NBME 24 ABO incompatibility (NBME Answers)

Retired NBME 24 Answers

nbme24/Block 4/Question#4 (reveal difficulty score)
A 63-year-old woman develops flank pain, ...
ABO incompatibility 🔍 / 📺 / 🌳 / 📖
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submitted by ∗neonem(630)

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This is acute hemolytic transfusion reaction, a type II hypersensitivity where pre-formed IgM antibodies bind to incompatible ABO antigens on donor RBCs, which causes intravascular hemolysis. Rh incompatibility, like colonelred_ said, comes more into play with Rh-compatibility of pregnancy and it is due to IgG antibodies, which more often cause extravascular hemolysis since splenic macrophages have those Fc-gamma-R receptors to bind whatever IgG has caught. Extravascular doesn't cause that hypotension, fever, flank pain associated with hemoglobinuria since the macrophages hold on to the degraded RBCs and convert it to biliverdin, which can safely be excreted by the liver.

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mousie Could you help me with understanding why this isn't a Type I HSR? I understand that ABO incompatibility is Type II HSR but I don't know how to tell the difference between a patient who is IgA deficient and having a Type I Reaction to an infusion vs ABO incompatibility .... +16

sympathetikey @mousie - https://imgur.com/QH5rCEX Basically, think of Type 1 HS like a normal allergic reaction (itchy, wheezing, etc.). Whereas, with ABO incompatibility you get the question's presentation. +10

medpsychosis When it comes to Acute hemolytic transfusion reactions, they are Type II hypersensitivity and divided into Intravascular (ABO) and Extravascular (host Ab against foreign antigen on donor RBC). The differentiating factor between them is simple. Intravascular (ABO) will present with hemoglobinuria alongside all the other common symptoms (fever,hypotension, tachypnea etc.) Extravascular hemolysis will stand out with Jaundice as one of the presenting symptoms. Hope this helps! +11

cassdawg Also just to add: Rh incompatibility causes a delayed hemolytic transfusion reaction, this reaction was immediate so it is indicative more of the ABO blood group incompatibility (FA2020 p114 has all the blood transfusion reactions) +4

ooooopss I just wanna say God bless all of you cuzI needed this +2

drdoom ^ linkify @sympathetikey https://imgur.com/QH5rCEX +1

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submitted by ∗lilmonkey(63)

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To conclude: 1) ABO: IgM -> Complement -> Intravascular 2) Rh: IgG -> Splenic Macrophage -> Extravascular

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submitted by ∗roygbiv(25)

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Why could this not be extravascular hemolysis? In FA it says acute hemolytic transfusion reaction can be due to ABO incompatibility or extravascular hemolysis.

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niboonsh because extravascular hemolysis is associated w jaundice. Intravascular hemolysis would have hemoglobinuria but that's not an answer +2

niboonsh i mean that is the answer lol +1

krewfoo99 According to pathoma: Intrvascular haemolysis will lead to haemoglobin binding to haptoglobin. This complex will travel to the kidneys and be excreted. This will lead to red colored urine and haemosiduria (Note: This can also lead to acute tubular necrosis) Extravascular haemolysis is when macrophages break down the RBC. Then the Haeme is converted to biliverdin then bilirubin and conjugated in liver, and then excreted. +5

paperbackwriter If you look under the "clinical presentation" column of the blood transfusion reactions chart (pg114), it says that hemoglobinuria is with intravascular hemolysis and jaundice is with extravascular. Makes sense because with extravascular hemolysis your splenic macrophages are are chewing up the RBCs and sequestering it in the spleen so you don't get "spillover" -- i.e. clean urine. +2

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submitted by ∗haliburton(224)

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FA 2017 states that extravascular hemolysis has jaundice where ABO incompatibility would not.

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submitted by ∗medstruggle(21)

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Why was the acute hemolytic transfusion reaction due to ABO incompatibility, but not Rh incompatibility?

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colonelred_ Rh incompatibility comes more into play with Rh- mother and Rh+ babies. +2

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submitted by ∗abhishek021196(120)

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Acute hemolytic transfusion reaction

Type II hypersensitivity reaction

Typically causes intravascular hemolysis (ABO blood group incompatibility)

During transfusion or within 24 hr (due to preformed antibodies)

Fever, hypotension, tachypnea, tachycardia, flank pain, hemoglobinuria (intravascular), jaundice (extravascular)

Donor RBC with A and/ or B group antigens react with Host anti-A, anti-B IgG, IgM.

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submitted by ∗weirdmed51(30)

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Blood transfusion reactions:

type I / Allergic Anaphylactic R- fever, urticaria, pruritus, wheeze, hypotension (2-3min) [Ig A defc]
type 2/ Ac. Hemolytic R- fever, hypotension,tachycardia, tachypnea, flank pain, Hburia, jaundice. (Within 1 hr)
Febrile non hemolytic R- Fever,chills, headache, but no Hburia or jaundice (1-6hr)
TRALI- Resp.distress, noncardio plum edema. (6hr)

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