Sample Volume

A frequent but preventable pre-analytical error is the under-filling and less frequently overfilling of EDTA or citrate tubes with blood. For example, excess EDTA (due to under-filling with blood) will alter the red blood cell indices, typically resulting in low MCV and/or high MCHC. Under-filling of citrate tubes prolong times of clotting assays. Overfilling the tubes, on the other hand, will result in clotting of the specimen. Therefore, the person collecting the sample should always attempt to fill the tubes at the appropriate ratio to avoid inaccurate results. This is typically accomplished by filling the tubes to the indicated mark on the collection tube. Additionally, there are different sizes of tubes available, including EDTA and citrate tubes that only require about 0.5 mL (500 uL) of blood. Maintaining the appropriate ratio of blood to anticoagulant is especially important for hemostasis testing!