on 25-Mar-2021 (Thu)

Hematocrit (HCT), also called packed cell volume (PCV), is the percentage of blood volume occupied by RBCs. It can be measured directly following centrifugation of a blood sample (picture 1 and picture 2).

When measured by an electronic cell counter, HCT is calculated from the RBC count (in millions/microL) and the mean corpuscular volume (MCV; in femtoliters [fL]): HCT = ([RBC x MCV]/10).

One set of normal ranges for hemoglobin, HCT, and RBC count is shown in the table (table 1). Other normal ranges have been proposed. World Health Organization (WHO) criteria for anemia in men and women are hemoglobin <13 and <12 g/dL, respectively [4]. However, these criteria were intended for use within the context of international nutrition studies and were not initially designed to serve as "gold standards" for diagnosing anemia [5].

The increased frequency of anemia seen with aging has led to suggestions that a different standard for the normal hemoglobin should be used in older adults [5]. Review of data from the National Health and Nutrition Examination Survey (NHANES) indicates that the mean hemoglobin values for men and women over 70 is within the usual normal range (14.5 g/dL and 13.4 g/dL, respectively), while the 5^th percentile is below the normal range (11.7 g/dL for men and 10.9 g/dL for women) [11]. This lower boundary may reflect an increased prevalence of comorbidities, especially chronic kidney disease [12].

In individuals with anemia, hemoglobin and HCT decrease in parallel, although the HCT/hemoglobin ratio (approximately 3 in most cases) may vary according to the volume (size) of the cells. The RBC count also usually parallels the hemoglobin and HCT, except in cases of extreme microcytosis such as thalassemia, in which the RBC count may be increased despite the presence of anemia. The RBC count is less commonly used to diagnose anemia for this reason. A finding of high RBC count in an individual with anemia suggests thalassemia.

Causes of lower values

• Intense physical activity – Values in endurance athletes may vary significantly from those in other healthy individuals. Various causes may contribute, including dilutional anemia from increased plasma volume, iron deficiency, and/or "march" hemolysis. (See "Exercise-related gastrointestinal disorders", section on 'Gastrointestinal bleeding' and "Non-immune hemolytic anemia due to systemic disease".)

• Pregnancy – During a healthy pregnancy, maternal red cell mass increases, but plasma volume increases to a greater degree, causing a relative decrease in hemoglobin and HCT [14]. By the criterion of RBC mass, the individual is not anemic, but hemoglobin, HCT, and RBC count frequently decrease to anemic levels (figure 1). The terms "physiologic" or "dilutional" anemia have been applied to this setting, although these individuals are not actually anemic and do not require evaluation as long as their hemoglobin remains ≥11 g/dL in the first and second trimester and ≥10 g/dL in the third trimester. (See "Anemia in pregnancy".)

• Older age – Values for hemoglobin and HCT in apparently healthy older adults are generally lower than those in younger adults, and differences between males and females that are seen in younger adults are lessened with aging [15-17]. (See 'Older adults' below.)

Causes of higher values (may occasionally mask underlying anemia)

• Smoking – Smoking causes an increase in hemoglobin, HCT, and RBC count due to increased levels of carbon monoxide, which reduces oxygen delivery. Thus, individuals who smoke or have significant exposure to secondary smoke or other sources of carbon monoxide may have HCT higher than normal [18]. A study of blood donors who smoke found a significant correlation between the patients' blood carboxyhemoglobin and hemoglobin values [19]. (See "Carbon monoxide poisoning", section on 'Pathophysiology'.)

• Hemoconcentration – Individuals with dehydration or hypovolemia related to vomiting or diarrhea will have a relative increase in hemoglobin and HCT due to hemoconcentration. Anemia will become apparent after volume replacement. This is particularly a problem in patients with severe burns, in whom substantial RBC loss may be masked by exudative loss of plasma volume until fluid resuscitation has occurred [20].

• High altitude – Persons living at high altitude have higher values than those living at sea level due to relative hypoxia [21]. (See "High altitude, air travel, and heart disease", section on 'Long-term altitude exposure'.)

Mean corpuscular volume (MCV) is the average volume (size) of the RBCs. It can be measured or calculated (MCV in femtoliters [fL] = 10 x HCT [in percent] ÷ RBC [in millions/microL]). RBCs with MCV in the normal range are roughly the same diameter as the nucleus of a normal lymphocyte on the peripheral blood smear.

Mean corpuscular hemoglobin (MCH) is the average hemoglobin content in a RBC. It is calculated (MCH in picograms [pg]/cell = hemoglobin [in g/dL] x 10 ÷ RBC [in millions/microL]). A low MCH is typically reflected in an enlarged area of central pallor in RBCs on the peripheral blood smear (greater than one-third of the RBC diameter), which defines "hypochromia" on the blood smear. This may be seen in iron deficiency and thalassemia.

