Color Coded Test Tubes

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.



As was mentioned in previous posts (here and here), whole blood can go into a test tube and then be separated into layers with cells in the bottom portion of the tube and plasma above.  It turns out that although some lab tests require whole blood or plasma, others need only blood serum (pronouced how rare steaks can be cooked – just sear ’em).

Serum is the result when clotting proteins are removed from plasma.

When a clot activator is added to the test tube in which blood is drawn, the sample can be centrifuged, leaving only serum at the top of the tube.  This serum can then be poured off and tested.

Still trying to break information down into bite-sized chunks.

Blood Cells Aren’t Boring

I remember my parents giving me a microscope for Christmas when I was six years old.  It was fascinating to look at hair, thread, grass, and anything else I could think of mounting on a slide to get a closer look.  What an astounding world!

As I’ve been searching for information about RA, I’ve become quite interested in blood.  Even better, since one of my children is studying anatomy & physiology for science this year, we ordered a bunch of slides and I get to investigate these things with my kid.  I’m amazed at how much more is known about blood now than when I was in school (back when dinosaurs roamed the earth).

Recall that blood components can be separated in a test tube, with watery plasma at the top of the tube, and different types of blood cells beneath.  It turns out that the plasma and blood cells can be investigated much more closely – and learning about them makes all those blood tests the doctor orders a lot more comprehensible!


The major types of blood cells were oh-so-creatively named based on their appearance.  Can you guess what color white cells and red cells appear to be when separated from plasma?

The third main cell type is platelet cells.  These look nothing like plates under the powerful microscopes used today, but in times past they were thought to look like small plates.  Platelets are also called thrombocytes.

Scientists seem determined to give everything both a common name and a scientific name – think about dog/canine and cat/feline.  Blood cells, too, have multiple names.  If I tell you that the Greek word for white is leukos, the Greek word for red is erythros, and the Greek word for clot is thrombos, I’m guessing you won’t have any trouble matching the common and scientific cell names (and will know which kind of cell helps blood to clot).

One of the things done in a CBC is to count the number of white cells, the number of red cells, and the number of platelets.  There’s a range of what’s considered a normal number of all these types of cells; anything far outside of the normal range tells the doctor that there’s something wrong.


Plasma, too, can be analyzed to determine the quantity of its various components.  If you read your lab results when the doctor orders bloodwork, you’re familiar with some of these words.

 To prevent posts from getting too long and overwhelming, I’ll periodically add short little bits of information about blood & blood tests.