MDRD Calculator

How does this MDRD calculator work?

This health tool calculates eGFR using the MDRD formula (Modification of Diet in Renal Disease) in order to provide insight into the overall function of the kidneys in adults. The parameters used are:

■ Serum creatinine – measured in mg/dL. The normal range for creatinine in the blood is between 0.84 and 1.21 mg/dL.

■ Age in years – older age poses higher risk of irregular kidney function.

■ Gender – significant gender difference is observed and accounted for in the MDRD equation.

■ Race – African Americans tend to have a higher GFR than Caucasians at the same level of serum creatinine thus an adjustment is required. This is due to the higher average muscle mass and creatinine rate for this ethnicity.

The IDMS-traceable MDRD formula used in this MDRD calculator is:

eGFR = 175 x (Serum Creatinine)^-1.154 x (Age)^-0.203 x (0.742 only if female) x (1.212 only if black).

The Working Group recommends that upper reporting limit to be placed at 90 mL/min/1.73 m² and all values above this threshold, to be referred to as "> 90 mL/min/1.73 m²".

While there aren’t any specific age related guidelines, for patients aged 70 and above an eGFR in the range 45-59 mL/min/1.73 m² without any other evidence of kidney damage can be considered typical for the age and not associated with chronic kidney disease-related complications, as these values might be in a lower age group.

The MDRD equation is used in estimating GFR in stable chronic kidney disease and cannot be used in acute renal failure.

Compared to other methods of estimating GFR, the one in question has been validated and shows better correlation than other equations such as the Cockcroft-Gault formula.

Example of an eGFR calculation

Taking the case of a patient with the following particularities:

■ Serum creatinine = 1.32 mg/dL;

■ Age = 34 years;

■ Gender: Male;

■ Race: Black.

The calculation looks like: eGFR = 175 x 1.32^-1.154 x 34^-0.203 x 1.212

In this case the estimated glomerular filtration rate is 75.25 mL/min/1.73 m².

eGFR interpretation

The results obtained in the calculator above do not need weighting as there are already adapted for 1.73 m² as standard body surface area.

■ GFR values above 90 mL/min/1.73 m² are considered normal in the absence of any kidney disease evidence. In case proteinuria, haematuria, polycystic kidney or any other abnormalities are present, even these values are consistent with chronic kidney disease (CKD) stage 1.

■ GFR values between 60 and 89 mL/min/1.73 m² associated with any of the conditions listed above, indicates CKD stage 2.

The National Kidney Foundation Kidney Disease Outcome Quality Initiative (K/DOQI) classification assigns the following CKD stages:

GFR guidelines

Glomerular filtration rate represents the amount of blood filtered by the capillaries of the kidney glomerular into the Bowman capsules during a certain amount of time. It basically shows how efficiently can kidneys filter waste products and toxins which are then disposed of through urine.

Serum creatinine levels are most often used as a mean to reveal GFR and most calculations test for creatinine in blood probe and in urine probe.

There is criticism of this method though because it is based on a 24h prediction of excretion rate which is highly dependent on muscle mass because creatinine is break down product of the creatinine phosphate from muscles.

Such a calculation would result in two different people, with different muscle mass but same serum creatinine receiving the same kidney filtration interpretation.

At the same time, creatinine clearance is known to overestimate the rate of glomerular filtration by 10 to 20%.

References

1) Levey AS, Coresh J, Greene T, Marsh J, Stevens LA, Kusek JW, Van Lente F; Chronic Kidney Disease Epidemiology Collaboration. (2007) Expressing the Modification of Diet in Renal Disease Study equation for estimating glomerular filtration rate with standardized serum creatinine values. Clin Chem; 53(4):766-72. Epub 2007 Mar 1.

2) National Kidney Foundation. (2002) K/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification. Am J Kidney Dis; 39(2 Suppl 1): S1-266.

3) Mathew TH, Johnson DW, Jones GR (2007) Chronic kidney disease and automatic reporting of estimated glomerular filtration rate: revised recommendations. The Medical Journal of Australia 187 (8): 459–63.

4) Guyton, Arthur; Hall, John (2006) Chapter 26: Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control. In Gruliow, Rebecca. Textbook of Medical Physiology (Book) (11th ed.) Philadelphia, Pennsylvania: Elsevier Inc.