John Muir Health

In 2004 the clinical laboratories of John Muir Health started reporting estimated glomerular filtration rate (eGFR) along with serum/plasma creatinine as recommended by the National Kidney Foundation. The rationale for this was to remind us that approximately 11% of the U.S. adult population has chronic kidney disease, which is defined as either kidney damage (i.e. persistent proteinuria) or decreased kidney function (i.e. decreased GFR) for 3 or more months. It was further noted that adverse outcomes of chronic kidney disease, such as development of kidney failure and cardiovascular disease, could often be prevented or delayed through early detection and treatment.

One of the early recommendations was that "Autoanalyzer manufacturers and clinical laboratories should calibrate serum/plasma creatinine assays using an international standard." An isotope dilution mass spectrometry (IDMS) reference method was chosen, but was not widely available back then. The manufacturer of our assay made reagent systems that are traceable to it available to us in late 2007. Since we have completed our internal validation studies, we will be bringing it on line soon.

In comparison to our old assay, the new IDMS-traceable creatinine runs slightly lower at values <2.00 mg/dL (range 0.04-0.20 mg/dL; average 0.07 mg/dL). This affects the reference range; new, Males: 0.61 - 1.24 mg/dL, Females: 0.44 - 1.10 mg/dL. In addition, the MDRD Study equation to calculate the eGFR also changes slightly. However, in comparison studies, the new IDMS-traceable calculated eGFR does not vary significantly from our old eGFR.

Regarding the laboratory determination of eGFR, the published National Kidney Disease Education Program guidelines also raise additional points:

1. The MDRD Study equation should only be used in individuals age 18 and older. Additionally, the equation has not been validated for use with the elderly (>70 years of age), pregnant women, patients with serious comorbid conditions, or persons with extremes of body size, muscle mass or nutritional status. For most patients, an eGFR is more accurate than a creatinine clearance calculation from serum and urine measurements. Therefore, it is not recommended to perform a measured creatinine clearance procedure for adults except when the patient's basal creatinine production is expected to be significantly abnormal (e.g. obese, severely malnourished, amputees, paraplegics, muscle-wasting diseases, strict vegetarian diets, muscle builders on creatine supplements, etc.).
2. The equation has been most extensively evaluated in people with chronic kidney disease and reduced GFR and is less accurate for persons with normal or mildly impaired kidney function. This is the rationale for the eGFR value of 60 being considered the cutoff for the reference range.
3. It is recommended that creatinine values in mg/dL be reported to two decimal places. This will reduce rounding errors that may contribute to imprecision in the eGFR value.

References

1. www.nkdep.nih.gov
2. Myers, GL, et al. Recommendations for Improving Serum Creatinine Measurement: A Report from the Laboratory Working Group of the National Kidney Disease Education Program. Clin Chem 2006; 52:5-18.
3. Ceriotti, F, et al. Reference Intervals for Serum Creatinine Concentrations: Assessment of Available Data for Global Application. Clin Chem 2008;54(3):in press.