The medullary pyramid index: an objective assessment of prominence in renal transplant rejection

Prominence of the medullary pyramids at sonography has been considered a sign of renal transplant rejection. A search of the literature reveals no previously published objective assessment of this phenomenon. Medullary pyramids of 67 normal kidneys, 53 nonrejecting transplanted kidneys, and 71 transplanted kidneys in rejection were measured. The area of the pyramid was related to the thickness of the overlying renal cortex by a "medullary pyramid index" (MPI): MPI (formula; see text) The median MPI was 4.17 for normal kidneys, 6.0 for nonrejecting transplanted kidneys, and 7.50 for transplanted kidneys in rejection. The results are significantly different (P = 0.0001) for all possible pairs. Overlap between rejection and nonrejection distributions is, however, considerable, rendering the discriminatory value of an individual observation quite low (0.69). Prominence of the medullary pyramids is therefore of very limited predictive value in the determination of transplant rejection in an individual patient.