Low iron is diagnosed through blood tests, most commonly a ferritin test that measures your body’s iron stores. A ferritin level below 15 ng/mL is the standard threshold for iron deficiency in adults, though newer research suggests iron stores may already be declining at levels below 25 ng/mL. Depending on your symptoms and initial results, your provider may order a single ferritin test, a full iron panel, or a complete blood count to get the full picture.
The Ferritin Test: First Line of Defense
Ferritin is a protein that stores iron in your cells, making it the most direct measure of how much iron your body has in reserve. It’s typically the first test ordered when iron deficiency is suspected and often the only one needed for a straightforward diagnosis.
The WHO and CDC both use a ferritin level below 15 ng/mL as the diagnostic cutoff for iron deficiency in healthy adults and children over six months. But a large 2025 study published in The Lancet Global Health found that hemoglobin levels (a marker of healthy red blood cells) actually start declining at ferritin levels around 23 to 25 ng/mL in women and children. That means your iron stores can be low enough to affect your body’s function well before they cross the traditional diagnostic line.
There’s one important caveat: ferritin rises during illness, infection, or any kind of inflammation in the body. If you’re fighting off a cold, managing a chronic inflammatory condition, or recovering from surgery, your ferritin can appear normal even when your iron stores are genuinely depleted. The WHO recommends using a higher cutoff in these situations: below 30 ng/mL for children and below 70 ng/mL for adults with active infection or inflammation.
The Full Iron Panel
When ferritin alone doesn’t tell the whole story, a full iron panel gives your provider four related measurements that together reveal how iron is moving through your bloodstream and whether enough of it is reaching your cells.
- Serum iron measures the amount of iron circulating in your blood at the moment of the draw. It fluctuates throughout the day and after meals, so it’s not very useful on its own.
- Transferrin measures the protein responsible for transporting iron through your bloodstream to where it’s needed. When iron is low, your body produces more transferrin to try to capture whatever iron is available.
- Total iron-binding capacity (TIBC) reflects how much room is available on those transport proteins to carry iron. A high TIBC means your transport proteins are mostly empty, a sign they’re not finding enough iron to pick up.
- Transferrin saturation is a calculated percentage showing how much of your iron-carrying capacity is actually being used. Below 20% indicates iron deficiency. Below 15% points to iron deficiency that has progressed to anemia.
These four values together help distinguish iron deficiency from other conditions that can look similar on basic blood work, like anemia caused by chronic disease or vitamin B12 deficiency.
What a Complete Blood Count Reveals
A complete blood count, or CBC, doesn’t measure iron directly. Instead, it looks at your red blood cells for signs that iron deficiency has been going on long enough to change how those cells are produced. Your provider may order it alongside an iron panel, or it may be the test that first raises suspicion of low iron.
In chronic iron deficiency, red blood cells become smaller and paler than normal. The CBC captures this as a low MCV (mean corpuscular volume, which measures cell size) and low MCHC (mean corpuscular hemoglobin concentration, which reflects how much oxygen-carrying protein each cell contains). Normal MCV falls between 83 and 97, and normal MCHC between 32 and 36. When both drop below range, it’s a strong signal that iron has been insufficient for weeks to months. Interestingly, a blood smear can reveal these shrunken, pale cells even before the MCV number formally drops below normal.
Another clue on the CBC: platelet counts often rise above 450,000 in iron deficiency anemia. This is a reactive response, not a separate problem, and it typically normalizes once iron levels are restored.
How to Prepare for Accurate Results
Iron blood tests are sensitive to what you’ve consumed in the hours before the draw. Mayo Clinic Laboratories recommends fasting for 8 hours beforehand (preferred but not strictly required) and stopping iron-containing supplements for a full 24 hours before your appointment. Taking an iron supplement just 3 hours before a blood draw can inflate your serum iron reading by 3 to 5 times its actual level, which could mask a genuine deficiency.
Multivitamins often contain iron, so check the label and skip your dose the day before testing. Morning appointments tend to work best since serum iron naturally fluctuates throughout the day and is most stable in the early hours after an overnight fast.
At-Home Iron Tests
At-home test kits, which use a finger-prick blood sample mailed to a lab, have become widely available. Most of these kits test only ferritin, which is useful as a screening tool but doesn’t give you the full picture of iron transport or red blood cell health that a clinical workup provides.
The accuracy question is nuanced. The lab analysis itself can be reliable, since many home kits send your sample to certified laboratories. But finger-prick blood collection introduces variability. A 2015 study found that finger-prick samples were less concentrated than venous draws, and blood counts could differ from drop to drop. For a definitive diagnosis, a venous blood draw (the standard arm-vein method) remains the gold standard.
Home kits can be reasonable for an initial check if you’re curious about your iron status or have symptoms like persistent fatigue, hair thinning, or feeling cold all the time. A low ferritin result on a home test is likely meaningful. A normal result, however, doesn’t necessarily rule out early depletion, especially if you’re dealing with any inflammation that could be propping up your ferritin number.
Which Tests You Actually Need
If you’re experiencing symptoms of low iron and haven’t been tested before, a ferritin test combined with a CBC is a practical starting point that catches both depleted stores and their downstream effects on red blood cells. If your ferritin comes back borderline (say, between 15 and 30 ng/mL) or if you have a chronic condition that complicates interpretation, a full iron panel with transferrin saturation gives the clearest answer.
For people with conditions like inflammatory bowel disease, rheumatoid arthritis, or chronic kidney disease, standard ferritin cutoffs are unreliable. In these cases, transferrin saturation below 20% becomes especially important as a diagnostic anchor, since it’s less affected by inflammation than ferritin.
Keep in mind that a single snapshot of your iron levels tells you where things stand right now but not why. If testing confirms low iron, the next step is usually identifying the cause, whether that’s dietary intake, blood loss, absorption issues, or something else, so the problem doesn’t simply recur after treatment.