On Tuesday, May 10, 2011, there will be several changes related to creatinine determination and glomerular filtration rate (GFR) calculation.  The first change is that the Clinical Chemistry laboratory will switch to an enzymatic creatinine method (from the older Jaffe method).  The enzymatic creatinine method is more accurate at low creatinine values and also less prone to interferences such as high bilirubin and cephalosporin antibiotics.

The second change is that automatic GFR determination will switch to a slightly different MDRD equation than currently used.  The new equation is appropriate for enzymatic creatinine methods that are traceable to isotope-dilution mass spectrometry (IDMS) “gold standard” methods for measuring creatinine.

The third change is that automatic GFR calculation will only be conducted for patients 18 years and older, in accordance with National Kidney Disease Education Program (NKDEP) recommendations.  Although GFR values will not be routinely calculated for children 17 years and younger, a separate Epic orderable for a GFR estimate by the Schwartz equation (which requires patient height) will be created, and will be the subject of a future broadcast message.

As is the current practice, if GFR cannot be calculated for a variety of reasons (e.g., creatinine measurement not determined, age or gender of patient not available in Epic, patient younger than 18 years old), then GFR will be resulted as “NOT DONE”.

The most notable practical implications of this switch are:

(1) The value of the creatinine concentration for a given patient measured by the new enzymatic assay will generally be within 5% of the value of the previous method.

(2) The reference ranges of normal values for creatinine concentration will be adjusted to reflect the new assay method.

(3) The new enzymatic creatinine method demonstrates no interference by bilirubin concentrations of up to approximately 55-60 mg/dL.  The current Jaffe method encounters substantial interference when bilirubin concentrations are in the 25-30 mg/dL range.  Estimation of renal function in patients with liver failure should thus be more accurate.  The enzymatic method is also less affected by hemolysis and by drug interferences such as the cephalosporin antibiotics.

(4) The calculated GFR values will be reported up to 90 mL/min/1.73 m² (as opposed to 60 mL/min/1.73 m²).  Values greater than 90 mL/min/1.73 m² will be reported as “> 90”.

(5) The new calculated GFR (using the enzymatic creatinine method and new MDRD equation) will be approximately 0-5% lower than the GFR estimated by the current method.

Questions should be directed to Matthew Krasowski, MD, PhD, Medical Director of the Clinical Chemistry Laboratory (384-9380, matthew-krasowski@uiowa.edu).