Most anemia is related to the digestive system by dietary deficiency, malabsorption, or chronic bleeding. We review the World Health Organization definition of anemia, its morphological classification (microcytic, macrocytic and normocytic) and pathogenic classification (regenerative and hypo regenerative), and integration of these classifications. Interpretation of laboratory tests is included, from the simplest (blood count, routine biochemistry) to the more specific (iron metabolism, vitamin B12, folic acid, reticulocytes, erythropoietin, bone marrow examination and Schilling test). In the text and various algorithms, we propose a hierarchical and logical way to reach a diagnosis as quickly as possible, by properly managing the medical interview, physical examination, appropriate laboratory tests, bone marrow examination, and other complementary tests. The prevalence is emphasized in all sections so that the gastroenterologist can direct the diagnosis to the most common diseases, although the tables also include rare diseases. Digestive diseases potentially causing anemia have been studied in preference, but other causes of anemia have been included in the text and tables. Primitive hematological diseases that cause anemia are only listed, but are not discussed in depth. The last section is dedicated to simplifying all items discussed above, using practical rules to guide diagnosis and medical care with the greatest economy of resources and time.

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Figure 1 Evaluation of microcytic anemia. TSI: Transferrin saturation index; MCV: Mean corpuscular volume; Hb: Hemoglobin.

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Figure 2 Evaluation of normocytic anemia. RPI: Reticulocytes production index.

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Figure 3 Evaluation of macrocytic anemia.

Erythrocytes and the digestive system are linked closely from the very beginning of life. The yolk sack is the origin of the first generation of erythrocyte precursors. Yolk-sac-derived progenitor cells may seed the developing liver via the circulation and produce mature red blood cells that are required to meet the metabolic needs of the fetus. By week 8, liver-derived red cells are evident and the liver is the only source of erythrocytes until the 18th week of gestation. Afterwards, the spleen and bone marrow take over, but a small percentage of hepatic erythropoiesis remains until the postnatal week 6.

In adult life, the esophagus, stomach, bowel and liver are involved in the pathogenesis of different types of anemia caused by nutritional deficiency, bleeding or chronic inflammation.

Several diseases of the esophagus may produce chronic or acute bleeding: varicose veins, diverticula, diaphragmatic hernia, tumors, ulcers, esophagitis and Mallory-Weiss syndrome. The stomach plays a decisive role in the protection of vitamin B12 until it is absorbed in the final portion of the ileum, and contributes to an optimum absorption of iron reducing Fe³⁺ to Fe²⁺. The stomach also may be a source of bleeding in peptic ulcer, gastritis and tumors, and treatment with acetylsalicylic acid and nonsteroidal anti-inflammatory drugs. The absorption of iron is a sophisticated and specialized process located mostly in the duodenum. The integrity of the small-intestinal mucosa is essential for folate absorption. Vitamin B12 absorption takes place chiefly in the ileum. In consequence, several gastrointestinal diseases, hereditary or acquired, including surgical resection, may produce nutritional deficiency anemia. Small-bowel inflammatory disease is a widespread cause of chronic disease anemia. Chronic bleeding is a crucial sign in the diagnosis of malignant and benign lesions (such as diverticula) in the colon and rectum.

Splenomegaly is a relatively common cause of chronic hemolysis, and Zieve’s syndrome is an infrequent cause of acute hemolytic crisis. Last, but not least, the liver plays an essential role in the control of iron metabolism through the hepcidin pathway, and accounts for most vitamin B12, folate and iron stores.

The links between anemia and digestive system are strong and multiple. Because of this, hematologist and the specialist in digestive system should work together to acquire a profound know of common pathogenic processes.

Anemia is defined as a lower than normal hemoglobin concentration. Low hematocrit is a subrogate value for anemia, but it is not measured directly by the hematological analyzer. Instead, it is calculated from hemoglobin and other parameters. The erythrocyte count may be misleading in the evaluation of anemia. In fact, in some cases of microcytic anemia, such as thalassemia, there is usually an elevated erythrocyte count (spurious polycythemia).

In order to make a generalized approach to the diagnosis of anemia, the World Health Organization (WHO) has established a reference range for normal blood hemoglobin concentration, depending on age and sex[1]. According to this criterion, anemia is present if the blood concentration of hemoglobin falls below 130 g/L in men or 120 g/L in women. This rule does not apply to infants, children and pregnant women, who have their own tables of lower limits of hemoglobin concentration. The WHO criterion has been accepted widely for diagnosis and publication, but its universal application has been questioned mainly because of racial differences. Beutler has proposed a lower limit of hemoglobin (1-2 g less) in African Americans than in Caucasians. The reference range of hemoglobin concentration in blood may vary depending on the population analyzed, age, sex, environmental conditions and food habits[2,3].

