The short answer
CKD-EPI and MDRD are equations used to estimate glomerular filtration rate (eGFR) from blood test results, usually serum creatinine. They can produce different eGFR values from the same creatinine result because they use different mathematical models.
The important point is that the difference does not necessarily mean your kidney function suddenly changed. It may simply reflect the equation used by the laboratory.
For adults today, the 2021 race-free CKD-EPI equations are preferred over the older MDRD equation in many clinical settings. The 2021 CKD-EPI creatinine equation uses age, sex, and serum creatinine; newer options can also combine creatinine with cystatin C. (NIDDK)
What is eGFR?
eGFR is an estimate of how well your kidneys filter blood.
It is usually reported as:
mL/min/1.73 m²
The number is calculated from a person’s blood creatinine and other characteristics rather than directly measuring the amount of blood the kidneys filter.
This makes eGFR convenient for routine kidney assessment, but it is still an estimate, not a direct measurement of GFR. Every estimating equation has some degree of uncertainty. (NIDDK)
What is the MDRD equation?
MDRD stands for Modification of Diet in Renal Disease.
It was developed using people with kidney disease and became widely used for estimating GFR from:
- Serum creatinine
- Age
- Sex
- Historically, a race coefficient
The older MDRD equation generally performed reasonably well in people with reduced kidney function but was less reliable when GFR was relatively high.
The MDRD equation is now considered an older equation. NIDDK lists it among previous equations rather than its preferred current adult equations. (NIDDK)
What is the CKD-EPI equation?
CKD-EPI stands for Chronic Kidney Disease Epidemiology Collaboration.
Several versions have been developed.
The important distinction today is between the older 2009 CKD-EPI equation and the 2021 CKD-EPI equation.
The 2021 CKD-EPI creatinine equation:
- Uses serum creatinine
- Uses age
- Uses sex
- Does not use a race coefficient
There is also a 2021 CKD-EPI creatinine-cystatin C equation, which uses both biomarkers. NIDDK notes that combining creatinine and cystatin C generally provides a more accurate estimate than using either marker alone. (NIDDK)
Why can CKD-EPI and MDRD give different numbers?
The equations were developed using different mathematical relationships between creatinine and measured kidney function.
Therefore, the same person can have different reported eGFR values depending on which equation a laboratory uses.
For example, an older laboratory report might show:
eGFR: 58 mL/min/1.73 m²
while a newer calculation using another equation might produce a somewhat different number from the same creatinine value.
This does not automatically mean the kidneys improved or deteriorated.
Before interpreting a change, check:
Which equation was used → whether creatinine changed → how much eGFR changed → over what time period → clinical context
Why did laboratories move away from older equations?
One major change was the move toward race-free eGFR equations.
The older 2009 CKD-EPI and MDRD equations included a coefficient for people identified as Black. The 2021 CKD-EPI creatinine equation removed that coefficient. Current NIDDK guidance supports race-free CKD-EPI equations for adult eGFR reporting. (NIDDK)
Therefore, comparing a historical eGFR calculated using an older race-based equation with a newer race-free result requires caution.
The numerical difference may partly reflect a change in calculation method rather than a biological change in kidney function.
CKD-EPI vs MDRD: the practical difference
| Feature | MDRD | CKD-EPI |
|---|---|---|
| Full name | Modification of Diet in Renal Disease | Chronic Kidney Disease Epidemiology Collaboration |
| Common older use | CKD assessment | CKD assessment |
| Creatinine-based versions | Yes | Yes |
| Older versions used race coefficient | Yes | Yes, in the 2009 version |
| Current 2021 version is race-free | No | Yes |
| Performance at higher GFR | Generally less accurate | Generally better suited |
| Current preferred adult approach | Older equation | 2021 CKD-EPI-based equations |
The newer CKD-EPI approach is generally preferred, but no equation is perfectly accurate for every person. (NIDDK)
Does a different eGFR mean I have a different CKD stage?
Not necessarily.
CKD categories use eGFR ranges, but the equation used can affect the reported number, especially when a value is near a classification threshold.
For example, if two equations produce results around an important boundary, the apparent CKD category could change even though the person’s underlying kidney function has not suddenly changed.
This is one reason clinicians consider the overall pattern, including:
- Previous eGFR values
- Serum creatinine trends
- Urine albumin-to-creatinine ratio (ACR)
- Blood pressure
- Medical history
- Other kidney findings
CKD is not diagnosed simply because one equation produces an eGFR below a particular number. Persistent abnormalities and clinical context matter.
Why can creatinine make eGFR imperfect?
Creatinine is produced partly from normal muscle metabolism.
Its blood concentration can therefore be affected by factors other than kidney filtration, including:
- Muscle mass
- Diet
- Certain medications
- Changes in creatinine production
- Some unusual body compositions
Someone with unusually high or low muscle mass may therefore have an eGFR that does not perfectly represent their actual filtration rate.
This is one reason cystatin C can sometimes provide useful additional information.
When is cystatin C helpful?
Cystatin C is another blood marker that can be used to estimate GFR.
When there is uncertainty about kidney function, a clinician may use an equation combining creatinine + cystatin C.
NIDDK reports that the combined 2021 CKD-EPI creatinine-cystatin C equation has higher accuracy and less bias than equations using creatinine alone. It can be particularly useful when eGFR is close to an important clinical decision point, such as certain medication-dosing decisions. (NIDDK)
It is not necessary for every person with CKD, but it can be valuable when a more precise estimate is needed.
Which eGFR should you trust?
Do not focus only on which equation gives the higher or lower number.
For current adult reporting, the 2021 race-free CKD-EPI equations are the preferred approach in many settings. If a laboratory report uses an older MDRD or CKD-EPI equation, the result should be interpreted according to the method and date of the test. (NIDDK)
If the result is close to an important threshold, your clinician may consider additional information, including cystatin C, rather than treating a small numerical difference as a definite biological change.
What matters more: one eGFR or the trend?
Usually, the trend is more informative than one isolated estimate.
For example:
Previous eGFR → current eGFR → size of change → time interval → creatinine change → urine findings
A single unexpected change can occur for several reasons and does not automatically prove that CKD is progressing.
Conversely, a stable-looking single value does not rule out kidney disease if there is persistent albuminuria or another kidney abnormality.
What should you do if your eGFR changed after your laboratory changed formulas?
First, check whether the eGFR equation changed.
If possible, compare:
- The creatinine values
- The dates of testing
- The reported eGFR
- The equation used
- Urine ACR or protein results
If the creatinine is essentially unchanged but eGFR changed after a laboratory switched calculation methods, the difference may be partly mathematical.
Do not change medications or assume that CKD has suddenly worsened based only on a formula-related difference. Discuss an unexpected change with your healthcare professional, particularly if it affects medication dosing or CKD classification.
Bottom line
MDRD and CKD-EPI are different mathematical methods for estimating GFR, so they can produce different eGFR values from the same creatinine result.
MDRD is an older equation. For adults, current practice generally favors the 2021 race-free CKD-EPI equations, with combined creatinine-cystatin C testing providing a potentially more accurate estimate when additional precision is important. (NIDDK)
Most importantly, do not interpret eGFR as an exact measurement. Look at the equation used, the creatinine trend, urine albumin, other kidney findings, and the clinical context. A different eGFR number does not automatically mean that your kidneys have actually changed.