Page 17 of 28

PA21.1-6 | Transfusion Reactions & Investigation — Part 1

CLINICAL SCENARIO

A 28-year-old woman, Group A Rh-positive, is 15 minutes into her second unit of packed red cells after a post-partum haemorrhage. She develops sudden rigors, low back pain, and her blood pressure drops to 80/50 mmHg. A nurse notices the unit label reads 'Group B'. What has happened, and what must you do in the next 60 seconds?

This scenario — an ABO-incompatible transfusion — kills within hours if not stopped immediately. Yet it is almost entirely preventable.

WHY THIS MATTERS

Transfusion is one of the most common clinical interventions in hospital medicine. In India, over 12 million units of blood are transfused annually. You will encounter transfusion reactions as a house officer, during your medical posting, and throughout any surgical or obstetric rotation. PA21.4 is examined in both theory and practical OSCEs — classification tables and investigation algorithms are recurring short-answer and MCQ themes. More importantly, recognising a reaction early and responding correctly is a core patient-safety competency.

RECALL

Before proceeding, consolidate your prior learning:

ABO blood group system — naturally occurring IgM antibodies (anti-A, anti-B) are present WITHOUT prior sensitisation. Recall why O-negative is the 'universal donor'.
Rh system — anti-D forms only after sensitisation (transfusion or pregnancy). Recall haemolytic disease of the newborn.
Direct antiglobulin test (DAT / Coombs test) — detects immunoglobulin or complement on the red cell surface; indirect Coombs detects free antibody in serum.
HLA antigens — expressed on leukocytes and platelets; relevant to febrile and TRALI reactions.
Complement cascade — the membrane attack complex (MAC) mediates intravascular haemolysis in ABO reactions.
DIC — disseminated intravascular coagulation; consumption coagulopathy triggered by massive cytokine and thrombin release.

Classification Framework

A 2 × 2 matrix classifies transfusion reactions by timing, acute or delayed, and mechanism, immune-mediated or non-immune-mediated, with key examples in each quadrant. — **Panel A:** 2 × 2 classification matrix showing acute immune, acute non-immune, delayed immune, and delayed non-immune transfusion reactions with examples: AHTR, FNHTR, allergic/anaphylactic, TRALI, TACO, septic reaction, metabolic complications, delayed haemolytic reaction, TA-GVHD, PTP, and iron overload.. **Panel B1:** Timing axis showing acute reactions within 24 hours and delayed reactions after 24 hours, sometimes weeks later.. **Panel B2:** Mechanism axis comparing immune-mediated antibody-antigen or T-cell responses with non-immune physical, metabolic, or microbial causes..

Transfusion reactions are classified along two independent axes:

Axis 1 — Timing
• Acute reactions occur within 24 hours of transfusion (most within the first few minutes to 6 hours).
• Delayed reactions occur after 24 hours, sometimes weeks later.

Axis 2 — Mechanism
• Immune-mediated — involve antibody–antigen reactions or T-cell responses.
• Non-immune-mediated — physical, metabolic, or microbial causes.

This 2 × 2 framework generates four quadrants:

	Immune	Non-Immune
Acute	AHTR, FNHTR, allergic/anaphylactic, TRALI	TACO, septic, metabolic
Delayed	Delayed haemolytic, TA-GVHD, PTP	Iron overload

Memorise this table — it structures every examination answer on this topic.

Classification table of transfusion reactions in 2×2 grid format with color-coded severity levels and accompanying pathophysiology flowchart for acute hemolytic transfusion reaction. — **Panel A:** 2×2 classification grid showing acute vs delayed (horizontal axis) and immune vs non-immune (vertical axis) transfusion reactions, with color-coded severity: AHTR, FNHTR, allergic reactions, TRALI, DHTR, TA-GvHD, transfusion-associated sepsis, and iron overload. **Panel B:** Pathophysiology flowchart of acute hemolytic transfusion reaction showing progression from ABO incompatibility through complement activation, intravascular hemolysis, and four major pathways leading to clinical manifestations.

Acute Haemolytic Transfusion Reaction (AHTR)

Four-panel infographic showing ABO-incompatible transfusion causing IgM complement activation, intravascular haemolysis, cytokine shock, DIC, renal injury, and the need to stop transfusion immediately. — **Panel A:** ABO-incompatible donor red cells, recipient circulation, pre-formed IgM anti-A or anti-B antibodies, wrong patient, wrong unit, labelling mistake, 10 mL fatal warning.. **Panel B:** IgM-coated donor red cell, classical complement pathway, C1 activation, C5b-9 membrane attack complex, intravascular haemolysis, free haemoglobin, haemoglobinaemia.. **Panel C:** Macrophage activation, TNF-alpha, IL-1, IL-6, IL-8, cytokine storm, bradykinin vasodilatation, hypotension, tissue factor release, DIC, fibrin strands, petechiae, IV site oozing.. **Panel D:** Kidney, renal vasoconstriction, nephron tubule haemoglobin precipitation, port-wine haemoglobinuria, oliguria or anuria, back pain, anaesthetised patient with unexplained hypotension and haemoglobinuria, stop transfusion immediately..

