Anemia: A Lab Testing Guide

Iron deficiency: it might be the most common nutritional disorder in the world, but diagnosing it isn’t always as simple as “you’re low on iron, eat steak” because we need to investigate the causation to be able to fix what is actually going wrong. It’s more like solving a mystery, with the laboratory playing detective to uncover the clues. The closer you look, the more you can deduce. From iron stores to red blood cell production, multiple tests work together to crack the case. For healthcare professionals, knowing how these tests work and when to order them is key to ensuring your patients don’t go through life feeling and looking pale.

Red blood cells (not an actual picture)

The first suspect in any iron deficiency investigation is the complete blood count (CBC). Hemoglobin and hematocrit values are often the red flags. If hemoglobin levels are low, it’s time to dig deeper. Red blood cell indices can lead you in the right direction and tell you what the cells actually look like, and a manual differential (looking at the blood under a microscope) can give you more information. So, let's start at the beginning, the CBC...

CBC and RBC Indices:

The CBC provides the first diagnostic step, with hemoglobin and hematocrit values confirming the presence of anemia. RBC indices — mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) — classify anemia as microcytic, normocytic, or macrocytic. Microcytic anemia (MCV <80) suggests iron deficiency, thalassemia, or anemia of chronic disease.

Normocytic anemia (MCV 80-100) may indicate acute blood loss, early iron deficiency, anemia of chronic disease, or hemolysis. Macrocytic anemia (MCV >100) points toward vitamin B₁₂ or folate deficiency, liver disease, hypothyroidism, or myelodysplastic syndromes. The red cell distribution width (or RDW, a measure of the variation in RBC size) can further refine the differential, as an elevated RDW suggests a mixed population of cell sizes, often seen in evolving nutritional deficiencies or recovery from anemia.

So, sum up the CBC:

Low hemoglobin and hematocrit tell you there is anemia present.

The MCV tells you if the anemia is microcytic (MCV <80), normocytic (MCV 80-100), or macrocytic (MCV >100).

A manual differential will look at the RBC morphology (Are there target cells, dacryocytes, spherocytes, etc...) that can give you further clues.

Peace, y'all.

Next up is the iron studies that may be ordered as the follow up to the CBC...

Iron Studies: Serum Iron, Ferritin, Transferrin, and TIBC

Iron studies assess systemic iron availability and storage. Serum iron reflects circulating iron bound to transferrin, though it is subject to diurnal variation and may be influenced by inflammation. Ferritin serves as the primary indicator of iron stores; low ferritin is diagnostic of iron deficiency, while elevated ferritin can indicate iron overload or, more commonly, an acute-phase response masking an underlying deficiency. Transferrin, the primary iron-binding protein, increases in iron deficiency and decreases in chronic inflammatory states. Total iron-binding capacity (TIBC) reflects the transferrin concentration; elevated TIBC with low serum iron supports iron deficiency, whereas low or normal TIBC in the setting of low serum iron suggests anemia of chronic disease. The transferrin saturation (serum iron/TIBC × 100) provides an integrated measure of circulating iron availability to the marrow.

Reticulocyte Count

The reticulocyte count, corrected for the degree of anemia, assesses the bone marrow’s response. An elevated reticulocyte count indicates appropriate marrow compensation, as seen in acute blood loss or hemolysis, whereas a low reticulocyte count suggests impaired RBC production from nutrient deficiency, bone marrow suppression, or chronic disease. In the context of iron studies, a low reticulocyte count with microcytosis often reflects iron deficiency or anemia of chronic disease, while macrocytosis with reticulocytopenia raises suspicion for B₁₂ or folate deficiency. These reticulocytes can be seen under a microscope and are noted as "polychromasia" when seen in a differential.

Again, not a real picture of red blood cells.

Integrating Patterns for Final Classification

The last step in the combines the indices, iron profile, and marrow activity into a cohesive pattern. For example:

Microcytosis + low ferritin + high TIBC + low reticulocytes → Iron deficiency anemia.

Normocytic + low serum iron + high ferritin + low TIBC + low reticulocytes → Anemia of chronic disease.

Normocytic + normal iron profile + high reticulocytes → Acute blood loss or hemolysis.

Macrocytosis + normal iron studies + low reticulocytes → Vitamin B₁₂ or folate deficiency.

This integrative step transforms raw data into an actionable working diagnosis, enabling targeted confirmatory testing, such as hemoglobin electrophoresis for suspected thalassemia or Coombs testing for suspected immune-mediated hemolysis.

Here is a Visual Summary of 10 Common Types of Anemia

Anemia Type	MCV	RBC Morphology	Serum Iron	TIBC	Transferrin Saturation	Ferritin	Reticulocyte Count
Iron Deficiency Anemia	Low (<80 fL)	Microcytic, hypochromic, ↑RDW	Low	High	Low	Low	Low or inappropriately normal
Anemia of Chronic Disease / Inflammation	Low–Normal (70–100 fL)	Normocytic or microcytic, mild anisopoikilocytosis	Low	Low or normal	Low	Normal or high	Low
Thalassemia Trait (α or β)	Low (<75 fL)	Microcytic, target cells, basophilic stippling, normal RDW	Normal or high	Normal or low	Normal or high	Normal or high	Normal
Sideroblastic Anemia	Low–Normal	Dimorphic population, basophilic stippling, Pappenheimer bodies	Normal or high	Normal	Normal or high	High	Low
Vitamin B₁₂ Deficiency	High (>100 fL)	Macrocytic, ovalocytes, hypersegmented neutrophils	Normal	Normal	Normal	Normal or high	Low
Folate Deficiency	High (>100 fL)	Macrocytic, ovalocytes, hypersegmented neutrophils	Normal	Normal	Normal	Normal or high	Low
Aplastic Anemia	Normal	Normocytic, decreased overall cell lines	Normal	Normal	Normal	Normal	Low
Hemolytic Anemia (general)	Normal	Normocytic, polychromasia, possible schistocytes or spherocytes	Normal or high	Normal	Normal or high	Normal or high	High
Acute Blood Loss	Normal	Normocytic, normal morphology early, polychromasia later	Normal	Normal	Normal	Normal	High (after 2–3 days)
Chronic Kidney Disease–related Anemia	Normal	Normocytic, unremarkable morphology	Normal	Normal	Normal	Normal or high	Low

So, you can see, the more information you have about a case of anemia, and if you know the patient history, you can piece together the causation of the anemia. Only then can you figure out an appropriate treatment for the anemia.