Blood collection tubes are specialized containers used in phlebotomy to collect and prepare blood samples for diagnostic testing. They feature a color-coding system on their caps, which distinguishes the specific additives they contain. Each additive preserves the blood sample for various laboratory analyses. This system helps healthcare professionals select the appropriate tube for required tests.
The Purple Tube’s Unique Additive
The purple-topped blood collection tube contains Ethylenediaminetetraacetic acid (EDTA). This chemical acts as an anticoagulant, preventing the blood sample from clotting. EDTA achieves this by binding to calcium ions in the blood, which are essential for the coagulation cascade.
By chelating these calcium ions, EDTA halts the clotting process, maintaining the blood in a liquid, whole-blood state. This mechanism is important for tests requiring intact cellular components. The purple or lavender cap universally signifies EDTA, ensuring the sample is correctly preserved for analyses where cellular integrity is important.
Key Diagnostic Applications
The purple tube, containing EDTA, is essential for laboratory tests requiring whole blood samples with preserved cellular components. A primary application is the Complete Blood Count (CBC), which measures red blood cells, white blood cells, and platelets. EDTA prevents clotting, ensuring accurate cell counts and morphology, vital for diagnosing conditions like anemia, infections, and blood disorders.
Another application is the Erythrocyte Sedimentation Rate (ESR), which measures how quickly red blood cells settle, indicating inflammation. While some ESR tests use citrate, EDTA is also a common anticoagulant for this procedure. The purple tube is also used for blood typing and cross-matching in blood banking, where preserving red blood cell integrity is important for safe transfusions.
Proper Handling and Significance
Correct handling of the purple-topped tube is important for obtaining accurate and reliable test results. An important aspect is ensuring the proper fill volume; underfilling or overfilling the tube can alter the critical blood-to-anticoagulant ratio, leading to inaccurate results. For instance, an underfilled tube can cause red blood cells to shrink due to an excess of EDTA, affecting measurements like hematocrit and mean cell volume.
After blood collection, gentle inversion of the tube, typically 8 to 10 times, is necessary to thoroughly mix the blood with the EDTA additive without causing hemolysis, which is the rupture of red blood cells. Vigorous shaking can damage cells and compromise the sample’s integrity. Adhering to standardized clinical protocols, such as those provided by the Clinical and Laboratory Standards Institute (CLSI), is important to prevent errors like sample rejection or misdiagnosis. These guidelines help ensure that the sample remains viable and accurately reflects the patient’s condition.