The Modification of Diet in Renal Disease (MDRD) Study equation is a mathematical formula used by healthcare providers to estimate kidney function. It provides the estimated Glomerular Filtration Rate (eGFR), the best single indicator of how well the kidneys filter waste from the blood. Developed from a study of patients with chronic kidney disease (CKD), the MDRD equation was widely adopted because it offered a standardized and relatively accurate way to screen for and monitor kidney health. It eliminated the need for the cumbersome 24-hour urine collection previously necessary to measure filtration rate directly.
Variables Used in the MDRD Equation
The calculation relies on four primary inputs, the main component being serum creatinine concentration. Creatinine is a byproduct of normal muscle breakdown that healthy kidneys filter out. Because creatinine levels are tied to muscle mass, simply measuring the concentration in the blood is not enough to determine kidney function accurately.
To adjust for variations in muscle mass, the MDRD equation incorporates three demographic variables. These variables are the patient’s age, sex, and, historically, a coefficient for “race” (specifically, African-American heritage). Age is included because muscle mass naturally declines over a person’s lifespan, while sex accounts for the average difference in muscle bulk between males and females. The inclusion of the race variable was based on observations that people of African descent often have higher average muscle mass and thus higher serum creatinine levels at the same measured GFR.
These factors allow the equation to provide a value normalized to a standard body surface area of 1.73 square meters, which is necessary to compare the kidney function of people of different sizes. The final eGFR is an estimate derived from these variables, not a direct measurement of the kidney’s filtering capacity.
Understanding Your Estimated GFR Result
The eGFR result represents the volume of blood, in milliliters, that the kidneys can clean per minute, per 1.73 square meters of body surface area. A normal eGFR is typically 90 mL/min/1.73m² or higher, indicating full kidney function. The primary purpose of the eGFR is to classify the stage of Chronic Kidney Disease (CKD), which is defined by a persistently low eGFR or the presence of kidney damage.
CKD is generally categorized into five stages based on the eGFR level, with lower numbers indicating a greater loss of kidney function.
- Stage 1 (eGFR 90+): Normal or high kidney function with evidence of kidney damage, such as protein in the urine.
- Stage 2 (eGFR 60–89): Mildly reduced function, still with evidence of kidney damage.
- Stage 3a (eGFR 45–59): Mild to moderate loss of function.
- Stage 3b (eGFR 30–44): Moderate to severe loss of function.
- Stage 4 (eGFR 15–29): Severely reduced function, often requiring preparation for kidney replacement therapy.
- Stage 5 (eGFR <15): Kidney failure, which necessitates dialysis or a kidney transplant.
For a diagnosis of CKD to be made, the eGFR must remain below 60 mL/min/1.73m² for at least three months. A single result is not sufficient; a physician looks at the eGFR trend over time to determine stability or progression. Tracking this number allows for the timely introduction of therapies designed to slow the progressive damage to the kidneys.
When the MDRD Equation May Be Less Accurate
Despite its widespread use, the MDRD equation has limitations, particularly concerning its reliance on serum creatinine as a proxy for filtration. The equation was developed primarily in a population with existing kidney disease, making it less accurate when estimating GFR values above 60 mL/min/1.73m². At these near-normal levels, the MDRD equation tends to systematically underestimate the true GFR, potentially leading to the over-diagnosis of mild kidney disease.
The formula’s dependence on variables that estimate muscle mass creates inaccuracies in patients with extremes of body habitus. For instance, individuals with very low muscle mass, such as those with severe malnutrition, amputees, or the very elderly, may have a falsely high eGFR because their creatinine production is low. Conversely, people with exceptionally high muscle mass, like competitive bodybuilders, may receive a falsely low eGFR despite having perfectly healthy kidneys.
The equation is also not validated for use in certain patient groups where creatinine levels are unstable or highly variable. These include pregnant women, people with acute kidney injury (AKI), children under 18, and those with rapidly changing kidney function. In these clinical scenarios, alternative methods or direct measurement of creatinine clearance may be necessary for a reliable assessment.
The Shift to Newer Kidney Function Formulas
The limitations of the MDRD equation, particularly its inaccuracy at higher GFR values, prompted the development of more refined formulas. The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation, introduced in 2009, is one such successor that has been widely adopted by many clinical laboratories. The CKD-EPI formula uses the same input variables as the MDRD equation but is considered more accurate, especially when the eGFR is above 60 mL/min/1.73m².
This newer equation has a reduced bias at these higher levels, meaning it is less likely to incorrectly categorize individuals with normal kidney function as having mild CKD. The evolution of GFR estimation also continues with the move toward race-neutral equations. The most recent CKD-EPI 2021 formula removed the race variable entirely from the calculation in response to concerns that its inclusion could contribute to health disparities.
The trend in clinical practice is shifting toward using these updated formulas to provide a more precise and equitable assessment of kidney function. Laboratories are also exploring alternative markers, such as cystatin C, which is less affected by muscle mass, often used alone or combined with creatinine to improve eGFR reliability.