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Human Physiology · Blood and immunity

Blood Groups and Transfusion

Blood transfusion saves lives but carries risks, and it depends on matching blood groups correctly. This page explains the ABO and Rhesus systems, compatibility, transfusion reactions, and what trials tell us about when to transfuse, including for recovery after hip fracture.

11Sections
5Figures
3Tables
10Questions

Part 6 · Blood and Immunity · Chapter 29 of 47

Learning objectives

By the end of this page you should be able to:

  • Describe the ABO and Rhesus blood group systems.
  • Explain compatibility for red cell transfusion.
  • Describe haemolytic disease of the newborn and its prevention.
  • Recognise transfusion reactions.
  • Summarise evidence on restrictive and liberal transfusion thresholds.
  • Plan physiotherapy around a transfusion.

Why it matters

Physiotherapists often treat patients receiving transfusions on surgical, orthopaedic and haematology wards. You need to know when to delay a session, what a reaction looks like, and why a patient may not be transfused even though their haemoglobin is low.

Large trials have changed practice: most patients now receive less blood than before. Understanding why helps you set expectations for rehabilitation.

The ABO system

Figure 1 · The ABO blood groups

Red cell antigens and plasma antibodies in groups A, B, AB and O Four red cells. Group A cells carry A antigens and the plasma has anti-B. Group B cells carry B antigens with anti-A. Group AB cells carry both and the plasma has neither antibody. Group O cells carry neither antigen and the plasma has both antibodies. Group Aantigens: Aplasma antibodies:anti-BGroup Bantigens: Bplasma antibodies:anti-AGroup ABantigens: A and Bplasma antibodies:neitherGroup Oantigens: noneplasma antibodies:anti-A and anti-BA antigenB antigen
You make antibodies against the antigens you lack. Mismatched red cells are attacked by the recipient’s antibodies.

Red cells carry sugar-based antigens on their surface. People naturally make antibodies, mostly IgM, against the ABO antigens they lack, without prior exposure. If incompatible red cells are given, these antibodies bind them and destroy them within minutes.

GroupRed cell antigensPlasma antibodiesCan receive red cells from
ONoneAnti-A and anti-BO only
AAAnti-BA, O
BBAnti-AB, O
ABA and BNoneAB, A, B, O

The Rhesus system

The Rhesus D antigen is present in most people (Rh positive). Rh-negative people do not have natural anti-D but can make it after exposure through transfusion or pregnancy. In a later pregnancy, maternal anti-D can cross the placenta and destroy the red cells of an Rh-positive baby.

This haemolytic disease of the newborn is prevented by giving Rh-negative mothers anti-D immunoglobulin during and after pregnancy.

Compatibility

Figure 2 · Who can receive whose red cells

ABO compatibility grid for red cell transfusion A four by four grid of donor red cell group against recipient group. Group O cells can go to all groups; A to A and AB; B to B and AB; AB only to AB. RecipientDonor red cellsOABABOyesyesyesyesAnoyesnoyesBnonoyesyesABnononoyesO negative red cells:emergency universal donorAB positive patients:can receive any groupAlways cross-matchwhen time allows
Red cells only. Plasma compatibility runs the other way. Rhesus D type must also match, especially for women who may become pregnant.

Before routine transfusion, the patient’s group is checked and a sample is cross-matched with the donor unit. Most serious errors come from giving the wrong unit to the wrong patient, so bedside identity checks are essential.

Transfusion reactions

ReactionTimingFeatures
Acute haemolytic (ABO mismatch)MinutesFever, back pain, low blood pressure, dark urine; can be fatal
Febrile non-haemolyticDuring or soon afterFever, chills; usually mild
AllergicMinutesItch, hives; rarely anaphylaxis
Circulatory overloadHoursBreathlessness, high blood pressure; older and cardiac patients
Lung injuryWithin 6 hoursSudden breathlessness, low oxygen

Figure 3 · Watching a transfusion

Monitoring during blood transfusion and warning signs of a reaction Three stages: baseline observations before, close observation in the first 15 minutes when severe reactions tend to start, and repeated observations during and after. A warning box lists fever, chills, back or chest pain, breathlessness, falling blood pressure and rash. Beforebaseline pulse, BP,temperature, breathingFirst 15 minutesmost severe reactionsstart early; observe closelyDuring and afterrepeat observations;watch for overloadStop the transfusion and call for help if:• fever, chills, rigors• back or chest pain• breathlessness or wheeze• falling blood pressure, rash
Defer active physiotherapy at the start of a transfusion. Gentle activity is usually fine later if observations are stable; follow local policy.

When to transfuse

For years, many patients were transfused to keep haemoglobin above 10 g/dL. Hébert and colleagues tested this in 838 critically ill patients. A restrictive threshold of 7 g/dL gave mortality at least as good as a liberal threshold of 10 g/dL [1].

