Renal Transplant RI: Differentiation of Rejection and Acute Tubular Necrosis
Why RI Does Not Differentiate Rejection and ATN
According to Diagnostic Ultrasound Vascular (2025), RI may be elevated in transplant dysfunction; however, the correlation is inconsistent, and the value is clinically useless for differentiating the causes of transplant dysfunction or type of rejection. RI depends not only on intrarenal vascular and inflammatory changes but also on recipient factors—age and systemic vascular pathology. RI weakly correlates with glomerular filtration rate and histology data.
Common Ultrasound Features
Gray-scale and Doppler changes in acute rejection and ATN partially overlap:
| Method | Acute Rejection (AR) | Acute Tubular Necrosis (ATN) |
|---|---|---|
| Gray-scale Ultrasound | Nonspecific enlargement of the transplant, urothelial thickening | Delayed graft function |
| Color Doppler | Possible decreased blood flow; in severe rejection, progression to thrombosis | May lack diastolic blood flow |
| Pulsed Doppler | RI (in segmental/interlobar arteries) may be elevated but is nonspecific | Absence of diastolic blood flow with delayed function |
RI is measured in segmental or interlobar arteries. It may be elevated in both acute and chronic rejection but remains nonspecific. In the provided clinical example, an RI of 0.81 six weeks post-cadaveric transplantation with absent diastolic blood flow on day 1 was accompanied by improved function; in another observation, the absence of diastolic blood flow was due to ATN with delayed graft function.
Prognostic Value of RI
Despite its low differential diagnostic value, an elevated RI > 0.80 in the early postoperative period or three months post-transplantation is associated with an increased risk of graft failure, chronic transplant nephropathy, and recipient death with a functioning graft (Diagnostic Ultrasound Vascular, 2025).
Differential Diagnosis and Role of Additional Methods
A high RI in the transplant may be due to postoperative complications—renal vein thrombosis, artery occlusion, acute tubular necrosis. However, it may also indicate subclinical (acute or chronic) rejection. For non-invasive diagnosis of subclinical rejection, a combination of high RI values with delayed peak enhancement on CEUS and elevated blood urea nitrogen (BUN) levels has been proposed. Early CEUS studies showed altered microperfusion and contrast kinetics.
Thus, isolated RI does not allow differentiation between rejection and ATN; interpretation should be conducted in conjunction with clinical and laboratory data and, if necessary, biopsy.
Frequently asked questions
Can RI distinguish acute rejection from ATN?
No. RI is nonspecific and clinically useless for differentiating the causes of transplant dysfunction or type of rejection; it increases in both conditions.
Why does RI weakly correlate with histology and GFR?
Because RI depends not only on intrarenal vascular and inflammatory changes but also on recipient factors—age and blood pressure.
What is the prognostic significance of RI > 0.80?
An elevated RI > 0.80 in the early postoperative period or three months post-transplantation is associated with an increased risk of graft failure, chronic nephropathy, and recipient death with a functioning graft.
What does the absence of diastolic blood flow in a transplant indicate?
It is a nonspecific sign: it may result from ATN with delayed function but is also described in improving graft function; in severe cases, thrombosis should be ruled out.
How to improve the accuracy of non-invasive diagnosis of subclinical rejection?
Use high RI values in combination with delayed peak enhancement on CEUS and elevated blood urea nitrogen (BUN) levels.