Delayed separation and the plasma amino acids arginine and ornithine.

When collecting blood for amino acid testing, leaving plasma in contact with cells at room temperature lowers the concentration of arginine and raises that of ornithine. This is presumably due to the arginase content of red blood cells. In contrast, the sum of arginine and ornithine is constant over the first hour, and defines a reference interval of 74-148 mumol/L (mean +/- 2 SD, n = 20) which is more insensitive to delayed separation. The ratio of arginine to the sum of arginine plus ornithine [arg/(arg + orn)] can be used to estimate the number of specimens not separated promptly. A ratio of 0.74-0.50 (mean +/- 2 SD, n = 20) is characteristic of specimens placed on ice and separated promptly, where delayed separation produces lower ratios. Of 91 adult specimens received for plasma amino acid analysis over five months, 35 (38 percent) showed a ratio < 0.50 suggestive of delayed processing.