How is transferrin saturation a reliable indicator for any parameter if serum iron is not reliable?

The question is a very good question. The Fe/TIBC must be drawn on an overnight fast including any vitamin pills containing iron. Otherwise, the serum iron is speciously elevated which in turn speciously elevates the TSAT. If those conditions are met, the TSAT is as good as the transferrin receptor but may be a tad more sensitive (surely no harm done). Since the ferritin is an acute phase reactant, a normal ferritin in no way rules out iron deficiency but a normal TSAT on a fasting sample (>20%)...