Anemia: Hemolysis Flashcards
Define hemolysis
Definition: decrease in red cell survival or increase in turnover beyond standard norms.
Intravascular hemolysis
Release Hgb into the circulation.
Tetramer is unstable and dissociates into αβ dimers which bind to haptoglobin.
Complex removed by liver.
Haptoglobin’s capacity may be overwhelmed:
- Fe in hemoglobin can be oxidized to form methemoglobin.
- Dissociation of globin releases metheme which may bind to albumin or hemopexin.
These two may be taken up by hepatic parenchymal cells and converted to bilirubin.
Alternately, the dimeric forms of methemoglobin or hemoglobin may be filtered and not reabsorbed by the kidney and appear in the urine.
Extravascular hemolysis
RBCs ingested by macrophages of the RE system.
Heme is separated from globin, Fe removed and stored in ferritin, and the porphyrin ring converted to bilirubin which is released from the cell.
Taken up by liver, bilirubin is made water soluble by addition glucuronic acid via Cyto P-450 in liver parenchymal cells.
Secreted into the biliary tract & small bowel, glucuronic acid is removed, bilirubin converted into urobilinogen and other water soluble pigments.
Urobilinogen may cycle between the gut and liver (entero-hepatic circulation) or excreted by the kidney into the urine.
Describe the biochemical pathways of breakdown of hemoglobin and the relevant clinical lab tests for hemolysis.
Inc retic count & index (most cases) Inc bilirubin (if too fast to be processed) --> inc in the unconjugated fraction (most of total)
INTRA: dec in serum haptoglobin
detection of Hgb in urine or plasma
Inc in metheme / methemalbumin
release of SGOT, LDH from damaged RBCs –> elevated serum levels may also provide evidence for increased red cell destruction (not specific)
Describe the major constituents of the RBC membrane and cytoskeleton. Identify the major defects in hereditary spherocytosis. Relate these to the clinical and laboratory findings of the disorder.
SPECTRIN, ANKYRIN or BAND 3 defects weaken the cytoskeleton and destabilize the lipid bilayer.
Loss of membrane and formation of the spherocyte leads to decreased deformability and entrapment in the spleen.
Conditioning in the red pulp leads to further loss of red cell membrane and, ultimately, removal by the macrophage.
Clinical: variable anemia, jaundice, splenomegaly, 1/3 has hyperbilirubinemia as neonates. Cholecystitis, cholelithiasis
Lab: Hct and Hgb (small % may have no or mild anemia), increased reticulocyte count and index, decreased MCV, spherocytes on smear, unconjugated hyperbilirubinemia and an abnormal osmotic fragility test.
Describe how G6PD def leads to hemolytic anemia
G6PD: enzyme in a pathway which provides protection against oxidant stress.
Early loss of enzyme activity in the RBC results in inability to make NADPH and restore reduced glutathione.
With oxidant stress, denatured hemoglobin attaches to the membrane and spectrin may be damaged.
The resultant decrease in deformability and presence of abnormal membrane results in splenic trapping and extravascular hemolysis.
G6PD MAY PROVIDE SELECTIVE RESISTANCE TO MALARIA!!
Describe how PK def leads to hemolytic anemia
Decrease in converting phosphoenolpyruvate to pyruvate results in decreased ATP
Increased 2,3-DPG
Loss of membrane plasticity and increase in rigidity
Destruction in the spleen
G6PD: inheritance patterns, clinical & laboratory findings
G6PD: X-linked recessive
Clinical: intermittent acute hemolytic anemia & hyperbilirubinemia associated with oxidant stress (infection, drugs or ingestion of specific foods including fava beans).
Rarely chronic hemolytic anemia punctuated by episodes of acute exacerbation of anemia.
May also be a cause for neonatal hyperbilirubinemia.
Smear MAY show microspherocytes and “blister” or “bite” cells.
PK: inheritance patterns, clinical & laboratory findings
PK: Auto recessive (usually, rarely dom)
Clinical: chronic anemia, hemolysis, splenomegaly, gallstones and aplastic crises.
Labs: increased reticulocytes, and no specific morphology.
Describe the pathophysiology and site of RBC destruction of immune-mediated hemlysis.
COLD: IgM binds and induces complement MAC C5-9 attack –> intravascular hemolysis
WARM: IgG binds and RBC is eaten by macrophage via Ab-induced phagocytosis via Fc
Describe the direct antiglobulin test (DAT or direct Coombs) and the indirect antiglobulin test (IAT or indirect Coombs).
DAT: Direct looks for IgG or C3d/C4d on cells surface (coombs reagent has Ab for these and causes agglutination)
IAT: Indirect detects the ability of patient’s serum to bind IgG and/or complement to test (normal) red blood cells. By definition, autoimmune hemolytic anemia should have a positive DAT.
Clinical / lab findings of AIHA
Clinical: acute or chronic onset of anemia, pallor, jaundice, and dark urine.
Splenomegaly may occur.
Variable degrees of anemia w/ inc retic count, inc bilirubin, hemoglobinemia/-uria (this latter finding depends on the extent of intravascular hemolysis).
Smear shows spherocytes and may also have teardrop or “bite” cells.
Distinguish warm antibody-induced autoimmune hemolytic anemia (AIHA) from cold antibody-induced autoimmune hemolytic anemia (AIHA).
Patients with warm (IgG) AIHA exhibit positive DAT (strong IgG ± weak complement), maximal reactivity at 37°C, and many times, a panagglutinin without antigen specificity.
Cold antibodies have a positive DAT (complement only, no IgG), maximal reactivity at 4°C, antigen specificity for I or i.
Donath-Landsteiner (cold IgG) shows positive DAT (complement only), maximal reactivity at 4°C, antigen specificity for P, and in vitro hemolysis (a specific in vitro test).
Associate w/ intravascular or extravascular hemolysis; suggests the possibility of underlying diseases (lupus, malignancy, etc.) and the diagnostic workup; and provides management strategies and prognosis.
Indications, risks and benefits of splenectomy. Explain when prophylactic antibiotics are indicated post-splenectomy and the role of vaccination.
Risk: overwhelming bacterial sepsis, particularly associated with S. pneumoniae, greatest in children <5 years of age
In adults, mortality from sepsis in splenectomized individuals is 200x that in the general population.
Pre-surgery protocol: vaccination against H. influenza b, S. pneumoniae and meningococcus.
Post-op, prophylactic antibiotics (penicillin) should be given daily at least during childhood; and the instruction to see a physician immediately for a febrile illness ≥38.5°C is a lifelong imperative.
Spleen is critical for clearance of intravascular microbes! Very important in children for development of the adaptive humoral response and is the origin of IgM agglutinins, especially for encapsulated organisms.
