Creatinine Standardization Recommendations

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Clinical laboratories are crucial partners in the successful implementation of the Creatinine Standardization Program. For clinical laboratories, the following steps are necessary:

Continue using the Original Modification of Diet in Renal Disease (MDRD) Study equation for routine methods that have not been calibrated to be traceable to IDMS. It is appropriate to use this equation because most methods in this category will produce creatinine results that have a bias similar to that of the method used in developing the Original MDRD Study equation. Contact the reagent and/or calibrator manufacturer with questions about the traceability of the calibration for the method used in your laboratory.


Coordinate with your creatinine method provider to ensure immediate use of the IDMS-traceable MDRD Study equation for estimating GFR once your lab begins using a creatinine method that has its calibration traceable to isotope dilution mass spectrometry (IDMS). During the transition to IDMS-traceable calibration, methods that produce results that have acceptable bias [as defined in Clinical Chemistry 2006;52(1):5-18] when compared to an IDMS-traceable method also should use the IDMS-traceable MDRD Study equation.


Communicate the following to health care providers, including pharmacists, when using a serum creatinine method that has its calibration traceable to an IDMS reference method:
Serum creatinine reference intervals will change, depending on the method. Provide the reference interval appropriate for the method.
All creatinine-based equations, other than the IDMS-traceable MDRD Study equation, used to estimate kidney function, such as Cockcroft-Gault, Schwartz, or Counahan-Barratt, will give values that, in most cases, are higher than the values obtained using traditionally calibrated creatinine methods. This change in calibration will affect interpretive criteria and drug dosage adjustments based on these estimates of kidney function. Guidance for using the Schwartz equation with creatinine measured by methods that have calibration traceable to IDMS is available in the GFR Calculators section.
Provide the relationship (including mathematical conversion factors, equations, or functions) between creatinine results when measured with a method that has IDMS-traceable calibration compared to the results obtained using a method with traditional calibration with emphasis on the 0.5 to 2.5 mg/dL (45 to 220 µmol/L) range that is most important for drug dose adjustments. This information will ensure that any of the pharmacy drug dosing approaches can adjust IDMS-traceable creatinine values for use with appropriate legacy dosing reference tables and algorithms (such as serum creatinine value; GFR or creatinine clearance based on estimating equations other than the MDRD Study equation. Read NKDEP’s recommendations for pharmacists and authorized drug prescribers.
Creatinine clearance calculated from serum and urine creatinine may increase depending on how the measurements for serum and urine are calilbrated. Reference interval and interpretive criteria for creatinine clearance may change.
For assessing kidney function in most patients, an estimated GFR (eGFR) using the MDRD Study equation is more accurate than a creatinine clearance calculated from serum and urine measurements. Therefore, NKDEP recommends against performing a measured creatinine clearance procedure for adults except when the patient's basal creatinine production is abnormal. This may be the case with patients of extreme body size or muscle mass (e.g., obese, severely malnourished, amputees, paraplegics or other muscle-wasting diseases) or with unusual dietary intake (e.g., vegetarian, creatine supplements).
Creatinine measurements at the low values usually observed in pediatric patients have greater measurement variability than for values seen in adults. Estimates of kidney function based on these values also will have greater variability than for adults.

Report Proficiency Testing and External Quality Assessment (PT/EQA) results for serum and urine creatinine using the correct instrument/method peer group for IDMS-traceable calibration. IVD manufacturers and NKDEP are cooperating to inform PT/EQA providers of the participant grading issues during the transition to standardized creatinine methods. (See NKDEP’s recommendations for PT/EQA providers.)


Monitor calibration performance of routine methods that have calibration traceable to an IDMS reference method through use of the College of American Pathologists' LN24 Survey (commutability validated), or comparable EQA with commutable samples and IDMS target values.

Commutability Study of Creatinine Reference Materials


In 2006, NKDEP’s Laboratory Working Group (LWG) conducted a commutability study of creatinine reference materials with the National Institute of Standards and Technology’s (NIST) Standard Reference Material (SRM) 967 to ensure commutability with native serum samples for most commonly used creatinine methods.

Reference materials are typically used to establish or validate the calibration traceability of a routine measurement procedure to a reference measurement procedure of higher order. When a reference material is intended to be measured by a routine clinical method, commutability* must be validated for all routine methods for which the reference material is intended for use.

Results
Fresh, frozen serum creatinine reference materials from NIST, SRM 967 (and from the College of American Pathologists’ Accuracy Calibration Verification/Linearity Survey LN24 used in 2006) were found to be commutable with a panel of native clinical serum samples for the manufacturers’ reagents and instruments listed below. Reference materials and clinical samples were measured by the stated analytical system and by a LC/IDMS reference measurement procedure at NIST. Commutability was evaluated using criteria described in Clinical and Laboratory Standards Institute (CLSI) document EP14-A2.1

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