FABP2 and infection: Concerning circulating levels of cytokines, FABP-2 and tryptophan metabolites, after controlling for sex, age, and BMI, we found a tendency to lower serum levels of IL-18 (p = 0.068), lower kynurenine (p = 0.026), and lower kynurenine/tryptophan ratio (p = 0.015) in post-infection than in non-post-infection ME/CFS patients (Table 5).