Glomerular Filtration Rate (GFR)

Chronic kidney disease is a worldwide problem that carries a substantial risk for cardiovascular morbidity and death.  Current guidelines define chronic kidney disease as kidney damage or glomerular filtration rate (GFR) less than 60 mL/min per 1.73 m2 for three months or more, regardless of cause.  The assay of creatinine in serum or plasma is the most commonly used test to assess renal function.  In addition to the diagnosis and treatment of renal disease and the monitoring of renal dialysis, creatinine measurements are also used for the calculation of the fractional excretion of other urine analytes (e. g., albumin, α-amylase).

Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass). It is freely filtered by the glomeruli and, under normal conditions, is not re-absorbed by the tubules to any appreciable extent. A small but significant amount is also actively secreted.  Since a rise in blood creatinine is observed only with marked damage of the nephrons, it is not suited to detect early stage kidney disease.

A considerably more sensitive test and better estimation of glomerular filtration rate (GFR) is given by the creatinine clearance test based on creatinine concentration in urine and serum/plasma, and urine flow rate.  For this test a precisely timed urine collection (usually 24 hours) and a blood sample are needed.  However, since this test is prone to error due to the inconvenient collection of timed urine, mathematical attempts to estimate GFR based only on the creatinine concentration in serum or plasma have been made.

Among the various approaches suggested to estimate GFR using serum creatinine, one based on the results of the MDRD (Modification of Diet in Renal Disease) trial has found wide recognition to calculate an estimated GFR (eGFR) in adults.  The first derivation of the MDRD equation was derived with the conventional JaffĂ© creatinine method.  A newer version of the MDRD equation has been developed for IDMS (isotope-dilution mass spectrometry)-traceable enzymatic creatinine methods.

GFR (mL/min/1.73 m2) = 175 x (Scr)-1.154 x (Age)-0.203 x (0.742 if female) x (1.212 if African-American)

where Scr is serum/plasma creatinine in mg/dL measured by an IDMS-traceable enzymatic creatinine method.

In children, there is not a single formula recommended for all ages.  However, the Schwartz formula is frequently used and appropriate for most ages, but should be used with caution or avoided in premature infants in the first several months of life.  In the premature infant population, more specialized equations are often used.  The pediatric Schwartz equation is

GFR (mL/min/1.73 m2) = (0.41 x Height) / Scr
were Scr is serum/plasma creatinine in mg/dL

At the University of Iowa Hospitals and Clinics, the Clinical Chemistry laboratory switched from the JaffĂ© creatinine method to the enzymatic method on May 10, 2011.  Upon the switch to the enzymatic method, the eGFR calculation by the MDRD equation underwent three changes:

(1) A change to the newer MDRD equation appropriate for IDMS-traceable enzymatic creatinine methods.

(2) The eGFR calculation is only performed for patients 18 years and older to follow NKDEP guidelines.

(3) Extended reporting of eGFR values up to 90 mL/min/1.73 m2

Additional resources:
(1) More detailed information on the MDRD equation for eGFR calculation in adults is found at the National Kidney Disease Education Program website.

(2) An on-line eGFR calculator for adults (note that this site also provides an additional calculation using the alternative CKI-EPI formula).

(3) Bedside eGFR calculator for children using the Schwartz equation.