Introduction
- 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.
Ethylenediaminetetraacetic Acid (EDTA)
Uses:
- 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:
- EDTA works by binding to calcium ions (Ca²⁺), essential for clotting. Without calcium, the coagulation cascade cannot proceed, preventing blood clotting.
Advantages:
- 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:
- 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.
Sodium Citrate
Uses:
- 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:
- 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:
- 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:
- 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.
Heparin
Uses:
- 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:
- 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:
- 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:
- 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.
Acid Citrate Dextrose (ACD)
Uses:
- 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:
- Citrate binds calcium, preventing blood coagulation.
- Dextrose provides cells energy, helping maintain cell viability during longer storage periods.
Advantages:
- 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:
- 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.
Sodium Fluoride
Uses:
- 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:
- 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:
- 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:
- 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.
Citrate Phosphate Dextrose (CPD)
Uses:
- 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:
- 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:
- 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:
- 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.
Clot Activator
Uses:
Used for biochemistry tests (e.g., liver function tests, renal profile, lipid profile)
Serology tests (e.g., HIV, Hepatitis, Dengue antibodies)
Immunological assays (e.g., hormone levels, thyroid function tests, CRP)
Drug level monitoring
Collection of serum for storage or transportation
Mode of Action:
The clot activator (commonly silica particles) promotes rapid clot formation by triggering platelet activation.
Once the blood clots, the sample is centrifuged to separate the serum, which is then used for testing.
Since there is no anticoagulant, the coagulation cascade proceeds naturally but is accelerated by the activator.
Advantages:
Quick clotting enables faster sample processing and reduced turnaround time.
Provides clean serum free from anticoagulants — ideal for most biochemical, immunological, and serological tests.
Does not interfere with most analytes in laboratory tests.
A stable sample if handled correctly (centrifuged promptly).
Disadvantages:
Not suitable for hematology or coagulation tests, as the clotting process removes fibrinogen and other clotting factors.
Once clotted, the sample cannot be re-used for tests requiring plasma or whole blood.
Fibrin strands may remain if the clotting is incomplete or if the sample is not handled properly — potentially interfering with automated analyzers.
Risk of hemolysis if the sample is not centrifuged in time or handled roughly.