Acute renal transplant rejection

Acute renal transplant rejection is a renal transplant complication that occurs within <5-7 days of the placement of the transplant. Although part of a spectrum of closely-related rejection disorders, the term is meant to distinguish this type of rejection from chronic renal transplant rejection, which is slower and progressive.

Terminology

Traditionally, acute renal transplant rejection has been divided into three types

• hyperacute (minutes-hours, often in the operating room)

  • this type of rejection is rarely imaged

• acute (5-7 days)

• accelerated acute (5-7 days)

There has been an evolution in understanding about acute transplant rejection, however, and some of the features blend with chronic transplant rejection. An updated set of terminology describes categories based on the underlying reason for the rejection, as found at biopsy.

Antibody-mediated rejection: 

• active 

• chronic active

• chronic

T-cell mediated rejection

• acute

• chronic active

The original terms still have clinical significance, however, and are used informally in discussing renal allograft dysfunction.

Pathology

Renal transplant rejection is not a radiologic diagnosis. It relies on histologic evaluation in combination with molecular, serologic, and clinical parameters. 

There are two main forms: antibody-mediated rejection and T-cell mediated rejection.

Radiographic features

Ultrasound

Renal transplant ultrasound is usually the first line imaging modality for evaluation of a renal transplant. Some findings suggest transplant dysfunction, but there are no sonographic findings specific for rejection.

• elevated resistive indices (or increasing resistive indices)

  • 0.8 is often used as a cut-off for "abnormal", but one should remember that any cut-off in this setting is along an ROC curve, the higher one sets the cut-off, the lower the sensitivity, but the higher the specificity

• parenchymal edema

  • enlargement of the transplant

  • loss of corticomedullary differentiation

Differential diagnosis

• acute tubular necrosis (ATN)

  • may be indistinguishable on imaging

• nephrotoxic drug effects

• delayed graft function

Practical points

The current role of imaging is to evaluate potential renal transplant dysfunction with a combination of grayscale and color Doppler findings, not to diagnose rejection directly. Rejection is diagnosed on biopsy.