Drawing blood for laboratory analysis requires strict standardization to ensure the collected sample accurately reflects a patient’s condition. Clinical laboratories rely on a standardized system of color-coded tubes, where the color of the stopper indicates the specific additive or anticoagulant inside. Selecting the correct tube is foundational to obtaining reliable results. The Complete Blood Count (CBC) is a frequently ordered hematology panel that provides a broad overview of a patient’s blood cell health. This test requires a specific type of tube to maintain the integrity of the blood cells for accurate counting and analysis.
Identifying the Correct Tube
The Complete Blood Count (CBC) with Differential requires blood to be collected into the Lavender (or Purple) top tube. The color of this stopper signals the presence of the required anticoagulant, Ethylenediaminetetraacetic acid (EDTA). This chemical is necessary for hematology testing because it preserves the cellular components of the blood sample. The EDTA is typically present as a potassium salt, dried onto the inner wall of the collection tube. Using any other color tube would compromise the sample, leading to inaccurate measurements.
Mechanism of the Anticoagulant
EDTA functions as an anticoagulant by a process called chelation, which involves binding to calcium ions (\(\text{Ca}^{2+}\)) in the blood. Blood coagulation is a complex cascade of reactions that relies on the presence of free calcium ions to act as a necessary cofactor. When blood enters the lavender tube, the EDTA rapidly sequesters these calcium ions, forming a stable chemical complex. By isolating the calcium, the EDTA effectively halts the entire clotting cascade, preventing the blood from coagulating inside the tube. This mechanism is necessary because clotting would physically trap and distort the blood cells, making it impossible for automated hematology analyzers to accurately count and differentiate the cells.
Components of the CBC with Differential
The Complete Blood Count test provides quantitative information on the three main cellular components of blood: Red Blood Cells (RBCs), total White Blood Cells (WBCs), and Platelets. The test also measures related parameters such as Hemoglobin, the oxygen-carrying protein, and Hematocrit (the percentage of blood volume occupied by red cells). The “Differential” component expands on the White Blood Cell count by analyzing and quantifying the five specific types of leukocytes:
- Neutrophils
- Lymphocytes
- Monocytes
- Eosinophils
- Basophils
Furthermore, the test generates red cell indices, which are calculated measurements like Mean Corpuscular Volume (MCV) and Mean Corpuscular Hemoglobin (MCH). These indices provide insight into the size, color, and hemoglobin content of the red cells, aiding in the diagnosis of conditions like anemia.
Preventing Collection Errors
The reliability of the CBC result depends significantly on following precise collection procedures to prevent sample contamination and degradation. Phlebotomists must adhere to a standardized Order of Draw when collecting multiple tubes, which dictates the sequence in which tubes are filled. The EDTA tube is typically collected toward the end of the draw sequence to prevent the potassium-rich additive from contaminating tubes used for chemistry tests. Once the blood is collected, the tube must be gently inverted eight to ten times to ensure the liquid blood fully contacts and mixes with the dried EDTA anticoagulant. Insufficient mixing can result in the formation of microscopic clots, which will lead to the rejection of the sample by the laboratory. Another potential error is under-filling the tube, which alters the carefully calibrated ratio of blood to anticoagulant, leading to inaccurate measurements of the Mean Cell Volume and Hematocrit.