Blood Products and Transfusion: The Components and When to Give Them
A unit of donated blood is almost never given whole. It is spun into parts, and each part is a different "drug" for a different problem: red cells carry oxygen, plasma and cryoprecipitate carry clotting proteins, platelets build the plug. Prescribe the wrong one and you either waste a scarce resource or fail to fix the bleed. This chapter treats each component like a medicine — its indication, its trigger, and its dangers.
The trauma bay doors burst open: a young man from a road crash, pale, cold, blood pressure sliding. The team calls for the massive transfusion pack. But notice what they DON'T do — they don't just pour in bag after bag of red cells. One nurse hangs red cells for oxygen. Another readies plasma and cryoprecipitate for clotting. Platelets are on the way for the plug. Three different problems, three different products, given in balance. Whole blood was one thing; modern transfusion is a pharmacy of parts, and knowing which bag fixes which failure is the whole skill.
First, the label: ABO, Rh and the crossmatch
Before any red cells flow, two questions must be answered: what group, and does it match? Red cells carry surface antigens. The two that matter most are the ABO system and the Rh (RhD) antigen. A person's plasma naturally contains antibodies against the ABO antigens they lack — so a group A patient has anti-B antibodies, and giving them group B cells triggers immediate destruction. Group O red cells (no A or B antigen) are the "universal donor" for red cells in an emergency; group AB plasma is the universal plasma. The crossmatch is the final safety check: the recipient's serum is mixed with the donor cells to confirm no reaction before release.
Note the mirror: group O is the universal RED CELL donor, but AB is the universal PLASMA donor. It flips because plasma carries the antibodies while red cells carry the antigens. Mixing this up is a classic exam trap — and a fatal one at the bedside.
Packed red cells: oxygen in a bag
Packed (concentrated) red cells are the workhorse. Their single job is to raise oxygen-carrying capacity — so the indication is symptomatic anaemia or ongoing major bleeding, not a number alone. Modern practice is restrictive: in a stable, non-bleeding patient we hold transfusion until the haemoglobin falls to about 70 g/L (7 g/dL), and even then transfuse one unit at a time and reassess. The old reflex of "top everyone up to normal" is gone — restrictive thresholds match or beat liberal ones on outcomes, and each unit carries risk.
In an average adult, one unit of red cells raises the haemoglobin by roughly 10 g/L (1 g/dL). So a stable patient at 68 g/L who you want in the mid-70s usually needs just one unit — then you recheck rather than assume.
Platelets: rebuilding the plug
When the problem is the primary plug, red cells won't help — you need platelets. Platelet concentrates are given for thrombocytopenia (a low platelet count) with active bleeding, or prophylactically before an invasive procedure or when the count is critically low. The threshold depends on the setting: a much lower count is tolerated in a stable patient than in one who is bleeding or about to have surgery. Platelets treat a problem of primary haemostasis — the initial plug — which is a different failure from the clotting-factor cascade that plasma products address.
Plasma and cryoprecipitate: the clotting proteins
Fresh frozen plasma (FFP) is the liquid part of blood, frozen to preserve its clotting factors. It contains ALL the coagulation factors, so it is used when several are deficient at once — liver disease (which makes most factors), disseminated intravascular coagulation (DIC), and as part of massive transfusion. Cryoprecipitate is a concentrated fraction pulled from thawed plasma; it is rich in fibrinogen, factor VIII, von Willebrand factor and factor XIII. Its main modern use is to replace a low fibrinogen (hypofibrinogenaemia), for example in DIC or major obstetric haemorrhage.
Broad, multi-factor deficiency (a bleeding cirrhotic, or DIC) → FFP, which carries everything. An isolated low fibrinogen → cryoprecipitate, the concentrated fibrinogen source. A single named factor missing (as in haemophilia) → not FFP or cryo but a specific factor concentrate — covered in the Haemophilia chapter.
Prothrombin complex concentrate: the fast warfarin reversal
One product deserves its own spotlight because students so often reach for the wrong one. Prothrombin complex concentrate (PCC) is a concentrated preparation of the vitamin-K-dependent clotting factors (II, VII, IX, X). Because it is concentrated, it works in a small volume and acts within minutes — which makes it the agent of choice for rapid warfarin reversal in serious or life-threatening bleeding (given with vitamin K). It replaces exactly the factors warfarin suppresses. FFP can supply the same factors but only in large, slow, volume-heavy bags — too slow when the bleed is in the brain. The full reversal logic for each anticoagulant lives in the Anticoagulant reversal chapter.
- Packed red cells = oxygen; restrictive trigger ~70 g/L Hb in stable, non-bleeding patients.
- Platelets = thrombocytopenia with bleeding, or before a procedure.
- FFP = multi-factor deficiency (liver disease, DIC, massive transfusion).
