Anticoagulants play a crucial role in ensuring that blood samples remain in a fluid state for testing, storage, and transfusion. Different anticoagulants are used depending on the test or procedure being performed. Each has a specific mode of action, advantages, and limitations. Below is a detailed description of various anticoagulants used in these settings, including their uses, mechanisms of action, merits, and demerits.
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Ethylenediaminetetraacetic Acid (EDTA)
Uses:
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- Primarily used for haematology tests, such as complete blood count (CBC), blood smears, and flow cytometry.
- Ideal for preserving blood cells, especially white blood cells (WBCs) and platelets.
- Used in blood typing and cross-matching in blood banks.
Mode of Action:
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- EDTA works by binding to calcium ions (Ca²⁺), essential for clotting. Without calcium, the coagulation cascade cannot proceed, preventing blood clotting.
Advantages:
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- Excellent preservation of cellular morphology, making it the preferred anticoagulant for haematology tests.
- Prevents platelet clumping, which can falsely lower platelet counts.
- Stable for long periods at room temperature, ensuring sample integrity for several hours.
Disadvantages:
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- Not suitable for coagulation studies, as it interferes with calcium-dependent clotting assays (e.g., PT, aPTT).
- EDTA can cause cell shrinkage if blood is stored for extended periods, which may affect red blood cell (RBC) indices like MCV (mean corpuscular volume).
- Interferes with electrolyte assays, especially calcium and potassium levels, as EDTA binds these ions.
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Sodium Citrate
Uses:
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- Commonly used for coagulation studies, including prothrombin time (PT) and activated partial thromboplastin time (aPTT).
- Employed in blood banks to prepare blood components like plasma for transfusion and in apheresis procedures.
- Also used in erythrocyte sedimentation rate (ESR) testing.
Mode of Action:
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- Sodium citrate binds to calcium ions, preventing their participation in the coagulation cascade.
- It reversibly binds calcium, meaning that clotting can resume if calcium is reintroduced, making it ideal for coagulation tests where calcium is added back to initiate clotting.
Advantages:
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- Reversible action: Calcium can be added back to initiate the coagulation process for tests, making it ideal for coagulation studies.
- Does not interfere with most biochemical tests or haematology assays.
- Minimizes clotting during apheresis procedures and plasma preparation.
Disadvantages:
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- Dilution effect: Sodium citrate dilutes the blood sample, so a correction factor must be applied in test results, especially if the patient has a high hematocrit.
- Not suitable for routine haematology tests as it alters the size and shape of red blood cells.
- Requires careful anticoagulant-to-blood ratio (usually 1:9) for accurate results, especially in coagulation assays.
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Heparin
Uses:
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- Commonly used in biochemistry and clinical chemistry tests, including blood gas analysis, electrolyte measurement, and hormonal assays.
- Heparinized tubes are used in some cases for cytogenetics, where other anticoagulants may interfere with the viability of cells.
- Used in plasma preparation in some blood banks.
Mode of Action:
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- Heparin inhibits the formation of clots by activating antithrombin, which in turn inactivates thrombin and other clotting factors (especially factor Xa).
- Unlike EDTA or citrate, heparin does not chelate calcium but directly interferes with the coagulation cascade.
Advantages:
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- Does not interfere with many biochemical analyses, including electrolyte testing.
- Fast-acting and preserves sample integrity for immediate testing, making it useful for urgent blood gas or electrolyte tests.
- It can be used for longer-term plasma storage without significantly degrading certain analytes.
Disadvantages:
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- Interferes with haematological analyses, especially in cell counting (it can cause WBC clumping).
- Not ideal for coagulation studies as it directly inhibits clotting factors, making the results unreliable.
- It can sometimes cause fibrin clots if used inappropriately or stored for too long, which can interfere with test results.
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Acid Citrate Dextrose (ACD)
Uses:
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- Used in blood banking for blood donation, apheresis, and platelet preparation.
- Employed in the preservation of blood for transfusion or tissue typing.
- Used to preserve blood for genetic testing and DNA analysis in research settings.
Mode of Action:
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- Citrate binds calcium, preventing blood coagulation.
- Dextrose provides cells energy, helping maintain cell viability during longer storage periods.
Advantages:
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- Maintains cellular integrity for extended periods, making it ideal for blood preservation and transfusion.
- Reduces hemolysis (breakdown of red blood cells), preserving blood for longer durations than other anticoagulants.
- Suitable for cellular assays, including platelet function studies and genetic testing.
Disadvantages:
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- Dilutes the blood sample, requiring volume adjustments in certain tests.
- Not suitable for routine clinical testing or haematological assays, as it alters blood cell morphology over time.
- Not commonly used for coagulation studies due to its action on calcium.
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Sodium Fluoride (with Potassium Oxalate)
Uses:
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- It is used primarily for glucose testing, as it prevents glycolysis (the breakdown of glucose by red blood cells).
- Suitable for lactate and ethanol testing, as it preserves these metabolites for extended periods.
Mode of Action:
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- Sodium fluoride acts as a glycolysis inhibitor, preventing red blood cells from metabolizing glucose.
- Potassium oxalate serves as the anticoagulant by binding calcium and preventing clot formation.
Advantages:
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- Excellent preservation of glucose levels, making it the anticoagulant of choice for tests requiring stable glucose levels over time.
- Suitable for blood samples where ethanol levels need to be preserved, as fluoride inhibits enzymes involved in ethanol metabolism.
- Stable for long periods, making it ideal for samples that cannot be processed immediately.
Disadvantages:
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- Not suitable for haematological studies, as it damages blood cell morphology and interferes with counting.
- Interferes with some enzymatic assays, particularly those involving glycolytic pathways, limiting its use to specific tests like glucose and lactate.
- Potassium oxalate can cause hemolysis, especially with improper storage, affecting the accuracy of some tests.
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Citrate Phosphate Dextrose (CPD)
Uses:
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- Used in blood collection bags for whole blood donation and preservation for transfusion.
- It is often combined with adenine (CPDA-1) to extend the shelf life of stored blood.
Mode of Action:
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- Citrate binds calcium, preventing blood coagulation.
- Phosphate helps maintain pH levels and provides a buffering system.
- Dextrose supplies energy to red blood cells, ensuring their viability during storage.
Advantages:
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- Prolongs red blood cell viability, making it suitable for preserving blood for transfusion for up to 21 days (or 35 days when combined with adenine in CPDA-1).
- Maintains cell integrity, reducing hemolysis and preserving blood for transfusion.
- Stable and widely used in blood banks.
Disadvantages:
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- Not suitable for laboratory testing, as it can interfere with many assays.
- Must be stored under appropriate conditions to ensure the viability of cells.
- Limited duration of preservation (21-35 days), after which the blood must be discarded or processed.