Expanded Explanation of the MDRD GFR Formula

The MDRD (Modification of Diet in Renal Disease) formula is a widely used equation for estimating glomerular filtration rate (GFR), a measure of kidney function. It was originally developed as part of the MDRD study to assess renal function in patients with chronic kidney disease (CKD). Below is a detailed breakdown of the formula and its background:

MDRD Formula

• Serum Creatinine : Measured in mg/dL, this is a key marker of kidney function. Higher creatinine levels indicate reduced kidney function.
• Age: As age increases, GFR decreases due to physiological changes in renal function.
• Black Race Adjustment : This factor increases the GFR estimate for Black patients, reflecting the observation that Black patients, on average, may have higher serum creatinine levels due to greater muscle mass. However, this adjustment has been debated due to concerns about race-based clinical algorithms.
• Female Adjustment: This reduces the GFR estimate for female patients, accounting for the generally lower muscle mass and creatinine production in women compared to men.

Background and Updates

From Levey 2006

This formula comes from the MDRD study and its subsequent validation and adjustment by Levey and colleagues in 2006. The study aimed to standardize GFR estimation across clinical settings.

• The original formula used a constant of 186, but this was revised to 175 to account for changes in the standardization of creatinine assays using IDMS (Isotope Dilution Mass Spectrometry).
• Despite the revision, the difference in clinical outcomes between using 186 vs. 175 is minimal, and many practitioners still use the older constant in some settings.

Comparison to CKD-EPI

While the MDRD formula has been widely adopted, it has limitations in accuracy at higher GFR values (i.e., in patients with normal or mildly impaired kidney function). To address these shortcomings, the same authors developed the CKD-EPI equation, which provides better accuracy across a broader range of GFRs and is now preferred in many settings.

Important Notes

1. Race Adjustment and Bias
   The race adjustment factor has been a topic of significant debate in recent years. Critics argue that it can introduce systemic bias into clinical decisions, potentially delaying diagnosis or treatment for Black patients. Many institutions are now moving toward race-neutral algorithms or providing both race-adjusted and race-neutral estimates to support equitable care.
2. Serum Creatinine Standardization
   • The use of standardized creatinine assays is critical for accurate GFR estimation. Variations in measurement methods can lead to significant differences in GFR estimates.
   • If using non-IDMS-standardized assays, adjustments to the formula may be needed.
3. Applicability
   • The MDRD equation is intended for patients with CKD and is not reliable for patients with acute kidney injury (AKI) or rapidly changing renal function.
   • For pediatric populations, separate equations (e.g., Schwartz formula) should be used.
4. Clinical Use
   GFR estimates from the MDRD formula are reported in units of mL/min/1.73m2, normalized for a standard body surface area (BSA). This allows for comparison across individuals of different body sizes.

Clinical Pearls

• Interpretation:
  • A GFR of 90 mL/min/1.73m² or higher is considered normal.
  • Values below 60 mL/min/1.73m² are typically indicative of CKD.
  • Values below 15 mL/min/1.73m² indicate kidney failure (stage 5 CKD).
• Race Optionality:
  • Patients and clinicians may choose whether to include the race adjustment factor. This decision should be guided by institutional policies and informed discussions about the potential impact on clinical care.
• Transition to CKD-EPI:
  • In many clinical settings, the CKD-EPI equation has replaced MDRD as the preferred method for estimating GFR due to its improved accuracy, particularly in patients with GFRs near normal levels.

Formula Summary

Inputs

• Serum Creatinine (mg/dL)
• Age (years)
• Sex (Male/Female)
• Race (Black/Non-Black, Optional)

Outputs

• Estimated GFR (mL/min/1.73m2): A measure of kidney function.