Clinical Protocol: The Order of Draw and Cross-Contamination
One of the most critical operational procedures in phlebotomy is the Order of Draw. Because a single needle is often used to fill multiple tubes, there is a risk of "carryover," where additives from one tube contaminate the next. For example, if EDTA (which binds calcium) is carried into a tube meant for calcium testing, the results will be falsely low.
To prevent this, international standards (CLSI) dictate a specific sequence:
Blood Culture (Sterile): Collected first to prevent microbial contamination.
Sodium Citrate (Light Blue): For coagulation studies.
Serum Tubes (Red/Gold): With or without clot activators.
Heparin (Green): For plasma chemistry.
EDTA (Lavender): For hematology.
Glycolytic Inhibitors (Gray): For glucose testing.
Following this sequence is mandatory to ensure that the chemical environment of each tube remains pure and the diagnostic data is reliable.