13. Hemoglobinopathies Flashcards
Hemoglobinopathies
- A genetic defect that results in abnormal structure of one of the globin chains of the hemoglobin molecule - Very few actually produce a “disease” or disorder - Most are asymptomatic abnormal hematologic findings
Pathophysiology of Hemoglobinopathies
- increased/ decreased O2 affinity - Methemoglobinemia (can’t bind to oxygen) - Unstable hemoglobin (hemoglobin precipitates) - Sickling and crystallization
Increased O2 Affinity
- Hemoglobin eagerly accepts oxygen but does not want to release it. - The kidney responds to the decrease in oxygen by producing more erythropoietin - Erythropoiesis is stimulated resulting in increased RBCs (high hematocrit) - The genetic disorder is rare; condition is more commonly acquired by stress, cigarette smoking, high altitudes, chronic lung disease
Decreased O2 Affinity
- Hemoglobin is reluctant to pick up oxygen = deoxygenated hemoglobin - If the level of deoxygenated hemoglobin exceeds 5 g/dL clinical cyanosis results (Blue lips/nails) - The genetic abnormality is rare; cyanosis can also result from pulmonary dysfunction
Methemoglobin
- Low O2 affinity hemoglobin variants characterized by the presence of heme that contains iron in the ferric (Fe+++) rather than ferrous (Fe++). - Genetic abnormalities are termed Hgb M - Enzyme deficiencies - Oxidizing drugs and compounds - “Brown” hemoglobin
Unstable Hemoglobin
- Genetic abnormalities cause an intrinsically unstable hemoglobin - Denatured hemoglobin forms RBC inclusions (Heinz bodies) -Denaturation due to enzyme deficiency. (ex. G6PD deficiency makes unstable hemoglobin.) ***Heinz bodies are not visible with Wright stain; must be stained with a supravital stain
Sickling and Crystallization
Common hemoglobopathies in the United States: - Hemoglobin S - substitution of valine for glutamic acid - Hemoglobin C - substitution of lysine for glutamic acid
Laboratory Tools for looking at Hemoglobinopathy
- CBC - Peripheral blood smear - Hemoglobin separation: Electrophoresis, HPLC (High Performance Liquid Chromatography), Isoelectric focusing - Sickle cell screening
Hemoglobin Electrophoresis
- Used to isolate, identify, and quantitate hemoglobin bands - When placed in an electrical field, hemoglobins will migrate according to their: pH (charge) and the Media used Typically (alkaline): 0 = origin A= accelerated (fastest so moves furthest down) F= fast S= slow C= crawl (slowest so it barely makes it of the origin/starting line)
Sickle Cell Screening
- Solubility Test (6 min) - Hemoglobin Electrophoresis (> 30 min; long)
Sickle Solubility Test
- Sickling hemoglobin in a deoxygenated state will precipitate in a concentrated phosphate buffer solution. - will not become soluble so you can’t see the lines through the tube (Turbid = +) - NYS does not recognize this test - Not specific for Hgb S - Cannot resolve trait vs disease - False positives with increased proteins, hyperlipidemia, high WBC, cold reagents (due to condensation) - Sensitivity approx 20-25% (Infants - have a low Hem count and won’t trip the test, Transfusions)
Epidemiology and Genetics of Hemoglobin S and Sickle Cell Disease
- Incidence of S trait in African populations is 40% - Incidence of S trait in African-Americans is 8% - Established itself as a protective mechanism against malaria (because Hem S doesn’t carry oxygen efficiently)
Pathophysiology of Hemoglobin S and Sickle Cell Disease
- Under low oxygen tension, hemoglobin S will precipitate forming tactoids - RBCs stretch around the tactoids, forming the characteristic sickled cells - The greater the proportion of Hgb S, the greater the propensity to sickle - Heterozygotes do not sickle except under extraordinary conditions
Clinical Findings of Sickle Cell Disease
- Chronic hemolysis: anemia, jaundice, cholelithiasis (gallstones), aplastic crisis (stop making RBC), hemolytic crisis - Vaso-occlusion: dactylitis - swollen and painful feet, auto-splenectomy, renal necrosis, infarctive crisis, leg ulcers and infection (due to poor circulation)
Laboratory Findings of Sickle Cell Disease
- Severe normocytic, normochromic anemia - Blood smear: target cells, sickle cells (sometimes), nRBC, schistocytes, siderotic granules (iron overload from transfusions), Howell-Jolly bodies - Reticulocytes increased (body trying to compensate because RBCs don’t last 120 days), except in aplastic crisis (retics are low) - Hemoglobin electrophoresis - Hgb S = 80-90%, Hgb F = 1-20%, Hgb A2 = 2-3% (Beta gene making Hgb S instead of Hgb A) Sickle test - positive
Laboratory Findings of Sickle Cell Trait
- normal CBC - normal blood smear - Hemoglobin electrophoresis - Hgb A = 55-65%, Hgb S = 35-45%, Hgb A2 = 2-3% - Sickle test - positive
Hemoglobin C: Epidemiology and Genetics
- Incidence in West Africa is 20-25% - Incidence in African-Americans is 2-3%
Hemoglobin C: Pathophysiology
- Hgb C forms intracellular blunt-ended crystalloids - Usually not seen with normal splenic function (Only seen if spleen is removed or non-functioning) - Decreased RBC survival
Hemoglobin C Disease
-hemolysis is not as severe as Hb SS and vaso-occlusive crisis generally does not occur; prognosis is excellent - Mild normo-microcytic, normochromic anemia (smaller end of normal) - numerous target cells - Hgb C crystals - post-splenectomy - Reticulocyte slightly increased - Hgb electrophoresis - Hgb C= 95%, Hgb F= <7%, Hgb A2 = 2-3%
Hemoglobin C Trait
-asymptomatic - target cells, mild microcytosis - Hgb Electrophoresis - Hgb A = 55-65%, Hgb C = 35-45%, Hgb A2 = 2-3%
Hemoglobin D
- Both homozygous and heterozygous states are asymptomatic - CBC/ Peripheral blood is normal - Hgb D migrates with Hgb S on alkaline electrophoresis and with Hgb A on acid electrophoresis
Hemoglobin E
- Occurs frequently in SE Asians - Currently the most frequent hemoglobinopathy in California - Hemoglobin E Disease - mild anemia, target cells, microcytic, hypochromic cells - Hemoglobin E Trait - clinically aymptomatic, microcytosis, target cells, - Hgb electrophoresis - Hgb A = 70%, Hgb E = 30%, Hgb A2 = 2-3%
Hemoglobin SC
-double heterozygote - generally less severe than S Disease - peripheral blood shows target cells, folded RBCs, and intracellular crystals - Hgb electrophoresis - Hgb S = 45%, Hgb C = 45%, Hgb F = 2%, Hgb A2 = 2-3%
Hemoglobin SD
- double heterozyhote - clinical severity falls between that of S Disease and S Trait - hemoglobins comigrate on alkaline electrophoresis - acid hemoglobin electrophoresis needed to differentiate (because looks like S on alkaline)
- SS Disease
- S Trait
- C Disease
- SC Disease
- AS (Trait)
- S disease
- C disease
- SC disease
- SS disease
- S trait
- C disease
- SC disease
- S trait
- D trait
* Note that an Acid electrophoresis must be run to destinguish the two.
- EE
- E trait
- E / beta-thal
