“Order of draw” in phlebotomy is a system of collecting more than one tube of blood at the same time from a patient while reducing instances of cross-contamination. Contamination may occur when the syringe contacts microorganisms, additives or blood mixed with additives in previous test tubes. Using this method, the phlebotomist can also identify stages in the testing process where issues occur, says the PJA School.
Phlebotomists use syringes to collect blood from patients and to dispense the blood into color-coded test tubes. Filling the collection tubes from the bottom up, phlebotomists using the “order of draw” method avoid touching the contents of the collection tubes while filling them. The recommended order of draw starts with a sterile tube, followed by coagulation and serum tubes. Next, the phlebotomist fills heparin and EDTA tubes with blood, followed by the glycolytic inhibitor tube. Any change in this order allows sample cross-contamination, states the PJA School.
Microorganism transfer occurs in tubes that have an issue with the sterility of the material. The sterile tube is the first in the series and if contamination is present there, the phlebotomist should question the validity of the entire test battery. The most likely source of cross-contamination is the test tube containing the additive EDTA, which is the lavender-, pink- or purple-colored test tube. The least likely opportunity for cross-contamination exists in the heparin tubes, which have green tops, states the PJA School.