Which type of analysis: urine or plasma?
The 24-hour urine amino acid analysis has the highest probability of detecting abnormalities if renal function is normal. The 24-hour test indicates what is high and low over the course of a day, reflects blood and tissue amino acid pools, and is not affected by circadian rhythm. Healthy kidneys efficiently conserve essential amino acids. Therefore, urine levels of amino acids decrease first and tend to give an earlier indication of inadequacy than do plasma levels.
A first morning void urine amino acid analysis, with results normalized per gram creatinine, provides an alternative when a complete 24-hour collection is not a viable option. The first morning void analysis is excellent for identification of marked abnormalities, particularly with respect to gastrointestinal health, inherited disorders in amino acid metabolism, and renal function, and can be used for protein challenge testing.
Plasma amino acid analysis measures what is being transported at the time of sampling. Abnormalities are deduced by comparison of measured levels with an established reference range. (The specimen should be collected after an overnight fast to reduce the influence of dietary protein). Plasma analysis is an excellent compromise if urine collection is difficult.