Blood drawing is a common and fundamental procedure in healthcare, providing essential samples for diagnostic testing. These tests offer valuable insights into a patient’s health status, helping medical professionals diagnose conditions, monitor diseases, and guide treatment decisions.
Understanding the Importance of Sequence
The specific order in which blood collection tubes are filled, known as the “order of draw,” is a standardized practice. This sequence preserves sample integrity and ensures accurate test results by preventing unintended contamination.
A primary reason for this precise order is to prevent additive carryover between tubes. Blood collection tubes contain various additives, such as anticoagulants or clot activators, which are crucial for the specific tests performed. If an additive-containing tube is drawn before a tube that should not have that additive, a small amount can be carried over by the needle, compromising its contents. For instance, anticoagulants like EDTA or heparin, if carried over, can interfere with clotting or alter electrolyte levels in other tubes.
Another significant consideration is minimizing microbial contamination. Blood cultures, used to detect bloodstream infections, must be collected first. This requires a sterile process to prevent contamination from skin flora or other tube additives, reducing the risk of false-positive results and ensuring accurate detection of infections.
The Standard Blood Collection Order
Following a universally accepted sequence for blood collection tubes maintains specimen quality. This standardized order minimizes potential additive carryover. The typical order involves different tube types, each identified by stopper color, unique additives, and designated tests.
Blood Culture Bottles
The process begins with blood culture bottles, which contain a nutrient broth to promote bacterial growth. These are drawn first to maintain sterility and detect bloodstream infections.
Light Blue Top Tube
Next in sequence is the light blue top tube, containing sodium citrate. This anticoagulant binds to calcium, preventing clotting. It is primarily used for coagulation studies, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT).
Serum Tubes
Following this, serum tubes are collected, typically identified by a red or gold stopper. Red top tubes often contain no additive or a clot activator like silica, while gold or speckled tubes usually have a clot activator and a gel separator. These tubes are used for a wide range of chemistry, immunology, and serology tests, where serum is required.
Green Top Tubes
After serum tubes, green top tubes are drawn, which contain heparin as an anticoagulant. Heparin inhibits thrombin, an enzyme involved in clotting, keeping the blood in its plasma state. These tubes are commonly used for general chemistry tests, especially when rapid results are needed.
Lavender or Purple Top Tube
The lavender or purple top tube is next, containing ethylenediaminetetraacetic acid (EDTA), a strong anticoagulant that prevents clotting by binding to calcium. This tube is primarily used for hematology tests, such as a complete blood count (CBC).
Gray Top Tube
Finally, the gray top tube is drawn, which typically contains sodium fluoride and potassium oxalate. Sodium fluoride acts as an antiglycolytic agent, preserving glucose levels, while potassium oxalate is an anticoagulant. These tubes are used for glucose testing, lactate levels, and blood alcohol levels.
Potential Issues from Incorrect Order
Deviating from the correct order of draw compromises laboratory results and, consequently, patient care. The primary concern is sample contamination from additives in tubes collected earlier in an incorrect sequence.
Additive carryover can cause inaccurate test results, leading to falsely elevated or decreased values for various analytes. For instance, if blood is drawn into an EDTA tube before others, EDTA can contaminate subsequent samples, causing falsely high potassium or falsely low calcium and magnesium levels. Similarly, clot activator carryover can interfere with coagulation tests, producing inaccurate clotting times.
Inaccurate results from an incorrect draw order can lead to patient misdiagnosis and inappropriate treatment. A false positive for an infection or an incorrect electrolyte reading might prompt unnecessary or delayed medical interventions, directly impacting patient safety. Such errors also necessitate inconvenient and uncomfortable re-draws for the patient, in addition to incurring extra costs.