Mean corpuscular hemoglobin concentration (MCHC) is the average hemoglobin concentration per RBC. It is calculated as (MCHC in grams [g]/dL = hemoglobin [in g/dL] x 100 ÷ HCT [in percent]). Very low MCHC values are typical of iron deficiency anemia, and very high MCHC values typically reflect spherocytosis or RBC agglutination. Examination of the peripheral blood smear is helpful in distinguishing these findings.

Red cell distribution width (RDW) is a measure of the variation in RBC size, which is reflected in the degree of anisocytosis on the peripheral blood smear. RDW is calculated as the coefficient of variation (CV) of the red cell volume distribution (RDW = [standard deviation/MCV] x 100).

A high RDW implies a large variation in RBC sizes, and a low RDW implies a more homogeneous population of RBCs. A high RDW can be seen in a number of anemias, including iron deficiency, vitamin B12 or folate deficiency, myelodysplastic syndrome (MDS), and hemoglobinopathies, as well as in patients with anemia who have received transfusions. Review of the peripheral blood smear often is helpful in identifying the cause.

Reticulocytes can be reported as a percentage of total RBCs or as an absolute count (table 2). Reticulocytes can be appreciated on a standard blood smear stained with Wright-Giemsa as RBCs with a blue tint (polychromatophilia) that are larger than mature RBCs, with irregular borders and a lack of central pallor (picture 3).

The appropriate count depends on the hemoglobin level. The "normal" reticulocyte count refers to the count in a non-anemic individual at steady state.

• Steady state – At steady state, approximately 1 to 2 percent of circulating RBCs are reticulocytes, corresponding to an absolute reticulocyte count of approximately 25,000 to 100,000/microL (0.25 to 1 x 10¹¹/L).

• Anemia – In anemia, the reticulocyte count rises. A normal bone marrow can increase the rate of RBC production approximately fivefold in adults (seven- to eightfold in children). Thus, under optimal conditions, reticulocyte percentages of at least 4 to 5 percent (often considerably higher) and absolute reticulocyte counts as high as 350,000/microL (3.5 x 10¹¹/L) are possible.

An increased reticulocyte count represents a normal bone marrow response to anemia. Incorporation of the reticulocyte count into the anemia evaluation can be especially helpful in diagnosing certain disorders including hemolytic anemias and multifactorial anemias

The reticulocyte stage of RBC development lasts for approximately four days. In the steady state, reticulocytes generally spend three days in the bone marrow and one day in the circulation. In severe anemia, reticulocytes can leave the bone marrow earlier and can circulate for two to three days, as illustrated in the figure (figure 2). This is the basis for the calculation of the reticulocyte production index [23]. (See "Regulation of erythropoiesis".)

If a laboratory does not report an absolute reticulocyte count or one of the corrected reticulocyte count parameters, calculators and other tools are available online (calculator 1).

While blood oxygen content is a function of hemoglobin, oxygen delivery to the tissues can also be affected by changes in the affinity of hemoglobin for oxygen, expressed as the partial pressure of oxygen at which hemoglobin is 50 percent saturated (P₅₀), as well as blood volume and tissue perfusion. (See "Oxygen delivery and consumption" and "Genetic disorders of hemoglobin oxygen affinity", section on 'Mutations that decrease the affinity of hemoglobin for 2,3-BPG'.)

Oxygen delivery is increased by:

• Decreases in pH

• Increases in RBC 2,3 bisphosphoglycerate (2,3 BPG) concentration

• Increased body temperature

Studies in animal models demonstrate that at any given HCT, systemic oxygen transport is lower with lower blood volume [25]. This is primarily a consequence of decreased tissue perfusion.

Other conditions besides hypovolemia that can impair tissue perfusion include:

• Hypotension

• Peripheral vasoconstriction

• Decreased cardiac output

• Bradycardia

• Coronary artery obstruction

Any or all of these can worsen symptoms at a given degree of anemia.

Symptoms of anemia also reflect the rate with which RBC mass declines, which determines the extent of compensatory changes. Following acute blood loss, an individual will initially have normal values for hemoglobin and HCT, but these values will decline over the ensuing 36 to 48 hours as most of the total blood volume deficit will be replaced by the movement of fluid from the extravascular into the intravascular space or with fluid resuscitation. Only then will the hemoglobin and HCT reflect blood loss. Thus, until the total blood volume deficit is fully repleted, the hemoglobin and HCT will underestimate the degree of blood loss [26]. The rapid onset of symptoms in these cases primarily reflects initial hypovolemia and hypotension and their effects on tissue oxygenation

Conversely, when anemia develops gradually over time (as with iron deficiency, vitamin B12 deficiency, or a myelodysplastic syndrome [MDS]), compensatory increases in blood volume and tissue adaptation to hypoxia may prevent symptoms from developing until the hemoglobin is very low.

Some of the more common ways to categorize anemia are based on:

● Obvious clinical features such as acute blood loss or known cause for malabsorption of nutrients needed for RBC production. (See 'Evaluation based on clinical presentation' below.)

● "Flags" on the initial CBC and chemistry panel, including other cytopenias, an abnormal differential, abnormalities of RBC shape, or evidence of kidney or liver dysfunction. (See 'Evaluation based on CBC/retic count' below.)

