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Anemia is a common finding in both inpatient and outpatient settings, defined as a decrease in circulating red blood cells (RBCs) or hemoglobin.[1] This condition is often inadequately evaluated or managed. While frequently listed as a standalone diagnosis, anemia is actually a clinical sign that indicates an underlying condition, requiring further investigation to determine its cause. Anemia leads to reduced tissue oxygenation and can worsen the progression of many coexisting diseases. Despite this condition's vast impact on health, inconsistencies remain in both its formal definition and the protocols for screening.[2][3] Symptoms of anemia vary widely and may include fatigue, weakness, lightheadedness, headache, pallor or jaundice, tachycardia, palpitations, chest pain, shortness of breath, cold extremities, and claudication. The prevalence and severity of these signs and symptoms differ among individuals. Anemia causes a reduction in the relative number of circulating RBCs or hemoglobin, leading to a corresponding decrease in oxygen delivery to tissues.[4] However, the hemoglobin concentration that defines anemia varies based on sex, ethnicity, and age. Additionally, no consensus has been established regarding which patient populations warrant routine anemia screening. Furthermore, the threshold for initiating treatment and the treatment goals differ depending on medical specialty and underlying conditions.[5][6] In 2010, the World Health Organization (WHO) set diagnostic criteria for anemia, defining it as hemoglobin levels below 12 g/dL in premenopausal women and below 13 g/dL in both postmenopausal women and men of all ages.[7] The journal Blood challenged these standards, citing limited WHO data and proposing new anemia thresholds based on race, gender, and age. According to these proposed standards, anemia should be defined as hemoglobin levels below 13.7 g/dL for White men aged 20 to 60 and below 13.2 g/dL for White men older than 60. White women of all ages were considered anemic at hemoglobin levels under 12.2 g/dL. Although the journal acknowledged significant differences in hemoglobin levels among Black men and women, it did not establish diagnostic thresholds for these populations. Most current literature continues to use WHO standards for consistency.

In 2010, the World Health Organization (WHO) set diagnostic criteria for anemia, defining it as hemoglobin levels below 12 g/dL in premenopausal women and below 13 g/dL in both postmenopausal women and men of all ages.[7] The journal Blood challenged these standards, citing limited WHO data and proposing new anemia thresholds based on race, gender, and age. According to these proposed standards, anemia should be defined as hemoglobin levels below 13.7 g/dL for White men aged 20 to 60 and below 13.2 g/dL for White men older than 60. White women of all ages were considered anemic at hemoglobin levels under 12.2 g/dL. Although the journal acknowledged significant differences in hemoglobin levels among Black men and women, it did not establish diagnostic thresholds for these populations. Most current literature continues to use WHO standards for consistency. Disagreement also exists among the U.S. Preventive Services Task Force (USPSTF), various medical academic institutions, and actual clinical practice regarding anemia screening guidelines. For example, the USPSTF evaluated anemia screening in asymptomatic children aged 6 to 24 months and concluded that evidence was insufficient to determine the benefits versus harms of screening in this population. In contrast, the American Academy of Family Physicians (AAFP) recommended universal anemia screening at 12 months of age, including hemoglobin measurement and an assessment of risk factors for iron deficiency and iron deficiency anemia (IDA).[8][9] A similar lack of consensus applies to screening in pregnant women. The USPSTF stated, "The evidence of the effect of routine screening for IDA in asymptomatic pregnant women on maternal health and birth outcomes is insufficient... and the balance of benefits and harms cannot be determined." The AAFP agreed with this position. However, the American College of Obstetricians and Gynecologists, in Practice Bulletin No. 95, advised, "All pregnant women should be screened for anemia and treated if necessary." At present, no guidelines recommend routine anemia screening for nongravid, otherwise healthy adults.

