Fatigue and plasma tryptophan levels in patients on chronic hemodialysis.

To the Editor: Mutsaers et al.1 have recently suggested that there is a direct link between tryptophan metabolism and the circadian rhythm in patients with chronic kidney disease (CKD) and that altered tryptophan metabolism as etiology for CKD-associated fatigue warrants further investigation. We have recently measured, by liquid chromatography with fluorometric detection previously described,2 plasma free tryptophan (FTRP) levels, large neutral amino-acid (LNAA) levels, and the FTRP/LNAA ratio in 50 patients on chronic hemodialysis and correlated with fatigue assessed through the SF-36 Vitality subscale.3 Patients were divided into two groups, fatigued (SF-36 Vitality subscale score 50) and not-fatigued (SF-36 Vitality subscale score 50).4 The two groups were similar for age, sex, primary cause of end-stage renal disease, body mass index, dialytic age, serum albumin, creatinine, and urea. As shown in the Table 1, the two groups did not differ significantly for the plasma levels of FTRP, LNAA, and FTRP/LNAA ratio. In addition, the correlation between fatigue and FTRP (correlation coefficient: −0.001; P=0.999), LNAA (−0.072; P=0.606), and FTRP/LNAA ratio (0.035; P=0804) was not statistically significant.