Sunday, November 11, 2012

ANTICOAGULANTS AND PRESERVATIVES FOR BLOOD



Chemical agents that prevent coagulation are routinely used when whole blood or plasma is required. Some of the commonly used anticoagulants are:

(1) Heparin         (2) Salts of Ethylene diamine tetra acetic acid (EDTA)
(3) Oxalates        (4) Sodium Fluoride 

Heparin

It is the most widely used anticoagulant and causes least interference with tests. It is available as sodium, potassium, lithium and ammonium salts. This anticoagulant accelerates the action antithrombin III which neutralizes thrombin and prevents conversion of fibrin form fibrinogen. 

It is used in 0.2mg/ml blood. Its disadvantage is high cost and short action, and produces blue background in blood smears. It is believed to inhibit the activity of acid phosphatase and affect binding of T3 and T4 to carrier proteins producing high free concentrations of these hormones.

EDTA

It is a chelating agent useful in hematology as it preserves the cellular component of blood. It is used as disodium, dipotassium or tripotassium salt. It is used as 1-2mg/ml blood. EDTA prevents coagulation by chelating calcium. 

Dry EDTA vials are used. It by chelating metallic cofactors, inhibits alkaline phosphatase, creatine kinase and other enzymes. It is not suitable to use in specimen for calcium and iron analysis.

Sodium fluoride

It is used when blood is collected for glucose estimations. It is a weak anticoagulant but is often added as preservative for blood glucose together with potassium oxalate as anticoagulant. It is effective at a concentration of 2mg/ml blood. It inhibits the enzyme enolase (sodium fluoride) thus inhibiting glycolysis. Na fluoride/K oxalates are mixed in the ratio 1:3.

Citrate

Sodium citrate solution at concentration 3.8g/dl in ratio or 1 part to 9 part of blood is widely used for coagulation studies because its effect is reversible by addition of calcium.

Oxalates

Sodium, potassium, ammonium, and lithium oxalates inhibit blood coagulation by forming insoluble complex with calcium. Potassium oxalate at concentration of 1-2 mg/ml of blood is widely used. Combined ammonium and/or potassium oxalate does not cause shrinkage of erythrocytes. It consists of three parts by weight of ammonium oxalate, which causes swelling of the erythrocytes, balanced by two parts of potassium oxalate which causes shrinkage. NH4+ & K+ oxalate mixture in the ratio of 3:2, and 2 mg / ml of blood is the required amount.


The disadvantage of the use of oxalate is the alteration of concentrations of plasma components.  Shrinkage of erythrocytes results from a water shift from the erythrocytes to plasma. This shift increases with increasing anticoagulant concentration, and if used in the same concentration on a weight basis, all anticoagulants will have this effect inversely proportional to their molecular weight.  Aside from the water shift there may be alteration of erythrocyte permeability, which may explain the varied and inconsistent effects of oxalates and other salt anticoagulants on certain plasma constituents.  Because of the difficulty, at times, in obtaining satisfactory preparation of heparin commercially, Heller and Paul introduced in 1934, a balanced oxalate mixture for use in hematocrit and sedimentation rate determinations.  It consists of three parts by weight of ammonium oxalate, which causes swelling of the erythrocytes, balanced by two parts of potassium oxalate which causes shrinkage.

Iodoacetate

Sodium iodoacetate at concentration of 2mg/ml is an effective antiglycolytic agent and a substitute for sodium fluoride.  It is a potent (suicide) inhibitor of G3P dehydrogenase
Formalin, toluene, 6N HCl, Boric acid, thymol, chloroform are used as urine preservatives for 24 hour urine. 
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