Key Structures and Mechanisms in Anemia Pathophysiology The development and regulation of RBCs involve complex interactions between the bone marrow, kidneys, and central nervous system (CNS). These interactions contribute to maintaining oxygen transport and vascular homeostasis. Understanding these mechanisms is essential for effectively diagnosing and managing anemia. Red blood cells RBCs are released from the bone marrow as reticulocytes. These immature cells contain a network of ribosomal RNA and mature into adult RBCs over 24 hours. The relative reticulocyte count helps determine whether the bone marrow is responding appropriately to anemia by increasing RBC production.[17] Each RBC consists of 2 α and 2 β chains, along with a single heme moiety that reversibly binds oxygen. Although numerous genetic variants can alter the configuration of these chains, most do not cause clinical consequences. However, sickle cell disease and thalassemia variants affecting the α and β chains can lead to anemia. Additional genetic abnormalities involving the cell membrane, metabolism, or morphology are also recognized causes of anemia.[18] Bone marrow The bone marrow requires approximately 21 days to develop a pluripotent stem cell into a reticulocyte ready for circulation. The initial stimulus for reticulocyte production is the renal release of erythropoietin, which must remain present for the transformation of a pluripotent stem cell into a proerythroblast. This initial stage takes approximately 10 to 15 days. The next step, which is iron-dependent, takes 3 to 4 days, during which iron is incorporated into the proerythroblast to form a heme moiety, completing reticulocyte formation.[19][20] Significant bone marrow-related causes of anemia include the following: Lack of essential substrates, such as iron, vitamin B12, or folate, needed for healthy reticulocyte production Direct suppression of bone marrow function due to medications, toxins, infections, or radiation exposure Replacement of bone marrow by neoplasm or fibrosis Kidneys The kidneys play a dual role in the pathophysiology of anemia. First, these organs produce 90% of the erythropoietin required to stimulate the transformation of pluripotent stem cells into proerythroblasts in the bone marrow. Any disruption in erythropoietin production or release leads to anemia.

Replacement of bone marrow by neoplasm or fibrosis Kidneys The kidneys play a dual role in the pathophysiology of anemia. First, these organs produce 90% of the erythropoietin required to stimulate the transformation of pluripotent stem cells into proerythroblasts in the bone marrow. Any disruption in erythropoietin production or release leads to anemia. Second, acute anemia caused by blood loss results in hypotension, which stimulates stretch receptors that send signals to the brain via the glossopharyngeal and vagus nerves. The resulting neural activity triggers several downstream effects, including the secretion of antidiuretic hormone, also known as arginine vasopressin or simply vasopressin. In response, the kidneys reabsorb water, reducing renal perfusion. The renin-angiotensin system is then activated, increasing vascular tone, stimulating aldosterone release, and ultimately raising intravascular volume.[21] Central nervous system The medulla, cerebral cortex, and pituitary gland coordinate the response to acute blood-loss anemia by increasing sympathetic tone. In addition, these CNS structures regulate volume changes by stimulating the secretion of antidiuretic hormone.[22] Physiologic Response to Acute versus Chronic Anemia Acute-onset anemia due to blood loss or rapid hemolysis is compensated by a CNS–directed, renal-mediated response to volume and perfusion loss.[23] This compensatory mechanism has an upper threshold, which is well-defined by the American College of Surgeons' Advanced Trauma Life Support protocols for managing volume loss: Class I hemorrhage: Up to 15% blood volume loss, no significant change in vital signs, no intervention required Class II hemorrhage: 15% to 30% blood volume loss, possible tachycardia, reduced pulse pressure, and peripheral vasoconstriction; volume repletion with crystalloids is typically sufficient, and blood transfusion is generally unnecessary Class III hemorrhage: 30% to 40% blood volume loss, resulting in hypotension, tachycardia, and shock; requires crystalloid resuscitation and blood transfusion Class IV hemorrhage: Greater than 40% blood volume loss, exceeding compensatory thresholds; lethal unless rapidly treated with blood products, crystalloids, and pressors

Class III hemorrhage: 30% to 40% blood volume loss, resulting in hypotension, tachycardia, and shock; requires crystalloid resuscitation and blood transfusion Class IV hemorrhage: Greater than 40% blood volume loss, exceeding compensatory thresholds; lethal unless rapidly treated with blood products, crystalloids, and pressors By contrast, very low hemoglobin levels may be tolerated in cases of chronic, slowly progressive anemia, where RBC mass is significantly reduced but circulating blood volume is preserved. The management of blood products and anemia-specific therapy, including RBC substrates or erythropoietin, depends on the underlying cause and clinical scenario.

The evaluation and management of anemia require an interprofessional approach. Anemia is a clinical sign that warrants investigation to determine its underlying cause. The impact of this condition extends beyond its direct medical effects, influencing comorbid disorders and overall patient health. A thorough patient history and physical examination guide diagnostic testing and treatment decisions. When clear clinical indicators are present, the workup may be streamlined, allowing for targeted diagnostic testing and appropriate therapeutic interventions. Healthcare team members who suspect anemia should pursue further evaluation and, when necessary, consult a hematologist for specialized expertise.