HPLC measurement of phenylalanine in plasma

Measurement of phenylalanine in plasma is required for the diagnosis and subsequent dietary management of phenylketonuria (PKU). We have developed an isocratic high-performance liquid-chromatographic (HPLC) method, with ultraviolet detection. For this measurement 10 microL of plasma is needed, and the complete analysis can be done in less than 15 min. Interbatch coefficients of variation for human plasma samples (phenylalanine 136-1541 mumol/L) ranged between 2.2% and 4.4%. The method has a wide linear range (10-3500 mumol/L), and results (y) correlated well with those (x) of an ion-exchange chromatographic method (r = 0.996; y = 1.014 x - 11.9; n = 64, range 22-1823 mumol/L). Only standard HPLC equipment is needed and no specialized sample preparation or detection system is required. The procedure has proved robust and reliable in regular routine use.