Interpreting the erythron (Proceedings)

Although one of the most frequently utilized diagnostic tools, the full power of the complete blood count and ancillary testing is often untapped. As a routine part the initial steps of patient work up to serial monitoring of a wide variety of diseases to determining efficacy of treatment, evaluation of the erythron can be extremely useful. The erythron is the sum total of the balance between stem cells, progenitor cells, precursor cells, mature erythrocytes and senescent erythrocytes

To fully understand the erythron and the tests that are available, basic physiology and kinetics should be reviewed. The main factor that controls total body iron is need, with control regulated by absorption. Diets contain ferric (Fe3+, better absorbed) and ferrous (Fe2+) iron. Iron is transported inside of transferrin. In health, only about 33% of transferrin's binding sites are occupied. Ferrotransferrin is made up of Fe3+ (one or two molecules) and apotransferrin. Less than 0.05% of total body iron is lost every day. Apoferritin in mucosal cells binds iron and forms mucosal ferritin, which can then be lost in the feces when that cell is sloughed. About 50-70% of total body iron can be found within hemoglobin (Hb). Each molecule contains four iron atoms and is 0.34% iron by weight. About 25 - 40% is in the storage pool (ferritin and hemosiderin). Ferritin is iron plus the apoferritin protein. This form is considered to be more available to the body. Ferritin is considered a positive acute phase protein; thus, the inflammatory status of an animal should be considered when evaluating serum ferritin. Hemosiderin is structurally similar to ferritin, but has a higher iron:protein ratio. This form of iron is is insoluble in water and can be seen on cytology and histology as an amorphous blue-grey or a golden pigment, respectively. If uncertainty exists when identifying this protein, Prussian Blue or Perl's staining can be used to readily identify it. Hemosiderin is primarily found in macrophages of the spleen, liver and bone marrow. In addition, approximately 3-7% is of body total is within myoglobin, with higher amounts seen in horses and dogs.

There are several avenues that can be taken to assess body iron. The first and most accessible route is to evaluate the peripheral blood. Blood is the largest pool of iron; however, iron is preferentially shunted to this pool for hemoglobin synthesis; therefore, this is the last place that may show abnormalities. You may be able to visually identify hypochromasia and/or microcytosis in cases of iron deficiency; however, this is a relatively insensitive method and somewhat unreliable in large animals. Other morphologic changes that may be seen include: leptocytes, codocytes, ovalocytes, polychromasia. Measuring or calculating data such as RBC, Hct, PCV and Hb allows for the determination of several indices, which can be used to evaluate iron status. The mean corpuscular volume (MCV)fl= (PCV x 10) / RBC (millions) indicates the average volume of the erythrocytes. With severe iron deficiency, hemoglobin synthesis is slowed in immature erythrocytes, while nucleated precursors continue to divide until a critical concentration of hemoglobin in present to stop DNA synthesis and cell division. The result is mature erythrocytes that are small and may have a central pallor. It is wise to consider that the MCV (as the name implies) is the mean; therefore, a significant population of Hypochromic cells can be present before the mean begins to shift. The mean corpuscular hemoglobin concentration (MCHC) g/dl = (Hb g/dl x 100) / Hct % indicates the amount of hemoglobin within each erythrocyte. It is the most accurate of the indices because it is not reliant on the RBC. This is not necessarily true; however, if the value for Hct is calculated. The MCHC may be decreased in cases of iron deficiency.

Serum Iron (SI) is generally a poor estimate of total body stores. It assesses the transport component, essentially all transferrin; however, serum ferritin can contribute slightly if there is a marked hyperferritinemia. Hypoferremia (Decreased Fe in blood) is most likely due to iron deficiency, usually due to chronic blood loss in adults; however dietary factors may play a part in young animals on iron poor diet. Serum iron will also decrease with a shift of Fe to storage sites or sequestration of iron in macrophages; therefore, blood is iron deficient but the body isn't. The classic example of this is an anemia of chronic inflammation. A decreased SI is also rarely seen with severe intestinal disease preventing Fe absorption. Hyperferremia rarely occurs and may signal excessive intake, iatrogenic administration of iron or repeated, overzealous transfusions.

The total iron binding capacity (TIBC) is essentially a measure of serum capacity to carry Fe. Since transferrin can bind more iron than is usually present, TIBC is greater than SI and the difference between the two is the unbound iron-binding capacity (UIBC). TIBC will often be increased in cases of iron deficiency and is often low with inflammation, as transferrin is a negative acute-phase protein. The TIBC may also be low due to lack of production by the liver or loss through glomerulus or GI tract (77Kd).

