SI measures circulating iron bound to transferrin and reflects total body iron. It is elevated in HH and acute hepatitis and decreased in iron deficiency anemia and chronic inflammation.
SI concentrations exhibit diurnal variation, with the lowest values occurring around midnight. In addition, specimens collected from the same individual at the same time may exhibit day-to-day variations as high as 40%. SI determinations are also affected by diet, menstrual cycle, pregnancy, ingestion of iron supplements, and oral contraceptive use. SI levels alone are considered insensitive indicators of HH.
SI is typically measured on automated analyzers using spectrophotometric methods. Iron in the sample is released from transferrin with an acid reagent, reduced to the ferrous state, and reacted with a chromogen such as bathophenanthroline or ferrozine.2 The intensity of the color change is proportional to the iron concentration. Interference can arise from using a hemolyzed sample and contaminating reagents and water with iron.
A typical reference interval for SI is 60–150 µg/dL.
SI is usually ordered along with its companion test, total iron-binding capacity (TIBC), or with transferrin (Tf).2
