DELAYED SEPARATION AND THE PLASMA AMINO-ACIDS ARGININE AND ORNITHINE

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