Lecture 7: Laboratory Eval of Erythrocytes

Indices, Coombs, Methemoglobin, Iron

What species of dog has a significantly higher normal mean HCT?

greyhounds

Why do we evaluate Hct, Hb, AND RBC count?

they all provide different insights into RBC status:

• Hct: proportion of RBCs in total blood volume

• Hb: how well RBCs are functioning (transporting oxygen)

• RBC count: # of RBCs in a sample

What has to be determine by manual calculation for camelids and non-mammals?

MCV

What type of animal has a high MCV to RBC count ratio?

reptiles

What is MCH, how is it determined, and what does it depend on?

the average amount of hemoglobin in a single RBC, determined by calculation, and depends on erythrocyte size and internal hemoglobin concentration

When is MCH the lowest?

with iron deficiency when MCV and MCHC are low

Why do reptiles have a higher MCH than mammals and birds?

they have a higher MCV and MCH is dependent on erythrocyte size

What is MCHC?

mean cell hemoglobin concentration determine by calculation and reported as g/dL of packed erythrocytes

What is macrocytosis and what is it indicated by, diagnostically?

increased volume in RBCs, high MCV

What conditions are associated with macrocytosis (high MCV)?

• regenerative anemia, especially hemolytic anemia

• FeLV-+ cats

• hereditary stomatocytosis in dogs

• dogs w/ nonregenerative anemia assoiated with immune-mediated anemia and/ormyelofibrosis

• Abyssinian and Somali cats with increased erythrocyte osmotic fragility

• congenital dyserythropoiesis in Hereford calves

What can cause erroneous macrocytosis (high MCV)?

• autoagglutination of erythrocytes

• prolonged blood storage before assay (RBCs swell with storage)

• persistent hypernatremia (dogs and cats): osmolality in blood higher than the analyzer diluent, erythrocytes swell when diluted for assay in the analyzer

What are some causes of microcytosis (low MCV)?

• chronic iron deficiency anemia

• portosystemic shnts

• some oriental dog breeds without anemia

• excessive erythrocyte fragmentation

• inhibitors of heme synthesis (lead, viramin B6 deficiency)

• hereditary dyserythropoiesis in english springer spaniel and labs

What can cause spurious microcytosis in animals?

• platelets counted as erythrocytes in severely anemic patients

• persistent hyponatremia in dogs (osmolality of blood is lower than hematology analyzer diluent)

What can cause low MCHC values?

• regenerative anemias

• chronic iron deficiency anemia (also microcytic)

• abyssinian and somali cats with erythrocyte osmotic fragility (also macrocytic)

• spurious in hypernatremia cases (water moves into RBC from analyzer diluent → swelling and dilution)

What artifacts can cause high MCHC values by decreasing HCT?

• Heinz bodies within erythrocytes (scatter light, skewing results)

• Lipemia

• Erythrocyte agglutination in electronic cell counts

What artifacts can cause high MCHC values by increasing Hb?

• IV hemolysis

• in vitro hemolysis

• erythrocyte agglutination in electronic cell counters

What is the coefficient of variation of erythrocyte volumes and an electronic measure of anisocytosis?

RDW

When would you expect to see increased RDW?

• iron deficiency anemia with increased numbers of microcytes

• after blood transfusion with different sized cells

• hereditary stomatocytosis

How can RDW spuriously increase?

erythrocyte agglutination counted as large erythrocytes or platelets counted as erythrocytes of severely anemic patients

If no autoagglutination of erythrocytes was present, what tests would you do?

• direct antiglobulin test (Coombs test)

• flow cytometry

What type of blood cross-matching is the most important for erythrocyte transfusions?

major cross match

What does a major cross match test for?

• RBCs of donor against serum of recipient

• RBC antibodies in serum of recipient

What blood cross match is most important for neonatal isoerythrolysis test or plasma transfusions?

minor cross match

What does a minor cross match test for?

• serum of donor against RBCs of recipient

• RBC antibodies in serum of donor

What serious reactions could be caused by an incompatibility of a minor cross match?

serious hemolytic reactions after colostrum absorption or plasma transfusion

What percentage of total body iron is stored in RBCs by hemoglobin?

65%

What percentage of total body iron is stored in tissues?

30%

What percentage of total body iron is stored in plasma?

0.1%

When is iron absorption increased?

• total body iron content is low

• erythropoiesis is increased

When is iron absorption decreased?

• total body iron is high

• inflammation is present

What is the general iron cycle?

circulating erythrocytes are engulfed by macrophages → iron stored as ferritin/hemosiderin is taken into plasma → transferrin-Fe complex forms after transferrin produced in liver → delivered to marrow erythrocyte precursors where it binds to hemoglobin

What is the peptide secreted by hepatocytes?

hepcidin

What is the function of hepcidin?

systemic control of iron metabolism by inhibiting iron release from enterocytes, macrophages, and hepatocytes

How is iron metabolism controlled systemically?

when more RBCs needed, EPO stimulates erythroblast production in bone marrow → erythroblasts produce erythroferrone to tell liver to stop hepcidin production, stabilizng ferroportin and increasing iron release for RBC production

When is hepcidin concentration in plasma decreased?

when iron is needed:

• total body iron content is low

• erythropoiesis is increased

When is hepcidin concentration in plasma increased?

when iron is not needed:

• total body iron is high

• inflammation is present (limits iron for bacterial growth)

Where is most of the plasma iron found and what is it used for?

bound to transferrin protein and most is used for hemoglobin synthesis

What percent of plasma iron is from duodenal enterocyte absorption?

only 0.2-4.5%

When are serum iron concentrations increased?

• hemolytic anemia and dyserythropoiesis (erythroferrone production increased)

• hypoplastic or aplastic anemia (decreased iron use for erythrocyte production)

• iron overload

• glucocorticoids in dogs and horses

When are serum iron concentrations decreased?

• iron deficiency

• anemia of inflammatory disease (increase hepcidin producition)

• portosystemic shunts

• marked erythropoiesis with inadequate iron mobilization

• glucocorticoids in cattle and goats

• intense exercise in dogs and horses

What does serum ferritin concentration correlate with?

total body iron stores

When does serum ferritin concentration decrease?

iron deficiency

When does serum ferritin concentration increase?

• iron overload

• hemolytic anemia

• inflammation

• malignant histiocytosis

• transiently in exercising horses and dogs

How would you expect the skin to appear in a methemoglobinemia case with normal pO2 in arterial blood?

cyanotic-appearing skin

What are the causes of methemoglobinemia?

• administration or consumption of oxidant chemicals

• hereditary erythrocyte methemoglobin reductase deficiency

What oxidant chemicals could cause methemoglobinemia?

• acetaminophen

• fermented bok choy (dogs)

• copepr toxicity in sheep and goats

• red maple toxicity in horses

• nitrite toxicity in ruminants eating nitrates

• skunk musk (dogs)

What are the clinical signs of methemoglobinemia?

• cyanotic-appearing skin and mucous membranes

• decreased exercise tolerance

• coma-like state

What are the clinical signs of toxicity?

• vomiting, anorexia, diarrhea

• subcutaneous edema, especially in the face, in cats with acetaminophen toxicity

What is the concentration of serum EPO dependent on?

production by the kdiney and utilization by erythroid cells

When would you expect serum EPO concentration to increase?

most anemias and secondary erythrocytosis

When would you expect serum EPO concentrations to decrease?

chronic renal disease and primary erythrocytosis