Manufacturers of evacuated blood collection tubes have provided a color-coding system to guide phlebotomists in identifying the appropriate tube when preparing to collect blood samples. The pre-analytical variability related to tube selection is broadly classified into two sources:
- Anticoagulant
- Gel barriers
The use of an incorrect anticoagulant can contribute directly to invalid laboratory results. For example, a plasma sample obtained from a lavender-top tube containing potassium-EDTA, would yield a falsely elevated potassium level, as well as a falsely decreased calcium level. Phlebotomists may also minimize the variability associated with the use of anticoagulants by allowing the tubes containing anticoagulants to fill to their intended volume. Underfilling tubes, either intentionally or due to interrupted flow, may lead to a higher ratio of anticoagulant to blood. In the case of plasma-based coagulation tests, the blood/anticoagulant ratio must be 9:1, which occurs when a 90% fill volume is achieved. If the fill volume is less than 90%, inaccurate test results may be obtained.
The use of gel-based serum/plasma separator tubes may be preferred to provide an inert barrier between the blood cells and the yielded serum/plasma, avoiding the continued metabolism of viable cells in the collected sample and increasing the stability of many analytes. However, certain small molecules (eg, therapeutic drugs and some specific proteins) may be trapped within the gel layer, which may yield falsely low levels. Laboratories should always evaluate the use of gel-based serum or plasma separation tubes for their entire menu of offerings prior to implementation, and consult with the tube manufacturers for literature on known interferences.