Figure 4 · Less blood can be better

Mortality with restrictive versus liberal transfusion in critically ill patients Paired bars. Thirty-day mortality was 23.3 per cent with liberal and 18.7 per cent with restrictive transfusion overall; 16.1 versus 8.7 per cent in less acutely ill patients; 13.0 versus 5.7 per cent in patients under 55. 23.3%Liberal (Hb <10)18.7%Restrictive (Hb <7)30-day mortality, all patients16.1%Liberal (Hb <10)8.7%Restrictive (Hb <7)Less acutely ill patients13.0%Liberal (Hb <10)5.7%Restrictive (Hb <7)Patients under 55
Restrictive transfusion was at least as safe. Possible exception: patients with acute coronary syndromes [1].

Mortality was lower with the restrictive strategy in less acutely ill patients and those under 55 [1]. Patients with acute coronary syndromes may be an exception [1].

Transfusion and recovery after hip fracture

Carson and colleagues asked whether extra blood helps older patients walk after hip fracture surgery. They randomised 2,016 patients with cardiovascular risk to a liberal threshold of 10 g/dL or a restrictive approach based on symptoms or haemoglobin below 8 g/dL [2].

Figure 5 · Transfusion and walking after hip fracture

Functional and survival outcomes with liberal versus restrictive transfusion after hip fracture Paired bars. Death or inability to walk across a room unaided at 60 days was 35.2 per cent with liberal and 34.7 per cent with restrictive transfusion. Death by 60 days was 7.6 and 6.6 per cent. 35.2%Liberal34.7%RestrictiveDeath or unable to walk across a room at 60 days7.6%Liberal6.6%RestrictiveDeath by 60 days2,016 older adults with cardiovascular risk after hip fracture surgery [2]
Extra blood did not help patients walk. A median of 2 units versus none made no difference to recovery [2]. Liberal: Hb below 10 g/dL; restrictive: symptoms or Hb below 8 g/dL.

Death or inability to walk across a room at 60 days was almost identical: 35.2% versus 34.7% [2]. Liberal transfusion gave a median of two extra units without improving recovery or reducing complications [2].

Practical skill · physiotherapy around transfusion

Avoid starting active treatment in the first 15 minutes of a transfusion. If observations are stable, gentle mobilisation within the limits of lines and drips is usually acceptable; follow local policy. A patient with low haemoglobin who is not being transfused can still mobilise, with closer monitoring of symptoms.

Clinical relevance for the physiotherapist

  • Timing. Delay active sessions at the start of a transfusion; watch for reactions.
  • Low haemoglobin. Restrictive thresholds are standard [1, 2]; adapt exercise rather than waiting for transfusion.
  • Hip fracture. Recovery of walking depends on rehabilitation, not extra blood [2].
  • Cardiac patients. Thresholds may be higher in acute coronary syndromes [1].
  • Overload. Breathlessness during or after transfusion may signal fluid overload; report it.

Assessment and application

SituationInterpretationAction
Hb 8.4 g/dL on day 2 after hip fracture surgery, no symptomsWithin restrictive range [2]Mobilise with monitoring
Rigors and back pain 10 minutes into a transfusionPossible acute reactionStop, call nurse or doctor immediately
Breathless, crackles after two units in an older patientPossible circulatory overloadSit upright; urgent medical review
Patient asks why they are not given bloodRestrictive strategy is evidence-based [1, 2]Explain; focus on safe activity

Common mistakes

  • Thinking group O plasma is universal; it is O red cells that are the emergency choice.
  • Expecting transfusion to speed rehabilitation [2].
  • Starting vigorous exercise at the start of a transfusion.
  • Assuming a higher haemoglobin target is always safer [1].
  • Missing early signs of a transfusion reaction.

Exam preparation

Long answer

Describe the ABO and Rh blood group systems. Add a note on transfusion reactions.

  • Antigens and antibodies (Figure 1 and table).
  • Compatibility and cross-matching (Figure 2).
  • Rhesus system and haemolytic disease of the newborn.
  • Transfusion reactions and monitoring (Figure 3).
  • Thresholds for transfusion [1, 2].

Short notes

  • Universal donor and recipient.
  • Haemolytic disease of the newborn.
  • Restrictive transfusion [1].
  • Transfusion after hip fracture [2].

Viva and practical

  • “Which antibodies does a group A person have?” — anti-B.
  • “What threshold did TRICC support?” — 7 g/dL [1].
  • “Did more blood help walking after hip fracture?” — no [2].
  • OSCE: decide whether to mobilise a patient during transfusion.

Current evidence

Exam and traditional teaching

Keep haemoglobin around 10 g/dL in sick or post-operative patients.