Acute haemolytic transfusion reaction (AHTR) is the most feared transfusion complication. It is almost always due to ABO incompatibility caused by a clerical error — wrong patient, wrong unit, or a labelling mistake — not a laboratory failure.

Mechanism:
1. Recipient has pre-formed IgM anti-A or anti-B antibodies.
2. IgM activates the classical complement pathway rapidly.
3. Intravascular haemolysis — the membrane attack complex (C5b-9) lyses donor red cells within the circulation.
4. Released haemoglobin, red cell stroma, and ADP activate:
- Macrophages → massive cytokine storm (TNF-α, IL-1, IL-6, IL-8)
- Bradykinin → hypotension and vasodilatation
- Tissue factor release → disseminated intravascular coagulation (DIC)
- Renal vasoconstriction + haemoglobin precipitation in tubules → acute renal failure

Clinical features (onset within minutes):
• Fever, rigors, flushing
• Loin / back pain (ureteric spasm from haemoglobin deposits)
• Hypotension, tachycardia
• Haemoglobinuria (port-wine urine)
• Haemoglobinaemia (pink/red plasma)
• DIC → oozing from IV sites, petechiae
• Oliguria / anuria

In an unconscious patient under general anaesthesia, the only signs may be unexplained hypotension and haemoglobinuria.

Key point: As few as 10 mL of ABO-incompatible blood can initiate a fatal reaction. STOP the transfusion immediately.

SELF-CHECK

A patient receiving a blood transfusion develops sudden hypotension, haemoglobinuria, and back pain 10 minutes after the transfusion starts. Which immunological mechanism best explains the intravascular haemolysis?

A. IgG-mediated extravascular haemolysis in the spleen

B. IgM-mediated classical complement activation leading to MAC formation

C. T-cell cytotoxicity against donor erythrocytes

D. IgE-mediated mast cell degranulation

Reveal Answer

Answer: B. IgM-mediated classical complement activation leading to MAC formation

ABO incompatibility triggers pre-formed IgM antibodies that rapidly activate the classical complement cascade, assembling the membrane attack complex (C5b-9) which directly lyses donor red cells intravascularly. IgG-mediated reactions (option A) cause extravascular haemolysis (spleen/liver phagocytosis) — slower and less catastrophic. T-cell cytotoxicity (C) is the mechanism of TA-GVHD. IgE/mast cell degranulation (D) causes anaphylaxis, not haemolysis.

Febrile Non-Haemolytic Transfusion Reaction (FNHTR)

⚑ AI image — pending faculty review (auto-QA score 8/10; best of 3 attempts)

Diagram showing mechanisms, clinical features, absence of haemolysis, and safety distinction of febrile non-haemolytic transfusion reaction. — **Panel A:** Transfusion bag, donor leukocytes, recipient anti-HLA / anti-leukocyte antibodies, cytokine release, stored cytokines IL-1β, IL-6, TNF-α, fever pathway. **Panel B:** Temperature rise ≥1°C, timing during or within 4 hours of transfusion, rigors, headache, myalgia. **Panel C:** Clear plasma, normal urine, intact red cells, no haemolysis. **Panel D:** Fever during transfusion, stop transfusion, investigate for haemolysis, exclude acute haemolytic transfusion reaction, FNHTR self-limiting course, leucoreduction prevention note.

Febrile non-haemolytic transfusion reaction (FNHTR) is the commonest transfusion reaction, occurring in 0.5–1% of transfusions.

Mechanism:
Two complementary mechanisms:
1. Anti-leukocyte (anti-HLA) antibodies in the recipient react with donor leukocytes in the unit — cytokine and pyrogen release.
2. Cytokine accumulation in stored blood — IL-1β, IL-6, TNF-α released by leukocytes during storage; even without recipient antibodies, pre-formed cytokines trigger fever.

Clinical features:
• Temperature rise ≥1°C during or within 4 hours of transfusion
• Rigors, headache, myalgia
• No haemolysis — plasma remains clear, urine normal
• Self-limiting; resolves within hours

Key distinction: FNHTR must be distinguished from early AHTR (which also causes fever). If in doubt, stop the transfusion and investigate for haemolysis before attributing fever to FNHTR.

Prevention: Leucoreduction (leukocyte-depleted blood) reduces but does not eliminate FNHTR — cytokines already accumulated in storage persist.