● Whether the RBCs are small (microcytic) or large (macrocytic). (See 'Evaluation based on MCV' below.)

● Whether the bone marrow is functioning appropriately (based on whether the reticulocyte count appropriately increased). (See 'Reticulocyte count' below.)

An experienced clinician will consider all of these frameworks simultaneously.

With some caveats, the following testing is appropriate in the initial evaluation of unexplained anemia:

● Reticulocyte count – Reflects the ability of the bone marrow to produce RBCs and can be used to categorize possible causes of anemia. (See 'Reticulocyte count' below.)

● Chemistry panel – Should include assessments of kidney and liver function, with creatinine, alanine aminotransferase (ALT), and aspartate aminotransferase (AST).

● Hemolysis labs – Lactate dehydrogenase (LDH), bilirubin, and haptoglobin (table 3), if the clinical history suggests hemolytic anemia and/or the reticulocyte count is increased. (See 'Hemolysis' below.)

● Blood smear – A review of the peripheral blood smear is always desirable in the initial evaluation of anemia. However, it is not always possible to obtain this immediately, and some workflows will direct the blood smear to an off-site reviewer who is unfamiliar with the patient. These practices may make blood smear review during the initial evaluation an unrealistic expectation.

In some settings, review of a blood smear by an experienced professional is critical to the evaluation and treatment; these settings are indicated in the following sections. Interpretation of specific findings is discussed separately.

The blood smear findings can in turn direct the subsequent laboratory tests needed to confirm or exclude a specific diagnosis.

As examples:

• Bite or blister cells (picture 5) – G6PD deficiency, evaluated by measuring the G6PD level.

• Microcytosis, target cells, teardrop cells (picture 6) – Thalassemia, evaluated by hemoglobin analysis or molecular (DNA) testing.

• Spherocytes (picture 7), elliptocytes (picture 8), or stomatocytes (picture 9) – Hereditary spherocytosis (HS), elliptocytosis (HE), or stomatocytosis (HSt).

Premenopausal women — Iron deficiency is common in premenopausal women, due to menses and/or pregnancies. RBCs are microcytic in some individuals but may be normocytic in others. Other conditions contributing to anemia can also be present.

● If the clinical history and examination are otherwise negative, we obtain iron studies (serum iron, serum transferrin or total iron binding capacity (TIBC), serum ferritin, and calculated transferrin saturation [TSAT]). (See "Iron requirements and iron deficiency in adolescents", section on 'Evaluation and presumptive diagnosis' and "Causes and diagnosis of iron deficiency and iron deficiency anemia in adults", section on 'Diagnostic evaluation'.)

● Findings that suggest another cause of anemia should also be pursued. This may include anemia during childhood, symptoms related to hemolysis, and other findings on the complete blood count (CBC) and blood smear.

The prevalence of anemia increases substantially in patients over the age of 60 [27,28]. As noted above, we evaluate the underlying cause rather than attributing anemia to aging.

Major causes of anemia in older adults include [29]:

• Nutritional deficiencies in approximately one third.

• Kidney disease or anemia of chronic disease/inflammation (ACD/AI) in approximately one third.

• Unexplained in the remaining third.

All individuals over the age of 60 should have testing for the following:

• Kidney function – Estimation of glomerular filtration rate (GFR). eGFR <45 mL/min/1.73 m² in the absence of another diagnosis implicates chronic kidney disease (CKD) as a cause.

• Iron stores – Iron studies (serum iron, serum transferrin or TIBC, serum ferritin, and TSAT).

• Vitamin B12 – Vitamin B12 level, with methylmalonic acid level if vitamin B12 deficiency is suspected and vitamin B12 level is equivocal.

• Folate – Folate level if at risk for malnutrition.

Further testing may be appropriate in certain settings:

• Monoclonal gammopathy – Testing is indicated if the mean corpuscular volume (MCV) is increased and/or if there is reduced eGFR or hypercalcemia. Serum free light chains and serum protein electrophoresis (SPEP) with immunofixation are obtained.

• Androgen deficiency – Testing with serum testosterone level is indicated in older men with an isolated normocytic anemia in whom the testing above did not provide a diagnosis [30].

• MDS and clonal cytopenias – Testing for a clonal bone marrow disorder is indicated if the MCV is slightly elevated and/or if there are other cytopenias or morphologic abnormalities on the blood smear. Molecular testing can be performed on peripheral blood using a gene panel for myeloid disorders (myelodysplastic syndrome [MDS] panel) or clonal hematopoiesis (CH) panel. Bone marrow can be examined for signs of dysplasia for possible diagnosis of MDS.

Unexplained anemia of aging is a poorly defined syndrome often seen in older individuals. The mechanism is unclear and appears to be cytokine-mediated [31]. This is a diagnosis of exclusion in older individuals with normocytic anemia and an unrevealing workup. The diagnosis should be reassessed periodically to avoid missing a correctable disorder.