The Glomerular Filtration Rate (GFR) is the primary measurement used to determine how well a transplanted kidney is working. It measures the rate at which the tiny filters in the kidney, called glomeruli, clean waste products from the blood. A stable GFR indicates that the new kidney is effectively performing its function.
Measuring Kidney Function After Transplantation
Determining the exact GFR after a transplant is complex because it is usually an estimation (eGFR) rather than a direct measurement. The eGFR calculation relies on the level of creatinine in the blood, a waste product from normal muscle breakdown that healthy kidneys filter out.
Standard eGFR formulas were primarily developed for the general population, making them less accurate for transplant recipients. Many factors influence creatinine levels besides kidney function, including lower muscle mass due to underlying disease or medications that affect how creatinine is handled.
Immunosuppressant medications necessary to prevent rejection can also affect creatinine levels, sometimes making the eGFR reading less reliable. For this reason, transplant centers focus more on the trend of the eGFR over time rather than a single number. If there is a major clinical concern, a measured GFR (mGFR) using an external marker may be performed for a more precise reading.
Expected GFR Goals Over Time
The optimal GFR for a transplanted kidney is generally lower than that of two healthy native kidneys. Immediately after the procedure, there is an initial phase of recovery, and function may be unstable or lower in the first one to three months, especially if there was delayed graft function (DGF).
Once the kidney settles into long-term function, the goal is stability. For recipients one year or more post-transplant, a GFR between 50 and 75 mL/min/1.73m² is often considered excellent and stable. It is uncommon for a transplanted kidney to consistently achieve a GFR above 90 mL/min/1.73m². In a large study of transplant recipients, the median eGFR at the one-year mark was around 51.8 mL/min/1.73m².
Over many years, a slow, gradual decline in GFR is an expected part of the natural aging process of the transplanted organ. A steady decline of around 1 to 2 mL/min/1.73m² per year is often observed. The transplant team monitors this gradual loss to ensure the rate of decline remains slow and predictable.
Common Factors That Affect GFR Readings
A single GFR reading can fluctuate significantly due to factors unrelated to the long-term health of the kidney. Understanding these variables helps prevent unnecessary anxiety when a lab result shows a temporary change.
Certain immunosuppressant drugs, particularly calcineurin inhibitors, can cause vasoconstriction, narrowing the blood vessels supplying the kidney. This effect temporarily reduces filtering pressure, leading to a slightly lower GFR even when the drug levels are therapeutic. The transplant team incorporates this known effect into their interpretation.
The body’s hydration status also has a direct impact on the GFR reading. Even mild dehydration can cause the GFR to drop temporarily as the kidney attempts to conserve fluid. Patients should ensure they are well-hydrated before having blood drawn for kidney function tests.
Dietary habits immediately preceding a blood test can influence the eGFR calculation. A high intake of protein increases creatinine production, which temporarily elevates serum creatinine and results in a lower eGFR reading. Infections or systemic illnesses can also cause a transient dip in function.
When to Alert Your Transplant Team
While minor, temporary fluctuations in GFR are common, certain changes require immediate communication with your care team. A sudden and unexplained drop of 20% or more from your established baseline GFR is considered a significant event that warrants prompt investigation.
This sudden drop may signal acute rejection, infection, or a complication from a medication change. Certain physical symptoms accompanying a GFR drop also require immediate attention:
- A significant decrease in urine output.
- New or increased swelling in the legs or feet.
- A fever of 101°F (38.3°C) or higher.
- Unexplained severe fatigue, shortness of breath, or feeling generally unwell.
The transplant team will likely order repeat blood work to confirm the trend and may check the levels of your immunosuppressant medications. Depending on the severity, they may arrange a more in-depth evaluation, such as an ultrasound or a transplant kidney biopsy, to determine the exact cause of the change.