Unbound iron binding capacity (UIBC) as a test for empty iron stores – results from the HUNT Study

BACKGROUND: When s-iron and s-transferrin are used to diagnose empty iron stores, the measurements are usually combined in the calculation of s-transferrin saturation. This may not be the best way to utilize the information in s-iron and s-transferrin, as s-transferrin alone has a better diagnostic accuracy than s-transferrin saturation. We suggest that unbound iron binding capacity (UIBC), which is s-total iron binding capacity (2 times s-transferrin) minus s-iron could be used for diagnosing empty iron stores. METHODS: To test this hypothesis, we used ROC curve analysis to compare the diagnostic accuracy of s-iron, s-transferrin, s-transferrin saturation and s-UIBC in diagnosing empty iron stores in 3029 women of childbearing age. Empty iron stores were defined as s-ferritin less than 10 μg/L or less than 15 μg/L. RESULTS: At both definitions of empty iron stores s-UIBC had a better diagnostic accuracy than the other tests, with area under the ROC curve of 0.80-0.87. This was also the trend in a subpopulation of 172 anemic women, where the area under the ROC curve of s-UIBC was 0.92. CONCLUSION: When diagnosing empty iron stores calculation of s-UIBC is a better way to utilize the information in s-iron and s-transferrin than is calculation of s-transferrin saturation.