What Is Anemia?
Anemia is defined as a lower-than-normal concentration of hemoglobin — the protein in red blood cells that carries oxygen — in the blood. Without enough hemoglobin, tissues and organs receive less oxygen than they need, producing the characteristic symptoms of fatigue, breathlessness, and cognitive difficulties.
According to the World Health Organization (2011 criteria), anemia is diagnosed when hemoglobin falls below:
- 12 g/dL in women (non-pregnant)
- 11 g/dL in pregnant women
- 13 g/dL in men
- 11.5 g/dL in children (6–11 years)
These thresholds vary slightly across guidelines and laboratory reference ranges, but they represent the approximate level below which most people experience physiological impairment from reduced oxygen delivery.
The WHO estimates that approximately 1.62 billion people globally have anemia — roughly 25% of the world population — making it one of the most prevalent nutritional and hematological conditions worldwide.
Types of Anemia
Anemia is a symptom, not a disease in itself. It has many causes, broadly grouped into three mechanisms:
1. Insufficient Red Blood Cell Production
- Iron deficiency anemia: The most common type worldwide. Iron is required to synthesise hemoglobin; without adequate dietary iron or with excessive loss, red blood cell production decreases.
- Vitamin B12 or folate deficiency anemia (megaloblastic anemia): B12 and folate are required for DNA synthesis in rapidly dividing cells, including red blood cell precursors. Deficiency causes abnormally large, dysfunctional red cells.
- Anemia of chronic disease: Inflammatory cytokines interfere with iron metabolism and erythropoiesis. Common in chronic infections, autoimmune conditions, kidney disease, and cancer.
- Aplastic anemia: Failure of the bone marrow to produce adequate blood cells; rare but serious.
2. Excessive Red Blood Cell Destruction
- Hemolytic anemias: Red blood cells are destroyed faster than they can be replaced. Causes include inherited conditions (sickle cell disease, G6PD deficiency, hereditary spherocytosis), autoimmune attack, infections, and medications.
3. Blood Loss
- Acute blood loss: Trauma, surgery, GI bleeding
- Chronic blood loss: Heavy menstrual bleeding (the most common cause of iron-deficiency anemia in women of reproductive age), slow GI bleeding from ulcers or polyps
Symptoms of Anemia
Symptoms reflect the reduced oxygen-carrying capacity of blood. They vary significantly with how severe the anemia is and how quickly it developed. Gradual-onset anemia (common with iron or B12 deficiency) may produce fewer symptoms at a given hemoglobin level than rapidly developing anemia.
Common symptoms:
- Fatigue and weakness: The most universal symptom; results from reduced oxygen delivery to muscles and the body’s general adaptation to low hemoglobin
- Pale or yellowish skin: Skin pallor occurs because less hemoglobin is present in surface blood vessels. Jaundice (yellow tint) can occur in hemolytic anemia from bilirubin release during red cell destruction
- Shortness of breath on exertion: The cardiovascular system compensates by increasing heart rate and cardiac output, but this compensation has limits; breathlessness on activity reflects the shortfall
- Dizziness or lightheadedness: Often worse on standing; reduced cerebral blood oxygen
- Rapid or irregular heartbeat (palpitations): Compensatory tachycardia to maintain oxygen delivery
- Cold hands and feet: Peripheral vasoconstriction preserves blood flow to vital organs at the expense of extremities
- Headaches: Reduced cerebral oxygenation and compensatory vasodilation
- Difficulty concentrating: Mild cognitive impairment is common; sometimes described as “brain fog”
Less common symptoms:
- Brittle nails (in iron deficiency)
- Glossitis (swollen, red tongue, in B12/folate deficiency)
- Pica (craving non-food items like ice or clay, particularly in iron deficiency)
- Restless legs syndrome (associated with iron deficiency specifically)
Risk Factors
Certain groups have substantially elevated anemia risk:
Dietary factors:
- Vegetarian and vegan diets: Non-haem iron (from plants) is less bioavailable than haem iron (from meat). Vitamin B12 is found almost exclusively in animal products; vegans require supplementation.
- Inadequate calorie or nutrient intake: Any diet restrictive in iron-containing foods or vitamins
Physiological factors:
- Heavy or frequent menstrual periods: The most common cause of iron-deficiency anemia in premenopausal women. Women with menorrhagia (heavy periods) lose significantly more iron per month than can typically be replaced through diet alone.
- Pregnancy: Increased iron requirements for fetal development; prenatal iron supplementation is standard practice in most healthcare systems.
- Infancy and early childhood: Rapid growth increases iron needs; breastfed infants may need iron supplementation.
- Older age: Decreased appetite, impaired absorption, and increased prevalence of chronic disease all contribute.
Medical conditions:
- Chronic kidney disease: Reduced erythropoietin production (the hormone that stimulates red cell production); also causes iron and folate losses
- Inflammatory conditions: Rheumatoid arthritis, IBD, chronic infections, cancer
- Gastrointestinal conditions: Celiac disease (impaired iron and B12 absorption); gastric bypass surgery
Genetic factors:
- Family history of anemia: Particularly relevant for inherited hemolytic anemias (sickle cell trait, thalassemia trait, G6PD deficiency)
How Anemia Is Diagnosed
Anemia cannot be reliably diagnosed from symptoms alone. A blood test is required.
Complete Blood Count (CBC) is the primary test. Key values:
- Hemoglobin (Hgb): The primary diagnostic criterion
- Hematocrit (Hct): Percentage of blood volume occupied by red cells
- Mean corpuscular volume (MCV): Size of red cells — low MCV suggests iron deficiency; high MCV suggests B12 or folate deficiency
- Red cell distribution width (RDW): Variability in cell size; elevated in mixed deficiencies
Additional tests (ordered based on CBC findings):
- Serum ferritin: Best marker for iron stores; low ferritin confirms iron deficiency before hemoglobin falls
- Serum iron and TIBC: Iron saturation of transferrin
- Serum B12 and folate: For suspected megaloblastic anemia
- Peripheral blood smear: Microscopic examination of red cell morphology
- Reticulocyte count: New red cell production rate — elevated in hemolytic anemia, low in aplastic or deficiency anemia
When to See a Doctor
The anemia risk checklist in the calculator is designed to help you recognize symptoms and risk factors that might warrant discussion with a healthcare provider. The following situations particularly warrant medical evaluation:
- Multiple symptoms from the checklist occurring together
- Symptoms that are worsening or have persisted for more than 2–3 weeks
- Heavy menstrual periods that are affecting daily life or that have worsened
- Known B12 deficiency risk (vegans, people over 50 with possible absorption issues)
- Family history of inherited blood disorders
- Any symptoms in the context of chronic illness, cancer treatment, or kidney disease
Anemia is treatable in the great majority of cases once the underlying cause is identified. Iron supplementation, B12 injections or oral therapy, dietary changes, or treatment of underlying conditions can restore hemoglobin to normal levels in most patients.