Anemia causes tissue hypoxia and triggers compensating mechanisms. Both processes together produce the signs and symptoms characteristics of anemic syndrome. Patients with anemia may present with fatigue, dizziness and dyspnea; however, mild anemia shows few clinical signs or symptoms. The sings of anemia include pallor of the conjunctivae, face, nail beds and palmar creases, although the absence of pallor does not rule out anemia[4].

Anemia is one of the most frequent causes of medical visits because of the high incidence in children, young women and elderly people, especially if malnutrition is present. Moreover, anemia is one of the leading sings in many diseases or is the first evidence of disease observed in routine blood cell enumeration. Anemia is unusually prevalent in developing countries because of malnutrition, and genetic, parasitic or infectious diseases[5,6]. The prevalence of anemia varies greatly, from 2.9% to 61%, depending on population, age, sex, and normal limits of hemoglobin used by the author[7].

The most frequent cause of chronic anemia is gastrointestinal pathology, and this explains why patients with anemia are often sent for consultation to a gastroenterological specialist[8].

Anemia is a syndrome, not a disease, and therefore, the etiology must always be investigated and therapy must be directed mainly to the causal disease, and not only to restoring a normal hemoglobin concentration. In a patient with anemia, whose clinical and laboratory data are insufficient to find the cause, examination of the digestive system is a priority for two reasons. The first one is the high frequency of anemia in bowel disease, and the second is the opportunity to diagnose a malignant disease before it is too advanced[9-11].

Anemia can be classified from three points of view: pathogenesis, red cell morphology, and clinical presentation. All are important to guide the diagnosis. Pathogenic mechanisms involved in the production of anemia are very simple: inadequate production and loss of erythrocytes a a result of bleeding or hemolysis. Based on these pathogenic mechanisms, anemia can be divided into two types. (1) Hypo-regenerative: when bone marrow production is decrease as a result of impaired function, decreased number of precursor cells, reduced bone marrow infiltration, or lack of nutrients; (2) Regenerative: when bone marrow responds appropriately to a low erythrocyte mass by increasing production of erythrocytes.

In practice, classification based on basic parameters of red cell morphology such as mean corpuscular volume (MCV), allows for a quicker diagnostic approach.

Anemia also can be classified according to the form of clinical presentation as acute (usually bleeding or hemolysis) or chronic.

Anemia can be classified as microcytic, normocytic or macrocytic, depending on MCV. As stated above, it can be hypo-regenerative or regenerative, which depends on the number of reticulocytes. Using both, the list of possible diagnoses in the individual patient is reduced considerably. Both parameters can be supplied routinely by most of the automatic hematological cell counters.

The reticulocyte count is useful to distinguish anemia in which there is an appropriate bone marrow response from that in which there is a decrease in the production of erythrocytes. The concentration of reticulocytes reports on the bone marrow response to anemia. This approach is especially useful when MCV is normal.

A decrease in hemoglobin stimulates erythropoiesis through an increase in circulating erythropoietin. Therefore, when the bone marrow shows a normal regenerative capacity, there should be an inverse relationship between the decrease in hemoglobin and the increased number of reticulocytes (regenerative anemia). The expected reticulocyte count is much higher than normal to compensate for the anemia. However, when the hemoglobin decreases and the bone marrow does not have regenerative capacity, the expected increase in reticulocytes fails, despite the increase in erythropoietin plasma level (hypo-regenerative anemia)[12].

The reticulocyte count is expressed as a proportion of the number of erythrocytes, which must be corrected for anemia, and for increased lifespan of reticulocytes in peripheral blood. Most hematological analyzers provide directly the number of reticulocytes per mm³, which is a better estimation of erythropoietic activity than a percentage. However, the absolute reticulocyte enumeration per mm³ overestimates the actual activity of erythropoiesis, since reticulocytes are released earlier and remain longer in the circulating blood. To avoid these confounding events, it is recommended to calculate the reticulocyte index.

The next step is the calculation of reticulocyte production index (RPI):

RPI = correcteds reticulocyte count (%) × (haematocrit observed/normal hematocrit) : F (reticulocyte maturation times in vivo)

RPI in a healthy person is 1. An RPI > 3 in a patient with anemia suggests an appropriate bone marrow response, and therefore, regenerative anemia[13,14]. Nevertheless, the best estimate of the actual erythropoietic activity[15], and easiest to calculate, is to divide by two the number of reticulocytes per mm³.

