Page 7 of 28
PA21.1-6 | Blood Components & Clinical Uses — Part 1
CLINICAL SCENARIO
A 28-year-old woman arrives in the emergency department after a road traffic accident — blood pressure 80/50 mmHg, haemoglobin 5.2 g/dL, platelet count 18,000/µL, and INR 2.8. The blood bank rings: 'What do you need — and in what order?' Knowing your blood components is the difference between a targeted rescue and a textbook catastrophe.
WHY THIS MATTERS
Component therapy is standard of care across surgery, oncology, haematology, and critical care. MBBS exit exams consistently test indications, storage, and complications. PA21.2 is also directly assessed in clinical postings where you will write (or countersign) transfusion requests — getting the component wrong can cause fluid overload, TRALI, or graft-versus-host disease.
RECALL
Before continuing, briefly recall:
• What is haemopoiesis and where does it occur in adults?
• Name the three main cellular components of blood and their primary functions.
• What is the coagulation cascade and which factors are vitamin-K dependent?
• From your Year-1 physiology: how does ABO blood grouping limit donor–recipient compatibility?
If any of these feel uncertain, skim the H10 SDL (Bleeding Disorders) before proceeding — this module builds on factor physiology covered there.
Why Component Therapy? The Efficiency Argument
Component Therapy from One Whole-Blood Donation
Component therapy means separating a single donated whole-blood unit into its constituent parts so that each component can be given to the patient who specifically needs it. One 450 mL whole-blood donation routinely yields:
• 1 unit of packed red blood cells (PRBC)
• 1 unit of platelet concentrate (random donor)
• 1 unit of fresh frozen plasma (FFP)
• 1 unit of cryoprecipitate (from the FFP)
Advantages over whole blood:
1. Targeted correction — the patient with anaemia does not receive unnecessary plasma proteins; the thrombocytopenic patient does not receive red cells.
2. Reduced transfusion-associated circulatory overload (TACO) — smaller volumes per component minimise cardiac stress, especially in elderly patients.
3. Optimised storage — each component requires different conditions; whole blood stored at 4°C destroys platelets and degrades labile coagulation factors within 24 hours.
4. Resource efficiency — one donation treats up to 4 different patients.
Whole blood is now reserved for specific scenarios: exchange transfusion in neonatal haemolytic disease, resource-limited settings, or military trauma ("walking blood banks").
Blood Component Separation by Centrifugation
SELF-CHECK
Which of the following is the PRIMARY rationale for preferring component therapy over whole blood in modern transfusion practice?
A. Whole blood is more likely to transmit transfusion-transmitted infections.
B. Component therapy allows targeted correction of specific deficits while reducing volume load.
C. Whole blood requires complex cross-matching that is not feasible in most hospitals.
D. Component therapy eliminates the risk of ABO incompatibility reactions.
Reveal Answer
Answer: B. Component therapy allows targeted correction of specific deficits while reducing volume load.
The central rationale is targeted therapy + reduced volume: the patient with isolated anaemia does not need plasma proteins, and unnecessary volume risks TACO. Option A is incorrect — both carry the same donor-pool infectious risk. Option C is wrong: cross-matching applies equally to both. Option D is wrong: ABO compatibility must still be checked for each component, especially FFP and platelets.
Packed Red Blood Cells (PRBC)
Packed Red Blood Cells: Preparation, Storage, Indications, and Dose
Packed red blood cells are prepared by centrifuging whole blood and removing most plasma, leaving a haematocrit of ~65–80%. An additive solution (SAGM — saline, adenine, glucose, mannitol) is added to prolong storage.
Storage: 2–6°C; shelf-life 42 days with additive solution (21 days without).
Clinical indications:
• Symptomatic anaemia: haemoglobin <7 g/dL in haemodynamically stable patients (transfusion trigger); <8–9 g/dL in patients with ischaemic heart disease or poor cardiopulmonary reserve.
• Acute haemorrhagic blood loss: restore oxygen-carrying capacity when haemoglobin drops with haemodynamic compromise.
• Chronic transfusion-dependent anaemias: thalassaemia major, aplastic anaemia, myelodysplastic syndrome.
Expected increment: Each unit of PRBC typically raises haemoglobin by approximately 1 g/dL (or haematocrit by 3%) in a 70 kg adult — a rule tested in exam MCQs.
Dose calculation: The formula Volume (mL) = (Target Hb − Actual Hb) × Weight (kg) × 3 applies in paediatric practice.
Key caution: Avoid transfusing solely based on a number — transfuse the patient, not the haemoglobin. Symptomatic status, ongoing losses, and cardiopulmonary reserve all modify the trigger.
Platelet Concentrates
Platelet Concentrates: Preparation, Storage, and Clinical Use
Two preparation routes yield functionally similar products:
Random-donor platelet concentrate (RDPC): Prepared from a single whole-blood donation via the buffy-coat method. Each unit contains ~5.5 × 10¹⁰ platelets in ~50–70 mL plasma. A therapeutic adult dose requires pooling 4–6 units ("pooled platelets").
Single-donor (apheresis) platelet concentrate: Collected by continuous-flow cell separation from one donor; equivalent to 4–6 RDPC units in one bag. Reduces donor exposure — important in multiply transfused patients at risk of HLA alloimmunisation.
Storage: 20–24°C (room temperature) with constant gentle agitation; shelf-life only 5 days. The short shelf-life and room-temperature storage (which risks bacterial growth) make platelets the most logistically challenging component.
Clinical indications:
• Active bleeding with platelet count <50,000/µL (surgical threshold: <100,000/µL for high-risk procedures).
• Prophylactic transfusion when platelet count <10,000/µL in stable patients (prevents spontaneous intracranial bleed).
• Qualitative platelet disorders with active bleeding: uraemia, drug-induced dysfunction (aspirin, clopidogrel), Glanzmann thrombasthenia.
• Prior to invasive procedures in thrombocytopenic patients.
Platelet refractoriness: Failure to achieve expected increment (≥10,000/µL rise at 1 h post-transfusion) — causes include HLA or HPA antibodies, fever, sepsis, DIC, and splenomegaly. HLA-matched or cross-matched platelets are used in immune refractoriness.
Expected increment: 1 unit RDPC raises count by ~5,000–10,000/µL; apheresis unit raises by ~30,000–60,000/µL.
SELF-CHECK
A patient with aplastic anaemia has a platelet count of 8,000/µL and no active bleeding. What is the most appropriate action?
A. Observe — platelets are only transfused for active bleeding.
B. Administer prophylactic platelet transfusion immediately.
C. Administer FFP to prevent spontaneous haemorrhage.
D. Transfuse platelets only if the count drops below 5,000/µL.
Reveal Answer
Answer: B. Administer prophylactic platelet transfusion immediately.
The standard prophylactic transfusion threshold in stable thrombocytopenic patients is <10,000/µL to prevent spontaneous intracranial bleeding. Option A is incorrect — prophylactic use is well-established evidence-based practice. Option C is wrong: FFP provides coagulation factors, not platelets. Option D uses too-low a threshold and unnecessarily risks spontaneous haemorrhage.