Pathophysiology of Anemia II Flashcards
Recall that two or more causes of anemia often co-exist. Iron deficiency in the context of a chronic GI bleed is the most common example. Ruling one etiology IN does not rule the others OUT.
Recall that two or more causes of anemia often co-exist. Iron deficiency in the context of a chronic GI bleed is the most common example. Ruling one etiology IN does not rule the others OUT.
What is Hemolytic anemia?
The term refers to anemias due to either intravascular rupture of red cells OR increased uptake of red cells by phagocytes (such as those in spleen in liver, often referred to as the “reticuloendothelial system”). The second type is sometimes called extravascular hemolysis.
What are some lab tools used to identify hemolytic anemia?
free hemoglobin
LDH
Unconjugated bilirubin
Haptoglobin
How is free hemoglobin measured?
you can just plain see it when you hold a tube of serum or urine up to the light.
“Visible hemolysis” is the usual term you’ll see attached to a lab report for this.
How else can free hemoglobin occur?
In blood specimens it can be caused by a “traumatic” blood draw. Free plasma hemoglobin usually gets filtered out by the kidneys, but in very severe hemolysis and/or renal disease it will be evident in urine as well.
Red cells, which can end up in urine via tissue damage anywhere between the kidneys and the urethra, will give a similar gross appearance; microscopy is needed to tell the difference.
Why is LDH a good indicator of hemolytic anemia?
There’s a ton of it in red cells, so if they are rupturing fast there will be a ton of it in serum.
What else can high levels of LDH be indicative of?
It can also be elevated due to lysis of other cells, such as lymphoblasts in the context of acute leukemia.
How is unconjugated bilirubin related to RBCs and when would it be elevated?
(end product of heme catabolism):
If free hemoglobin is being dumped into serum faster than the liver can metabolize it, this serum value will be up.
Can you visually see unconjugated bilirubin in blood draws?
Yes, this one is visible as well – the serum turns yellow.
Is elevated levels of unconjugated bilirubin specific to hemolytic anemia?
No, it can also be elevated due to some types of liver disease or Gilbert’s syndrome, a benign liver anomaly.
What is haptoglobin?
Is it elevated or decreased in hemolytic anemia?
The hemoglobin recycling transporter, and if lots of free hemoglobin is being cleared the net haptoglobin level can be reduced.
But this can be masked by the increase in haptoglobin seen in association with infectious/inflammatory states (it’s an “acute phase reactant.”)
T or F. There is often evidence of hemolysis on the peripheral blood smear and via the reticulocyte count.
T, if you recall that the workup of any hematologic disease begins with a review of the peripheral blood smear, you will not miss this.
What is plasma? What is serum?
Plasma is the liquid portion of blood.
Serum is the liquid that is left over if you let a tube of blood clot.
Most lab assays of enzyme activities can be done on serum, but those involving the assessment of blood coagulation obviously cannot.
How can defects in red cell membrane components lead to hemolytic anemia?
The red cell membrane can be thought of as that of a pressurized balloon with a number of anchor points on its surface. “Cables” (spectrin) run between the anchor points on the interior of the membrane to stabilize the structure. Genetic defects in the cables, anchor points (Band 3), and associated proteins (ankyrin) result in misshapen, and often unstable, red cells.
What is an osmotic fragility test?
an assay of what fraction of the red cells survive after they are placed in a hypotonic environment.
How can variations in the AA/protein structure of hemoglobin result in hemolytic anemia?
Minor changes in the globin amino acid sequence can cause hemoglobin to crystallize, resulting in precipitation of the hemoglobin and resultant instability of the red cell.
Because these mutations can also confer some resistance to malaria in heterozygotes, they have persisted in the gene pool – more so among populations in malaria-endemic areas.
T or F. Without active membrane transport of sodium out of the cell, the cell would get in trouble fast. Its metabolic needs are otherwise minimal, so after it has matured in the bone marrow it gets rid of its mitochondria and sustains itself by glycolysis only.
T.
Thus, defects in glycolytic enzymes are therefore particularly lethal to red cells. What is the most common?
pyruvate kinase deficiency.
What does pyruvate kinase deficiency cause?
As you could predict, it leads to inadequate transport of sodium out of the red cell, an influx of water down the osmotic gradient, swelling of the red cell, and ultimately rupture.
What would pyruvate kinase deficiency result in, in bone marrow?
Bone marrow’s increases efforts to keep up with rapid red cell loss: lots of red cells that have just left the marrow, showing bluish discoloration (“polychromasia”) which the hematology analyzer will count as reticulocytes.
Even less mature red cells (nucleated ones) may also be seen
How do red cells protect against oxidized species?
The red cell uses a sulfhydryl-containing molecule, glutathione, which becomes oxidized as it functions to reduce dysfunctional, oxidized proteins and, more importantly, the O2-derived small molecule (peroxide) that generates them.
How is glutathione recycled in red cells?
To recycle it, the red cell generates NADPH via a biochemical alternative to glycolysis called the pentose phosphate pathway. Glucose 6 phosphate dehydrogenase (G6PD) is one of the first enzymes in this pathway, and genetic defects in G6PD are particularly bad for red cells.
G6PD deficiency is one of the more common genetic defects in the human population Why is it so common?
It has been selected for during recent human evolution because it imparts slight resistance to malaria (probably because the cells rupture before the parasite can make full use of their contents).
What are Heinz bodies?
G6PD deficiency is usually only detected when hemolysis is observed following certain dietary intakes (fava beans are a well known culprit) or after exposure to any of several different drugs.
The resultant oxidized hemoglobin forms precipitates in the red cells, which can be visualized with a stain called methylene blue- the cells are called Heinz bodies.
What else can be oxidized in a red cell?
Their cargo (O2) is carried next to a Ferrous (+2) iron atom; if the latter gets oxidized to the ferric form (+3) it no longer works.
Extensive oxidation of heme iron is called ______. When can this occur?
Methemoglobinemia; it can occur (in conjunction with hemolysis) in G6PD deficiency, drug toxicity, and in kids who lack the enzyme that normally reduces it back to the ferrous state (cytochrome B5 reductase).
Write out the anti-oxidant pathway of red cells.
1) G6PD oxidizes G6P into 6-phosphogluconate as NADP is reduced
2) GSSG is reduced to GSH via glutathionine reductase as NAPDPH is re-oxidzed
3) H2O2 is reduced to 2H20 via glutathione peroxidase as GSH is re-oxidized to GSSG
Which drugs should be avoided in G6PD deficient patients?
- dapsone and primaquine (malaria)
- methylene blue
- nitrofurantoin
- rasburicase
So G6PD testing probably should be (and often is) required before starting any patients in those regions on those drugs.
What is the ‘alternative pathway’ in relation to complement proteins and RBCs?
The plasma proteins that initiate complement fixation do in fact bind to red cells at a low rate all the time (via the “alternative pathway”).
If the process went to completion (and formed the dreaded membrane attack complex, or MAC) every time this happened, you would lose a lot of red cells. How is this avoided?
Red cells deploy at least one surface protein that blocks the steps in complement fixation that occur immediately after the first few components have latched on.
The key protein is decay accelerating factor (DAF) because it accelerates the decay of surface-bound complement proteins before they can put together the MAC