What the evidence adds

Restrictive thresholds were at least as safe in critical care [1].

After hip fracture, liberal transfusion did not improve walking or survival [2].

What the student should understand

Blood is a treatment with risks, used sparingly. Function returns through activity and rehabilitation.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. In the TRICC trial, the restrictive strategy transfused red cells when haemoglobin fell below:
  1. (A) 5 g/dL
  2. (B) 7 g/dL
  3. (C) 10 g/dL
  4. (D) 12 g/dL

Answer: (B) Restrictive transfusion was triggered below 7.0 g/dL [1].

Q2. Overall 30-day mortality in TRICC was:
  1. (A) Much higher with restrictive transfusion
  2. (B) Similar: 18.7% restrictive vs 23.3% liberal
  3. (C) Zero in both
  4. (D) Only reported for surgery

Answer: (B) Mortality was 18.7% vs 23.3%, not significantly different overall [1].

Q3. In TRICC, restrictive transfusion had significantly lower mortality among:
  1. (A) Patients with acute heart attack
  2. (B) Less acutely ill patients and those under 55
  3. (C) Children
  4. (D) Nobody

Answer: (B) Lower mortality was seen with APACHE II ≤20 and age <55 [1].

Q4. A possible exception to the restrictive approach in TRICC was patients with:
  1. (A) Fractures
  2. (B) Acute myocardial infarction or unstable angina
  3. (C) Asthma
  4. (D) Diabetes

Answer: (B) The authors noted a possible exception for acute coronary syndromes [1].

Q5. FOCUS enrolled patients:
  1. (A) Under 30 with sports injuries
  2. (B) Aged 50 or over with cardiovascular risk after hip fracture surgery
  3. (C) With asthma
  4. (D) Undergoing dialysis

Answer: (B) 2,016 older patients with cardiovascular history or risk after hip fracture surgery [2].

Q6. The FOCUS primary outcome was:
  1. (A) Haemoglobin at discharge
  2. (B) Death or inability to walk across a room unaided at 60 days
  3. (C) Length of stay only
  4. (D) Wound infection

Answer: (B) Function and survival at 60 days [2].

Q7. Rates of the FOCUS primary outcome were:
  1. (A) 35.2% liberal vs 34.7% restrictive
  2. (B) 10% vs 50%
  3. (C) 50% vs 10%
  4. (D) 0% vs 0%

Answer: (A) There was no meaningful difference [2].

Q8. How many red cell units did the median patient receive in the restrictive FOCUS group?
  1. (A) None
  2. (B) 2
  3. (C) 5
  4. (D) 10

Answer: (A) A median of 2 units in the liberal group and none in the restrictive group [2].

Q9. A key message for rehabilitation from FOCUS is that:
  1. (A) Transfusing to a higher haemoglobin improves walking
  2. (B) Liberal transfusion did not improve walking at 60 days
  3. (C) Patients should not walk until Hb is 12
  4. (D) Transfusion replaces rehabilitation

Answer: (B) Liberal transfusion did not reduce death or inability to walk [2].

Q10. Which statement about transfusion thresholds is best supported by these trials?
  1. (A) Always transfuse below 10 g/dL
  2. (B) Restrictive thresholds are generally as safe as liberal ones
  3. (C) Never transfuse
  4. (D) Thresholds do not matter in heart disease

Answer: (B) Both trials found restrictive strategies at least as effective in most patients [1, 2].

Quick review

Key takeaways

  • ABO: you have antibodies to the antigens you lack.
  • O-negative red cells: emergency universal donor; AB-positive: universal recipient.
  • Rh-negative mothers receive anti-D to protect future babies.
  • Watch closely in the first 15 minutes of a transfusion.
  • Restrictive threshold of 7 g/dL at least as safe in ICU [1].
  • Liberal transfusion did not improve walking after hip fracture [2].

Further reading

SourceWhy it earns its place
Hébert PC et al. — NEJM, 1999The trial that changed transfusion thresholds [1]
Carson JL et al. — NEJM, 2011Transfusion and function after hip fracture [2]
Hoffbrand AV — Essential HaematologyBlood groups and transfusion

References

  1. Hébert PC, Wells G, Blajchman MA, et al. A multicenter, randomized, controlled clinical trial of transfusion requirements in critical care. New England Journal of Medicine. 1999;340(6):409–417. doi:10.1056/NEJM199902113400601 PMID 9971864 Randomised controlled trial
  2. Carson JL, Terrin ML, Noveck H, et al. Liberal or restrictive transfusion in high-risk patients after hip surgery. New England Journal of Medicine. 2011;365(26):2453–2462. doi:10.1056/NEJMoa1012452 PMID 22168590 Randomised controlled trial

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Reviewed by the Physiotherapist India Team. · Human Physiology contents