Methemoglobin Interferes with the Hemocue B-glu

linear regression coefficients (r) were 0.981 for tryptophan (slope 1.004) and 0.905 for kynurenine (slope 1.21). The presented method is well suited for high throughput of samples. Approximately 150 specimens can be measured with one chromatographic system per day. On the one hand, it allows measurement of tryptophan and kynurenine concentrations with high sensitivity. Determination of tryptophan may be especially important for psychiatric clinics when the background of increased susceptibility for mental depression needs to be characterized more precisely. On the other hand, the parallel measurement of kynurenine concentrations will allow ruling out whether low tryptophan is attributable to insufficient dietary intake of the essential amino acid tryptophan and allows an estimate of IDO activity in vitro and in vivo when the kynurenine-to-tryptophan ratio is calculated. The sensitivity of the method can be easily enhanced by increasing the injection volume to 100 L, which is necessary when analyzing cerebrospinal fluid or cell culture supernatants in which kynurenine concentrations are one order of magnitude lower than in serum or plasma.