation, simulations to assess differences among equations in recommended drug dosages, and simulations to assess differences in prediction of toxicities and outcomes.

The CKD-EPI Study "Development" data set was used to determine impact of different GFR estimating equations on drug dosage by doing a simulation on drug dosing recommendations for 15 medications. The study concluded that the MDRD Study equation had the highest rate of concordance with measured GFR, and for specific drug dosing, concordance rates among equations was high, with lower concordance for drugs with a greater number of dosing levels. (The results of this study have been published: Stevens et al. Am J Kidney Dis. 2009; 54(1):33-42.)

The NKDEP recommendations for drugs that have been proposed in the draft are:

Discussion:

4. Update: Cystatin C Standardization, Greg Miller for Anders Grubb (Chair, IFCC WG-SCC)
University Hospital, Lund, Sweden

Greg Miller reported for Anders Grubb, who was not able to attend. The project has successfully prepared a secondary reference material, which is pooled human serum to which a recombinant cystatin product has been added. The value assignment is complete, and they are beginning the commutability validation process. All methods used to evaluate the material gave results that are in close agreement. It is estimated that the product will be available in 2010. This group is planning a large study to develop a more universal equation once the standardization is in place.

5. Performance of CKiD Pediatric Equation with Different Cystatin C Methods, George Schwartz
University of Rochester Medical Center

Background: The original Schwartz equation used height/serum creatinine to estimate GFR. Blood urea nitrogen (BUN) tends to vary inversely with GFR and urea clearance represents approximately one to two thirds of the GFR. Cystatin C is a new marker and is inversely proportional to GFR. The purpose of this new study is to use these variables to develop a new, more precise and accurate equation for estimating GFR in children.

Study: There were 349 pediatric CKD patients; generally developmentally immature, mostly male, and stunted, which is a stigmata of children with kidney disease. GFR was measured by iohexol disappearance. (The results of this study have been published: Schwartz et al, J Am Soc Nephrol. 2009; 20:629-637.) The cystatin variable in the equation was investigated further by comparing the Dade Behring PENIA (nephelometric) method to the DAKO PETIA (turbidometric) method.

Results: Univariate correlations are higher when cystatin C is measured by Dade-Behring method. Cystatin C and height/serum creatinine describe the entire variability and BUN used in the original publication is not important with a change in cystatin C method. A new equation is being developed.

Issues for NKDEP-LWG: Standardization of creatinine at low concentrations is critical for pediatric needs. Standardization of cystatin C methodology is required for generalized application of GFR estimating formulas; and current new pediatric estimating equations are valid only in the range of iohexol GFR (15-80 mL/min per 1.73 m2).

Discussion:

Part 2: Urine albumin and creatinine (Refer to written summaries sent with agenda)

1. Urine Albumin Biological Variability and Albumin Adsorption Study, Mary Robinson, Centers for Disease Control and Prevention

Study Design: Phase 1 is the container study; Phase 2 is the biologic/pathologic variability study.

Containers were made of the following materials: polypropylene (PP), polystyrene (PS), polyethylene (PE), polyethylene terephthalate (PET), and PL; and sample cups were made of PE and PS; circular discs punched from containers and cups were used; the discs were spiked with 125I-alb loaded on discs and rinsed; the raw data presented have not been statistically analyzed; all sets of containers showed the same pattern for pH and albumin concentration; PE and PE-terephthalate were lowest albumin binders.

Summary: All six materials have greater adsorption at higher albumin concentrations and longer exposure; adsorption at lower pH is greater for most materials; addition of Triton X and Tween significantly reduced the binding; adsorption in sample cups was essentially the same for PE and PS; and addition of Triton X and Tween reduced the adsorption.

Discussion:

2. Urine Albumin Method Harmonization Study, Lori Bachmann, Virginia Commonwealth University

There was not a presentation about this study, but comments on the basic approach given in the handout would be appreciated. The project is expected to start in January 2010.

3. Urine Albumin Molecular Forms Study, Glen Hortin, University of Florida

A summary of the proposal is found in the outline. Comments are appreciated.

4. Urine Albumin Reference Material (JSCC), Yoshi Itoh, Asahikawa Medical College, Hokkaido, Japan

5. Reference Material for Urine Creatinine and Urine Albumin, David Bunk, National Institute of Standards and Technology

6. Urine Albumin IDMS Candidate Reference Measurement Procedure, John Lieske, Mayo Clinic

John Lieske presented data on the validation of an LC-MS/MS procedure for quantification of albumin in a urine matrix. A 15N-labeled human serum albumin internal standard was developed and two separate peptides compared very well. Comparison of the LC-MS/MS to three clinical albumin assays showed good but not perfect agreement and demonstrated that there is variability between assays. The proposal for this project has 2 aims: Aim 1 is to validate the LC-MS/MS procedure as a reference method procedure to quantify intact albumin in urine; and Aim 2 is to compare clinical sensitivity and specificity of LC-MS/MS assay performed at Mayo and NIST, as well as with other immunoassays, using a population of patients with or without overt diabetic nephropathy. The sample requirement for this procedure is about 50 µL.

Discussion:
John Eckfeldt asked which of the two peptides was used for the comparison with the clinical assays because this may have an effect on some of the outliers. He speculated that the ratio of two different peptides may be different for the outliers and explain some of the lack of correlation. John Lieske replied that they suspect that some samples may have different fragments and this may be a reason to look for other fragments to include in the assay.

Part 3: Summary and Next Steps

Discussion:

Summary of actions agreed upon during meeting:

Page last updated: March 1, 2012