Pathogenic classification is very important to understand the mechanisms involved in the genesis of anemia (Table 2). However, in daily clinical practice, it is more useful to start with the analytical parameters of the hemogram. MCV allows us to classify anemia as microcytic (MCV < 82 fL), normocytic (MCV = 82-98 fL) and macrocytic (MCV > 98 fL)[21,22].

MCV has a relationship with mean corpuscular hemoglobin (MCH), which reports on the mean hemoglobin per erythrocyte expressed in picograms (normal range: 27-32 pg). Therefore, MCV and MCH decrease (microcytic, hypochromic anemia) or increase (macrocytic, hyperchromic anemia) jointly. The MCH concentration (MCHC) reports on the average concentration of hemoglobin in each erythrocyte expressed as a percentage (normal range: 32%-36%), and its variations are very small, even in the presence of hypochromia. MCHC increases only in a few rare diseases such as hereditary spherocytosis, and therefore, its practical utility is scarce.

It should always be borne in mind that MCV is an average value and therefore does not provide information about the homogeneity of the erythrocyte population. To resolve this problem, hematological analyzers provide the erythrocyte distribution curve, with an index of dispersion: red blood cell distribution width (RDW) (normal range: 10%-14%). RDW is a rough indicator of anisocytosis and is an essential complement to MCV (Table 3).

The strategy for establishing a diagnosis of anemia by gastroenterologists must follow a classic pattern that includes a clinical interview, a basic physical examination, and careful selection of complementary tests.

The clinical history should emphasize the causes of blood loss, including non-gastrointestinal diseases, recent history of gastrointestinal symptoms, gastrointestinal surgery, use of nonsteroidal anti-inflammatory drugs, acetylsalicylic acid and anticoagulant therapy, family history of hematologic diseases and colorectal neoplasia[53-55]; all symptoms should be recorded, and not only the gastrointestinal ones.

A single blood sample can give information on the concentration of hemoglobin (essential for the diagnosis of anemia according to WHO criteria), the erythrocyte indices MCV and MCH (useful for the morphological classification of anemia), RDW (estimate of anisocytosis) and ESR, which reports on possible ACD.

The hematologist should examine a blood smear if any abnormal erythrocyte indices have been detected. A clotted blood sample (to obtain serum) allows the determination of the rest of the analytical parameters that are useful in diagnosis, which are iron metabolism (mainly ferritin), urea, creatinine, bilirubin and total protein. In cases of suspected IDA, the concentration of soluble transferrin receptor should be included as a key parameter in differentiation between IDA and ACD[56]. The most probable causes of microcytic anemia are IDA, thalassemia or thalassemia trait. RDW and iron metabolism are clues to differentiate between these processes in the initial step, as has been mentioned above. The most likely causes of normocytic anemia are ACD, renal failure and primitive hematological diseases (least frequently). Iron metabolism and routine biochemical tests may help to guide the diagnosis, but in some cases, erythropoietin serum levels and bone marrow examination are necessary.

In cases of macrocytic anemia, serum levels of vitamin B12 and serum and/or erythrocyte levels of folate will guide the diagnosis to deficiency of one of these. Most causes of folic acid deficiency are nutritional, although malabsorption should always be kept in mind. In contrast, vitamin B12 deficiency is almost always the result of malabsorption[49,57]. The few exceptions are in strict vegetarians and hereditary transcobalamin II deficiency (a very infrequent disease). Among malabsorption causes of B12 vitamin deficiency, the most frequent is pernicious anemia. Therefore, the first step is to establish whether the serum levels of gastrin are highly elevated, because they are > 1000 pg/mL only in atrophic gastritis type A (pernicious anemia) and Zollinger-Ellison syndrome. In pernicious anemia, pepsinogen I levels are low, gastric pH is high and gastric acid secretion does not respond to stimulation with histamine or pentagastrin. Serum antibodies against intrinsic factor and parietal cells are not sensitive or specific enough to confirm the diagnosis. The exploration that leads directly to diagnosis is gastroscopy, with mucosal biopsy and study of antiparietal cell antibodies. Gastric biopsy has the added advantage of giving information about possible metaplasia or precancerous lesions[58]. In the absence of gastric disease, the most likely cause of malabsorption of vitamin B12 is Crohn’s disease, with involvement of the terminal ileum[59]. Hereditary causes of malabsorption (Immerslund syndrome) or transport (transcobalamin II deficiency) are extremely infrequent.

The list of causes of anemia is long and cumbersome, but most can fit into a very small number of diseases on which we must concentrate as a first diagnostic option (Table 6)[60-66].