- Cryoprecipitate = concentrated fibrinogen (also VIII, vWF, XIII) for low fibrinogen.
- PCC = fast replacement of factors II, VII, IX, X — the rapid warfarin reversal.
- O is the universal red-cell donor; AB is the universal plasma donor.
When a transfusion turns dangerous
Every transfusion carries risk, and the reactions cluster into a few patterns worth memorizing. An acute haemolytic reaction — usually an ABO mismatch from a clerical or bedside identification error — is the true emergency: within minutes the patient develops fever, loin/back pain, dark urine and shock as the transfused cells are destroyed. Milder febrile non-haemolytic and allergic (urticarial) reactions are common and usually manageable. Infection transmission is now rare because of donor screening. And chronic, repeated transfusion brings its own slow danger: iron overload.
Two reactions share the same moment — new breathlessness during a transfusion — but they are opposites in cause and cure. TACO — transfusion-associated circulatory overload — is exactly what it sounds like: too much volume too fast, so the circulation is overloaded and fluid backs up into the lungs. It looks like heart failure (hypertension, raised jugular pressure) and responds to slowing/stopping the transfusion and giving a diuretic. TRALI — transfusion-related acute lung injury — is an immune-mediated injury to the lung capillaries; the patient is often HYPOtensive with non-cardiogenic pulmonary oedema, and the treatment is supportive respiratory care, NOT a diuretic. Same breathlessness, opposite blood pressure, opposite management.
Each unit of red cells carries a large iron load, and the body has no way to excrete the excess. Patients on lifelong transfusion (as in thalassaemia major) accumulate iron in the heart, liver and endocrine glands — which is why they need iron chelation therapy. The full story is in the Thalassaemia and iron chelation chapter.
Rh, anti-D and the massive transfusion protocol
Rh matters most in Rh-negative individuals, especially women of childbearing potential: exposure to RhD-positive cells (a mismatched transfusion, or a Rh-positive fetus) can provoke anti-D antibodies that attack future Rh-positive red cells — the basis of haemolytic disease of the newborn. Anti-D immunoglobulin, given to a Rh-negative mother, mops up any fetal Rh-positive cells before her immune system can react, preventing sensitization. Finally, when bleeding is catastrophic, the massive transfusion protocol delivers red cells, plasma and platelets in balanced ratios (approaching 1:1:1) — replacing whole-blood function rather than red cells alone, so you don't correct the oxygen deficit while leaving the patient unable to clot.
The logic of the 1:1:1 ratio is the scene from the trauma bay in one line: red cells alone dilute out the patient's own clotting factors and platelets, so you must give plasma and platelets alongside — otherwise you resuscitate the oxygen but manufacture a coagulopathy.
- Acute haemolytic reaction = ABO mismatch = emergency: fever, loin pain, dark urine, shock.
- TACO = fluid overload, hypertension → slow/stop + diuretic.
- TRALI = immune lung injury, hypotension → supportive care, NOT diuretic.
- Chronic transfusion → iron overload → needs chelation.
- Anti-D immunoglobulin prevents RhD sensitization in Rh-negative mothers.
- Massive transfusion = balanced red cells : plasma : platelets (~1:1:1).
- Reaching for FFP to reverse warfarin urgently. PCC is faster, smaller-volume and more reliable — FFP is the fallback, not the first choice.
- Confusing TACO with TRALI. Both cause breathlessness, but TACO is overload (hypertensive, give diuretic) and TRALI is lung injury (hypotensive, no diuretic).
- Forgetting that platelets are contraindicated in HIT and TTP — transfusing them there can fuel thrombosis rather than help.
- Transfusing to a haemoglobin number in a stable patient instead of to symptoms/the restrictive ~70 g/L trigger.
A patient on warfarin arrives with an intracranial bleed and a very high INR. Which product gives the fastest, most reliable reversal (with vitamin K)?
- Blood is split into parts: red cells (oxygen), platelets (the plug), FFP/cryo (clotting).
- Red cells: restrictive ~70 g/L trigger in stable patients; platelets for low count + bleeding.
- FFP for multi-factor deficiency; cryoprecipitate for low fibrinogen; PCC for fast warfarin reversal.
- Know the reactions: ABO haemolytic (emergency), TACO vs TRALI, and chronic iron overload.
- Hoffbrand AV, Moss PAH. Hoffbrand's Essential Haematology — Blood transfusion: components, grouping, crossmatch and complications.
- British Society for Haematology (BSH) Guidelines — Administration of blood components and management of transfusion reactions.
- BSH Guidelines — Use of fresh frozen plasma, cryoprecipitate and prothrombin complex concentrate.
- Klein HG, Anstee DJ. Mollison's Blood Transfusion in Clinical Medicine — Red cells, plasma products and transfusion reactions.
- NICE NG24 — Blood transfusion: red cell and platelet thresholds and restrictive strategy.
