Anticoagulants in Packed Red Blood Cells
Packed red blood cells (PRBCs) contain citrate-based anticoagulants that prevent coagulation by chelating calcium ions, which are essential for the coagulation cascade to function. 1, 2
Primary Anticoagulant: Citrate
The standard anticoagulant used in PRBC preparation is citrate-phosphate-dextrose (CPD) or its variants, which prevents clotting through calcium chelation. 2, 3 Specifically:
- Trisodium citrate at 0.109 M final concentration is the most commonly used anticoagulant in blood component preparation 2
- Blood is initially drawn into CPD anticoagulant solution before the red cells are separated and resuspended in additive solutions 3, 4
- A standard PRBC unit (300-400 mL) contains up to 3 grams of citrate 2
Mechanism of Action
Citrate prevents coagulation through a specific biochemical mechanism:
- Citrate chelates (binds) calcium ions, making them unavailable for the coagulation cascade 1, 2
- Calcium ions are required for coagulation factors (II, VII, IX, X) to bind to cell membranes, which is a prerequisite for thrombin formation 1
- Without available calcium, the tenase and prothrombinase complexes cannot form, effectively blocking both the intrinsic and extrinsic coagulation pathways 1
Additive Preservation Solutions
After initial collection in CPD, PRBCs are typically resuspended in additive solutions that maintain cell viability during storage:
- SAGM (saline-adenine-glucose-mannitol) - a hypertonic solution (376 mOsm/L) 3
- PAGGSM (phosphate-adenine-glucose-guanosine-saline-mannitol) - an isotonic solution (285 mOsm/L) 3
- Adsol (AS-1) - containing saline, adenine, dextrose, and mannitol 4
These additive solutions do not provide anticoagulation themselves but rather support red cell metabolism and reduce hemolysis during the 42-49 day storage period. 3, 4
Clinical Significance
The citrate anticoagulant has important clinical implications during transfusion:
- Citrate toxicity can occur during massive transfusion, as the citrate load overwhelms the body's ability to metabolize it, leading to hypocalcemia 2
- Hypocalcemia from citrate chelation causes coagulopathy with platelet dysfunction and decreased clot strength 2
- Risk is exacerbated by liver dysfunction, hypothermia, hypoperfusion, and renal insufficiency, which all impair citrate metabolism 2
- Fresh frozen plasma and platelet products contain particularly high citrate concentrations compared to PRBCs 2