This report describes two individuals, one with known propionic acidemia and one undergoing evaluation for an IMD, who were found to have marked hyperargininemia (>500 μM; reference 10–140 μM) by plasma amino acid (PAA) analysis of a specimen collected ~1.5–3 h after clearance of an indwelling catheter with tPA. The gene discussed is PLAT; the disease is arginase deficiency.