Which Tubes to Use?
Most facilities are using blue top tubes, which contain 3.2% sodium citrate. The reasons for this are many and include the fact that this concentration provides a closer osmolality to plasma, has less binding of calcium, and provides a more favorable environment for heparinized samples.
Many variables affect coagulation results such as medication, physical and emotional stress, and patient age and personal habits. These factors cannot be controlled by the laboratory. A patient's hematocrit, however, is something that can be adjusted for when drawing a coagulation sample. For patients who have hematocrits that are >60% (neonated, polycythemia), falsely prolonged results will occur if the anticoagulant is not adjusted, since there is too much anticoagulant for plasma. For patients who have hematocrits of less than 22%, results will be falsely decreased as a result of too little anticoagulant because of increasing plasma volume. The standard formula for adjusting the volume of anticoagulant is:
New volume of sodium citrate = (1.85 X 10)-3 X (100 - Hct) X volume of sample
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