Table 2. Characteristics of transplant centers over the seven year period of study.

Number of kidney transplant centers	194
Volume category 1: Range of mean annual overall     kidney transplants (n=65centers) *	11.4 – 41.3
Volume category 2: Range of mean annual overall     kidney transplants (n=65 centers) *	41.4 – 76.7
Volume category 3: Range of mean annual overall     kidney transplants (n=64 centers) *	76.9 – 313.4
Mean annual live donor kidney transplants	25.5 +/- 23.9
Range	0.7 – 153.6
Mean live donor transplants/all kidney transplants (%)	34.7 +/- 13.4
Range	2.5 – 77.1
Mean laparoscopic donor nephrectomies/all nephrectomies (%)	23.3 +/- 15.3
Range	0 – 69.0
Mean unrelated donors/all donors (%) **	8.6 +/- 4.7
Range	0 – 24.4
Mean Herfindahl-Hirschman index (HHI) ***	0.16 +/- 0.26
Number of centers performing any donor exchange   transplants (%) ****	36 (18.6)
Number of centers performing any blood group   incompatible transplants (%)	74 (38.0)

^*Calculated by dividing all US kidney transplant centers into 3 groups of approximately equal size

^**Live donors who were not parents, siblings, children or spouses were categorized as “unrelated”

^***The HHI is a measure of market competition. Categories were defined by prior published literature(34)

^****Donor exchange transplants were living donor kidney transplants in which a live donor provided a kidney to a recipient through an exchange, for example, with another living donor-recipient pair; these exchanges are usually performed due to biological incompatibility between a donor and his or her intended recipient.