We established a highly reproducible, simple and rapid simultaneous measurement method in which blood plasma can be directly injected into high-performance liquid chromatography (HPLC) without conducting conventional extraction of kynurenine (Kyn) and tryptophan (Trp) from blood plasma. In this method, Kyn and Trp were first separated from proteins present in blood plasma using a pre-column. Then, by column-switching, the separated Kyn and Trp were automatically injected into a separation column. The recoveries of Kyn and Trp determined in blood plasma by this method were 100.0 ± 0.9 and 100.0 ± 1.4%, respectively.
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