Iron deficiency anemia is diagnosed through a combination of blood tests, starting with a complete blood count and confirmed with iron-specific markers like ferritin and transferrin saturation. A single test isn’t enough on its own. Your doctor will typically order a sequence of labs that first identify anemia, then confirm iron deficiency as the cause, and finally look for why you’re losing iron in the first place.
The Complete Blood Count Comes First
The initial screening tool is a complete blood count, or CBC. This is a routine blood draw that measures several components of your blood at once. The key number is hemoglobin, the protein in red blood cells that carries oxygen. The World Health Organization defines anemia as hemoglobin below 130 g/L in men, below 120 g/L in non-pregnant women, and below 110 g/L in children over five and pregnant women. In pregnancy, the threshold dips slightly lower in the second trimester (below 105 g/L) because blood volume naturally expands.
A CBC also reveals clues that point specifically toward iron as the problem. When your body doesn’t have enough iron, it produces red blood cells that are smaller and paler than normal. The lab report captures this as MCV (a measure of cell size) and MCHC (a measure of how much hemoglobin is packed into each cell). In iron deficiency, MCV typically drops below about 84 fL, and MCHC falls below normal range. Your hematocrit, the percentage of blood volume made up of red cells, also drops. These patterns together create a recognizable fingerprint: small, pale red blood cells combined with low hemoglobin.
Ferritin Confirms the Diagnosis
If your CBC suggests anemia, the next step is measuring your iron stores directly. Ferritin is the most reliable single marker for this. It reflects how much iron your body has tucked away in reserve. A ferritin level below 30 µg/L is the most widely used cutoff for iron deficiency in adults and adolescents, with 92% sensitivity and 98% specificity when compared against bone marrow iron stores (the definitive but rarely needed gold standard). For children under five, the threshold is lower: below 12 µg/L.
There’s an important catch. Ferritin is also an inflammatory marker, meaning it rises when your body is fighting infection, dealing with chronic illness, or experiencing significant inflammation. If you have conditions like inflammatory bowel disease, chronic kidney disease, or heart failure, a “normal” ferritin level can be misleading. In these situations, current guidelines raise the diagnostic threshold considerably. Iron deficiency can be diagnosed with ferritin below 100 µg/L, or with ferritin between 100 and 300 µg/L if transferrin saturation (a separate marker) is below 20%. Your doctor may also check inflammatory markers like C-reactive protein to determine whether ferritin is artificially elevated. The WHO recommends using a ferritin cutoff of below 70 µg/L for older children and adults with active inflammation.
Transferrin Saturation and Iron Binding Capacity
Transferrin is the protein that carries iron through your bloodstream. Transferrin saturation measures what percentage of that carrier protein is actually loaded with iron. In a healthy state, about 33% of your transferrin is carrying iron. When iron stores are depleted, saturation drops to 16% or less.
A related test is total iron-binding capacity, or TIBC, which measures how much iron your transferrin could carry if fully loaded. In iron deficiency, TIBC rises above the normal range of 240 to 450 mcg/dL. Think of it this way: your body produces more “empty trucks” to scavenge whatever iron it can find. High TIBC combined with low transferrin saturation is a classic pattern that helps distinguish iron deficiency from other types of anemia.
Telling Iron Deficiency Apart From Other Anemias
Several chronic conditions, including kidney disease, autoimmune disorders, and chronic infections, cause a type of anemia that can look similar to iron deficiency on basic labs. This is sometimes called anemia of chronic disease, and sorting the two apart matters because the treatments differ.
The most useful tool for making this distinction is the soluble transferrin receptor (sTfR) test. When your bone marrow is genuinely starved for iron, cells ramp up production of transferrin receptors to grab every available iron molecule. The sTfR level rises in true iron deficiency but stays normal in anemia of chronic disease. A calculated ratio called the sTfR-ferritin index (sTfR divided by the log of ferritin) sharpens the picture further. An index above 1.5 points toward iron deficiency even in patients without inflammation, and values above 0.8 are suggestive when inflammation is present.
Another newer marker gaining traction is reticulocyte hemoglobin content, which measures how much hemoglobin is in the youngest red blood cells your bone marrow just released. Because these cells are only a few days old, their hemoglobin content reflects your iron supply right now, not weeks ago. A value below about 27 pg/cell suggests iron-restricted red blood cell production. This test can flag iron deficiency days before traditional markers shift, making it especially useful for monitoring patients who are receiving treatment and need real-time feedback.
Diagnosis in Pregnancy and Children
Pregnant women present a diagnostic challenge because pregnancy itself changes blood composition. Blood volume increases dramatically in the second trimester, diluting hemoglobin and making mild anemia appear on paper even when iron stores are adequate. The hemoglobin thresholds are adjusted accordingly: below 110 g/L in the first and third trimesters, below 105 g/L in the second. Ferritin below 15 µg/L in the first trimester indicates iron deficiency, though physiological changes later in pregnancy make ferritin harder to interpret.
In infants under two years, iron deficiency is defined by ferritin below 12 µg/L (or below 30 µg/L if infection or inflammation is present). School-age children and adolescents follow the same thresholds as adults: below 15 µg/L when healthy, below 70 µg/L when inflammation is a factor. Pediatricians often screen for iron deficiency around 12 months of age, when the iron stores a baby was born with are running low and dietary intake becomes critical.
Finding the Underlying Cause
Confirming iron deficiency anemia is only half the job. The more important question is why you’re iron deficient, because replenishing iron without addressing the root cause means the problem will return.
In premenopausal women, heavy menstrual bleeding is the most common explanation, and the diagnostic workup may stop there if periods are clearly heavy. In men and postmenopausal women, the concern shifts to the gastrointestinal tract. Blood loss from the stomach or intestines is the leading cause in these groups, and it can be slow enough that you never notice it. Guidelines from the American Gastroenterological Association recommend both an upper endoscopy (examining the esophagus, stomach, and upper small intestine) and a colonoscopy for men and postmenopausal women with iron deficiency anemia when no other source is obvious.
Up to 15% of patients have contributing lesions in both the upper and lower digestive tract at the same time, which is why both procedures are typically performed rather than just one. Your doctor may also test for celiac disease (through a blood antibody test) and Helicobacter pylori infection, both of which impair iron absorption without necessarily causing visible bleeding. If endoscopy in both directions comes back normal and iron deficiency persists, the small intestine becomes the next area of investigation, usually with a capsule endoscopy, a procedure where you swallow a tiny camera that photographs the entire length of your gut as it passes through.
Dietary insufficiency is another possibility, particularly in vegetarians, vegans, and people with restrictive eating patterns. But because treatable and sometimes serious conditions like colon cancer can present first as iron deficiency anemia, identifying the cause is a step your doctor will take seriously, especially if you’re over 50 or your anemia is unexplained.