NHA Phlebotomy Certification Practice Exam

Which additive is used in tubes designated for a Complete Blood Count (CBC)?

1. Sodium citrate

2. EDTA (purple tube)

3. Gel separator

4. Heparin

The correct answer is: EDTA (purple tube)

The additive used in tubes designated for a Complete Blood Count (CBC) is EDTA, which is typically contained in purple-top tubes. EDTA, or ethylenediaminetetraacetic acid, is a chelating agent that binds calcium in the blood, effectively preventing clotting. This is crucial for ensuring that the cellular components of the blood remain intact for accurate analysis during the CBC. The choice of EDTA is significant because it preserves the morphology of blood cells, allowing for reliable counting and evaluation. In contrast, other additives might interfere with the assessment of cellular characteristics or lead to clot formation, which would render the sample unsuitable for a CBC. For example, sodium citrate, often used in coagulation studies, functions differently and is not appropriate for a CBC, while gel separators are typically associated with serum separation and are not intended for cellular analysis. Heparin is primarily used for certain chemistry tests and can alter the cellular structure, making it unsuitable for a CBC as well. Thus, EDTA is the clear choice for preserving the integrity of blood cells during the analysis.