Iron testing is a routine diagnostic tool used to assess the body’s iron stores and metabolism, helping identify conditions like iron-deficiency anemia or iron overload disorders such as hemochromatosis. Accurate sample collection is paramount because the wrong tube type can lead to misleading results, potentially affecting diagnosis and treatment. Obtaining a reliable measurement of iron status begins with selecting the correct blood collection container.
The Correct Tube Color for Iron Testing
The most common tests for iron status, including Serum Iron, Total Iron Binding Capacity (TIBC), and Ferritin, typically require a serum sample for analysis. Serum is the liquid portion of the blood that remains after clotting. To obtain serum, blood is collected into a tube that does not contain an anticoagulant, which would prevent clotting.
The tubes used for iron testing are identifiable by their colored tops, which denote the additive inside. The preferred tubes are the Gold Top or Tiger Top, known as Serum Separator Tubes (SST), which contain a clot activator and a gel barrier. A plain Red Top tube, which only contains a clot activator, is also acceptable. These tubes allow the blood to clot, and centrifugation separates the serum from the blood cells, with the gel acting as a stable barrier.
What Iron Tests Measure
A complete assessment of iron status involves a panel of tests that measure different aspects of iron metabolism. The Serum Iron test directly measures the amount of iron circulating in the blood, primarily bound to the protein transferrin. This measurement indicates the current level of available iron, but it can fluctuate significantly throughout the day.
Total Iron Binding Capacity (TIBC) is an indirect measure of the transferrin concentration in the blood. TIBC measures the maximum amount of iron that the blood can carry. The ratio between the serum iron and the TIBC is calculated to determine the transferrin saturation percentage, showing how much of the transferrin is bound with iron.
Ferritin measures the body’s stored iron and is often considered the most reliable single indicator of iron deficiency or overload. Ferritin levels reflect the iron reserves in tissues like the liver, spleen, and bone marrow. Interpreting these three components together provides a comprehensive picture of the body’s iron balance.
Why Other Tubes Must Be Avoided
Using the wrong blood collection tube can lead to inaccurate results because the additives in other tubes interfere with the chemical process of the iron assay. Tubes containing anticoagulants are designed to prevent clotting, but many of these substances are metal-chelating agents. Chelators are compounds that bind tightly to metal ions, including iron.
The Lavender or Pink Top tubes, commonly used for hematology tests, contain Ethylenediaminetetraacetic Acid (EDTA), a powerful chelator. If a sample is collected in an EDTA tube, the EDTA binds the iron, causing the test result to be falsely low. Similarly, Light Blue Top tubes contain sodium citrate, which chelates calcium, but its presence can also interfere with the chemical reactions used to measure iron.
In general, any tube containing an anticoagulant other than perhaps lithium heparin should be avoided for iron studies. The presence of these interfering substances can sequester the iron, making it unavailable for the laboratory test to measure accurately. This pre-analytical error means the lab result would not reflect the patient’s true biological iron status.
Preparing for an Iron Test
Obtaining the correct tube is only one part of ensuring an accurate iron test result; patient preparation is equally important. Iron levels naturally vary throughout the day (diurnal variation), with levels typically highest in the morning. For this reason, the blood sample is ideally drawn before noon to capture the most representative levels.
Iron tests often require the patient to fast for a specific period, typically eight to twelve hours, consuming only water. Eating before the test can introduce dietary iron that temporarily skews the serum iron result, making it appear artificially high. Patients are also routinely instructed to stop taking iron-containing supplements for at least 24 hours before the blood draw. Certain medications, such as some antibiotics or hormone therapies, can also affect iron test results. Patients should always consult their doctor before stopping any prescribed medicine.