The Red Knot (Calidris canutus) is a long-distance migrant breeding on tundra in the high Arctic and wintering along temperate and tropical coasts. Preflight fueling rate is a major determinant of successful migration, yet individual fueling rates are impossible to determine because Red Knots cannot be recaptured easily. These problems can be overcome by estimating changes in body mass from plasma metabolites. Plasma metabolites are, however, sensitive to stress and time since last meal, limiting studies to situations where birds can be bled almost immediately after capture. Such sampling is almost impossible in the field, where Red Knots are often captured with mist nets in darkness. This study on captive Red Knots investigates whether plasma metabolites obtained from blood samples taken up to 3 hr after capture can be used to predict individual long-term (weekly) body-mass changes during the natural spring preflight fueling period. Triglyceride decreased and β-hydroxybutyrate increased with time since capture, and these changes varied with time since start of the spring fueling period. β-Hydroxybutyrate and uric acid were correlated with weekly body-mass change, but triglyceride was not. Triglyceride was correlated with overall body mass. Weekly body-mass change was best predicted with a model including all metabolites and body mass. Time of blood sampling (immediately or 3 hr after capture) did not affect the accuracy of the predictions. The predictions were not accurate enough to allow comparisons of individuals; they should be used only to compare groups.