Anemia can be generally defined as a reduction in hemoglobin level or as a decrease in circulating red blood cell (RBC) mass to below age-specific and gender-specific limits. Anemia can be a single entity or, most commonly, one symptom of a more complicated or chronic disease.

In determining the seriousness of the anemia, the level of hemoglobin is less important than the underlying cause; there are more than 200 types of anemia.

There are three main ways to classify anemias.
- Cytometric types depend on cell size and hemoglobin-content parameters, such as mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC).
- Erythrokinetic types take into account the rates of RBC production and destruction.
- Biochemical/molecular types consider the cause of the anemia at the molecular level.

The first task in the workup for a patient with anemia is to classify the case in one of the three major cytometric categories.

Normocytic, normochromic anemia (normal MCV, normal MCHC)
- Anemias of chronic disease
- Hemolytic anemias (characterized by accelerated destruction of RBCs)
- Anemia of acute hemorrhage
- Aplastic anemias (characterized by disappearance of RBC precursors from the marrow)

Microcytic, hypochromic anemia (low MCV, low MCHC)
- Iron deficiency anemia
- Thalassemias
- Anemia of chronic disease (rare)

Macrocytic, normochromic anemia (high MCV, normal MCHC)
- Vitamin B12 deficiency
- Folate deficiency

The severity of clinical symptoms is related more to the period of time over which the condition develops and not the absolute value of the hemoglobin.
An acute hemorrhagic condition may produce life-threatening symptoms (for example, tachycardia, lightheadedness, shortness of breath, postural hypotension, syncope) with loss of as little as 20% of the total blood volume (or 20% of the total red cell mass).
Conversely, anemias developing over periods long enough to allow compensatory mechanisms to operate will be associated with much greater loss of red cell mass before symptoms are manifested.

The signs and symptoms of anemia occur when the oxygen-carrying capacity of the blood is unable to meet the oxygen requirements of body tissues:
- Dyspnea on exertion
- Easy fatigability
- Fainting, lightheadedness
- Tinnitus, roaring in the ears
- Headache
- Palpitations
- Exacerbation of pre-existing cardiovascular conditions
- Angina pectoris, intermittent claudication, and nighttime muscle cramps are some of the effects of anemia on already-compromised perfusion.

Physical findings
For slowly developing anemia:
- Pallor
- Tachycardia
- Systolic ejection murmur

In rapidly developing anemia (as from hemorrhage and certain catastrophic hemolytic anemias), additional symptoms and signs are noted:
- Syncope on rising from bed
- Orthostatic hypotension (i.e., the blood pressure falls when the patient is raised from a supine to a sitting or standing position)
- Orthostatic tachycardia

If anemia develops through rapid bleeding, the hematocrit and hemoglobin will be normal (because in hemorrhage the loss of RBCs and plasma is proportional). Therefore, the appreciation of the clinical signs will be of more value in diagnosing this type of anemia than will the results of laboratory tests.

See reference for details.

Adapted from Health Canada. Clinical practice guidelines for nurses in primary care. Adult Care - Chapter 10 - Hematology, Metabolism and Endocrinology. Internet. Accessed on May 17, 2016.