IDA is the most prevalent form of anemia worldwide, especially in women and children. Thirty-percent of the world’s population, some 1300 million people, suffer from anemia. However, the prevalence of anemia worldwide is unequal (36% in underdeveloped and 8% in developed countries) About half of the patients with anemia have IDA, which is most prevalent in the general population and in outpatients. The most likely cause of IDA is malnutrition in children, bleeding in adult males (especially gastrointestinal), menstruation or lactation in fertile women, and bleeding in the elderly. The distribution of nutrient-deficiency anemia in the elderly is: 48% iron alone, 19% folate alone, 17% vitamin B12 alone, and the rest have combined deficiencies. Therefore, in young male adults and in both sexes older than 65 years, the most likely cause of IDA is chronic bleeding, especially from gastrointestinal lesions[67].

The hemogram, the concentration of iron, ferritin and transferrin in blood and investigation of fecal occult blood are sufficient to obtain a diagnosis in most cases.

ACD is the most prevalent form of anemia after IDA in the general population[40,68], but is even more common in hospitalized patients, regardless of sex and age[42,67,69,70].

The underlying causes of ACD vary greatly: acute and chronic infections, 18%-95%; cancer, 30%-77%; autoimmune diseases (including chronic inflammatory bowel disease), 8%-71%; rejection of solid organ transplantation, 8%-70%; and chronic renal disease, 23%-50%. ACD is found in 20% of elderly people with anemia (30% excluding nutritional deficiency), and a further 4.3% suffer from ACD and kidney failure. Therefore, we should keep in mind ACD as the most likely diagnosis in elderly and inpatients, and as the second most likely diagnosis in outpatients under 65 years of age[68]. The laboratory tests needed to confirm the diagnosis include serum iron, transferrin, transferrin saturation, ferritin, urea and creatinine.

The gastroenterologist must rule out chronic inflammatory bowel disease, tumors of the digestive tract and liver disease. In some patients, the cause of anemia is complex (chronic disease and chronic hemorrhage). In these cases, it is difficult to confirm the diagnosis and additional testing is required: soluble transferrin receptor and microscopic examination of bone marrow with special stains for iron.

The most common type of anemia after IDA and ACD is megaloblastic anemia (vitamin B12 deficiency, folic acid or both). It is important in gastroenterology because intestinal malabsorption and pernicious anemia are common causes of this form of anemia. The diagnosis is suspected when MCV is elevated, and is confirmed by measuring serum vitamin B12 and serum and erythrocyte folate.

Hemolytic anemia follows next in order of frequency. Acute hemolysis presents a distinctive clinical picture, but chronic hemolysis may go unnoticed. If hemolytic anemia is suspected, the appropriate laboratory tests are antiglobulin test (direct and indirect), LDH, haptoglobin, total and conjugated bilirubin in serum, and salts and bile pigments in urine. These tests can give us the generic diagnosis of hemolytic anemia, immune or not immune, but the etiological diagnosis should be resolved in collaboration with the hematologist.

Anemia is common in hematological malignancies, but with limited gastrointestinal symptoms, except for gastric or intestinal bleeding caused by thrombocytopenia or lymphoma. It is rare, but it should be noted that gastric or intestinal lymphoma sometimes presents as mild anemia with nonspecific gastrointestinal symptoms. Endoscopy usually provides the diagnosis.

Hereditary anemia is extremely rare, except for thalassemia and sickle cell disease in certain geographical areas. Sickle cell disease is restricted to blacks. When one suspects hereditary anemia, the cooperation of a hematologist should be sought, because the diagnosis is difficult and the laboratory tests should be carefully selected.

More than 100 diseases may cause anemia, but 90% belong to three groups: nutritional deficiencies (iron, vitamin B12 and folic acid), ACD (chronic inflammation, tumors), and bleeding (excluding chronic bleeding, which produces iron deficiency). Hemolytic anemia, although less frequent than the other, is the last option before considering the diagnosis of rare diseases.

A few laboratory tests, such as blood count, ESR, serum ferritin, and serum iron and transferrin, are sufficient to focus the diagnosis. According to initial results, additional laboratory tests should be ordered: serum vitamin B12 and serum and erythrocyte folate (in case of macrocytosis), tumor markers and acute phase reactants (if ACD is suspected), LDH, haptoglobin and antiglobulin test (if hemolytic anemia is suspected).

With these key data, the doctor can put in place more specific tests: imaging, endoscopy and additional laboratory tests. The next step should be to correct the cause of anemia when possible or correct anemia itself when the cause cannot be treated. The advantages are clear if the cause is a cancer that can be diagnosed at an early stage. On the other hand, we can improve the quality of life of patients by correcting the anemia with medical treatment or regular transfusions.