Serum ferritin is a fairly good estimate of total body iron. There are species-specific antibody-driven assays, which are not widely available, but have been measured and correlate significantly with non-heme iron in the spleens of horses, pigs and cattle. Apoferritin is a positive acute-phase protein, therefore inflammation can introduce considerable error in interpreting results. Likewise, liver disease causing ineffective production can produce errors in data interpretation. Increased ferritin can be seen with hepatocellular damage or necrosis as well as with hemolysis. This change represents a shift of ferritin from tissue to plasma. Iron overload can cause increased serum ferritin. Serum ferritin is generally low with iron deficiency, unless concurrent inflammation or other source of "false elevation".

Bone marrow iron stores can also be used to evaluate iron stores in dogs.. Formalin fixed tissues (histology) or air-dried cytologic samples are adequate for evaluation. The results are subjective and require an experienced evaluator to determine. Hemosiderin within macrophages is more easily identified using the special stains Prussian Blue or Perl's stain. Iron deficiencies should not have stainable iron stores present. Cats will not have stainable iron in their bone marrow. Healthy younger animals often have less abundant bone marrow stores of iron. Anemia of chronic inflammation or iron overload should have increased stores of the less-readily available hemosiderin.

Comparative iron profile results

Erythropoiesis is influence by numerous stimulatory and inhibitory factors results in mature erythrocytes from pluripotent stem cells. Most growth factors synergize with erythropoietin (EPO), but are not capable of significant in vitro erythropoiesis alone. Erythropoietin is primarily produced by the kidneys, likely from peritubular interstitial cells. However, as much as 10-15% of plasma EPO comes from the liver, likely from centriacinar hepatocytes and Ito cells. EPO has a half-life (T½) of 6-10 hrs and a molecular weight of 30 kDa. Transcription of EPO is mediated by Hypoxia-Inducible Factor-1 (HIF-1), in response to sustained low arterial pO2. EPO is regarded primarily as a survival factor (i.e. protects against apoptosis via Bcl-XL), but also stimulates Hb synthesis in already dividing erythroid cells. From stimulation of progenitor to release of reticulocyte is approximately five (5) days.

The first way to evaluate the serum and red blood cell mass is the packed cell volume (PCV). The PCV generally refers to microhematocrit tube centrifugation. Microfilaria can easily be found by microscopic examination of the plasma just above the buffy coat. Hct determined electronically and is 1-3% lower than PCV due to trapped plasma in microhematocrit tube. Some automated cell counters are not calibrated for erythrocytes that are different sizes than human, therefore investigation of lab equipment is warranted prior to data interpretation. Using a refractometer, the total plasma protein (TPP, TP) can be determined. The hemoglobin concentration (Hb) is often measured by the machine. Heinz bodies, hemolysis, lipemia or exogenous hemoglobin (e.g. Oxyglobin) cause false elevations. Hb should be roughly 1/3 of the Hct if erythrocytes are normal size. An incongruence can help identify subtle hemolysis. New technology can provide Hb concentration histograms that are even more sensitive than MCHC to detect shifts in populations. The Hb will go down with anemia and increase with erythrocytosis. The red blood cell count (RBC) is also measured by the machine. Similar to Hb, the RBC will go down with anemia and increase with erythrocytosis. Having the RBC allows calculation of RBC indices. MCV can go up with increased numbers of immature cells in circulation. Agglutination can falsely increase the MCV when group of cells interpreted as one big cell. Increases in MCHC are not physiologically possible. If seen, they are artifacts due to heinz bodies, lipemia, exogenous hemoglobin administration, agglutination (if using automated cell counter) or hemolysis (intravascular or in vitro). Decreases are seen with regenerative anemias, as reticulocytes are not fully hemoglobinized. Decreases in MCHC are also seen in iron deficiency, usually after MCV decreases.

In addition to utilizing the number produced by the hematology analyzer, evaluation of a blood smear by human eyes is absolutely necessary. RBC morphology is many times non-specific; however, certain shape changes such as spherocytes, acanthocytes and schistocytes can signal significant systemic disease. RBC inclusions can also alert the clinician to significant diseases. Finally, the identification of erythrocytic parasites is something that a hematology analyzer can not do. Antibody titers, PCR and other molecular techniques are available and may be more sensitive; however, if identified in-house rapidly, valuable time can be saved and treatment initiated immediately.

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