AIIMS November 2011 – MCQ 133

The anticoagulant of choice for coagulation testing is sodium citrate, which reversibly chelates calcium. Evacuated blue-top tubes containing sodium citrate are available commercially with a 3.8% (129 mmol/L) or3.2% (109 mmol/L) citrate concentration. Blue-stoppered tubes are manufactured to draw nine parts of whole blood to one part of liquid sodium citrate already present in the tube. Thus, when using an evacuated system, blood must be allowed to flow into the tube until it stops automatically. This provides for the 90% fill ratio required for coagulation testing. Ideally the plasma specimen is tested within four hours of collection. If this is not possible then the samples should be frozen until testing. Gross hemolysis is usually a criterion for sample rejection