Iron deficiency anemia (IDA) is a common problem in older patients, but it is one that is frequently difficult to diagnose. Blood tests to identify IDA can be unreliable. Serum ferritin levels naturally increase with age, and underlying chronic medical conditions can further compromise the results. An examination of bone marrow aspirate is definitive, but this test is expensive and invasive. What's needed, say a group of Israeli researchers, is an "intermediate" test -- one that is more accurate than standard blood tests, but less invasive than bone marrow examination. They propose one such solution in a recent issue of the Archives of Internal Medicine.

Alternative measurements

The researchers recruited 49 adults (>age 80) with IDA confirmed by means of bone marrow examination. A group of 14 similar adults with signs of IDA but whose bone marrow exams were normal served as controls. All were tested for IDA using standard laboratory tests -- serum iron, transferrin saturation, and ferritin -- and then for serum transferrin receptor (TR) levels. To make the test more specific, a ratio of TR to log ferritin (TR-F) level was calculated. Levels of this index higher than 1.5 indicated iron deficiency anemia.

The standard laboratory tests identified IDA in only 8 of the patients (16% sensitivity), whereas the TR-F index identified IDA in 43 of the 49 cases (88% sensitivity). Neither set of tests could confirm IDA in the remaining 6 patients, all of whom had elevated levels of ferritin and C-reactive protein. In the control group, the TR-F index correctly ruled out iron deficiency as the cause of anemia in 13 of the 14 patients.

A cost-effective option?

Transferrin receptor is a transmembrane glycoprotein found in most cells, especially those that require high iron levels, such as immature erythroid cells. TR levels reflect the amount of membranous transferrin receptor, which is inversely correlated with iron storage levels. Serum TR levels increase in IDA, but not in response to inflammation.

A positive TR-F index (>1.5) can identify IDA and eliminate the need for a bone marrow test. On the other hand, a normal result in a person with signs of IDA would indicate the need for further testing. The TR-F is five times more expensive than usual blood tests, say the authors, but much less expensive than an examination of bone marrow aspirate.