Have you ever wondered about the rainbow assortment of test tubes that the lab uses for blood draws? I remember the time my rheumatologist marked twenty-five different boxes on the lab order slip, and the
vampire phlebotomist took forever counting out specific numbers of different colored tubes. That was when I realized that color matters. Every color stands for a specific additive, so it’s important to get the right one.
Test tubes usually have additives to make blood behave in specific ways until it can be tested. Some additives cause blood to clot, others prevent it from clotting. Different chemicals are used for different lab tests.
Tests requiring serum are drawn into a test tube containing a clot activator. The tube will then be centrifuged, and the clot activator ensures that it’s serum (instead of plasma) at the top of the tube. Depending on the lab, these are generally gold-topped (or plastic red-topped) test tubes (although one lab tech claims that green can sometimes be used, depending on the specific test ordered). Part of the difference is the lab’s machinery and preferences.
Notice that the first thing the phlebotomist does after a draw is to invert the test tube. This is to make sure that all the chemical on the tube’s stopper is mixed into the blood.
Other colors of tubes contain an anticoagulant to stabilize different blood components so that tests can be run later. Since those different colors indicate different additives for different tests, and since it’s possible for small amounts of additive from one tube to carry over to the next when tubes are switched, the order in which tubes are drawn is important. Otherwise, the blood sample could be contaminated and the test results wouldn’t be valid.
Before labs were computerized, the person doing the blood draw had a book to reference so she’d know which color of tube to use. Now many labs have computers which will print labels stating the patient’s name and date of birth; those labels can also indicate which type of tube should be used to obtain the sample.
According to the Clinical and Laboratory Standards Institute, order of draw is:
- Sterile Blood Cultures are drawn first
- Light Blue – for coagulation tests (ie when a person is taking blood thinners)
- Gold (or Red plastic tubes*) – for tests needing serum instead of whole blood or plasma**
- Green – for some of the chemistry tests
- Purple/Violet– hematology tests (CBC, ESR…)
- Grey– for blood sugar tests
Some labs vary the sequence slightly (ie gold/red before blue), but this is CLSI’s order of draw. Some people, unfortunately, pay no attention to the importance of order, and bounce back and forth between different colors of tubes. When that happens, it might be worth taking the time to ask some questions or find a different lab.
My rheumatologist usually orders tests for gold, green, and purple test tubes. Have you noticed what yours orders?
*Red-topped glass test tubes don’t have any additives and are used for different tests (ie antibody or drug tests) and are a different place in the draw sequence
**Some labs use red/black tiger-striped tops for serum collection. STAT serum collection can be drawn into tubes with orange or yellow/grey tiger-striped tops. The tubes that a lab has on-hand will depend on the types of tests that they normally run.