Transferrin saturation is calculated from serum iron concentration and TIBC. All those factors which can affect the serum iron and TIBC such as diurnal variation, iron status, drugs, food, alcohol can affect the measured transferrin saturation. This test cannot be recommended for screening purpose. It is an indicator of iron-deficient erythropoiesis rather than iron depletion; hence, it is less sensitive to changes in iron stores than is serum ferritin concentration.

On the other hand this test is one of the most sensitive tests for detecting early hemochromatosis. A transferrin saturation greater than 45 percent should be investigated further.  

In the context of Haemochromatosis :

http://www.medscape.com/viewarticle/535644_3