You go to the doctor, and they order some blood tests to see what’s going on inside you. You sit down with a phlebotomist to get some blood drawn, and a week later you get results back, but what happens in between? I hope that I do a good job explaining things – please let me know in the comments if there’s anything I missed or that you’d like to know more about.

The question for today is:

Are the colors of caps consistent across labs? I seriously always just figured it was an internal thing, and the bar codes on the sides were the important parts of communicating info to other labs if blood had to be sent out there.

I’m going to break this into two parts, about tube colors and bar codes, and address each one in a separate post. I’ll start with the colors.

If you’ve ever had lab work drawn, whether at a doctor’s office, a hospital, or an external collection site like LabCorp, you may have noticed that when they take more than one tube of blood, the caps on the tubes are usually different colors. That’s because there are different requirements for how the blood is treated and transported before it gets tested.

 

Cap color is indeed consistent across labs. More accurately, you could say that it’s consistent among the major manufacturers of blood collection tubes. It’s possible that the manufacturers are doing this voluntarily, but I suspect there may be a federal entity like the Food and Drug Administration (FDA) involved, since it could hugely affect patient safety. Even if there is no official rule about cap color in the Code of Federal Regulations, I suspect that someone trying to market an EDTA tube with a green cap would find a very grumpy FDA inspector on their case.

Color coding the blood collection tubes means that blood will always be collected and tested properly for every type of blood test, no matter where it’s drawn and what lab it’s sent to for testing.

 

Now, I know that EDTA (ethylenediaminetetraacetic acid) probably doesn’t mean much to you, yet. I’m here to tell you that it’s an anticoagulant, one of many, used in blood collection tubes. Depending on what test is being run, we want the blood in the tubes to act in different ways. I’ll go over the most commonly used tubes and explain what the cap colors mean and what sort of tests each type is normally used for.

 

Pink or Lavender – EDTA

EDTA tube – from bd.com

 

Both pink and lavender tubes contain EDTA, which is a chemical that binds with and ties up calcium ions. Because blood needs calcium ions present in order to initiate the clotting process, blood that is collected into a pink or lavender top tube (and well mixed) will remain liquid.

Lavender tubes are generally used for complete blood counts (often shortened to CBC), which includes things like white cell and platelet count, and hemoglobin. This makes sense – if we’re trying to do a platelet count, we need the blood to remain liquid, since a clot is going to tie up a bunch of platelets. If a number of blood cells are tied up in a clot and unavailable for the instrument to count, then the count will seem a lot lower than it really is. A CBC is one of the most commonly ordered tests, because it can tell a doctor about infection (high white cell count) or anemia (low red cell count), which are common reasons for doctor visits. You’re very likely to have a lavender tube drawn if you’re getting lab work done.

The main difference between the two colors is that the pink top tubes are generally bigger, and get spun down in a centrifuge to separate the plasma from the cells. The pink tubes are primarily used in the blood bank, because we run tests on both the cell part and the plasma part of the blood. We could use lavender top tubes, but we like to have a bigger volume of specimen to work with, in case we need to start cross-matching blood for the patient, which will use up the plasma. Also, the rules for labeling blood bank specimens are usually more strict, and having a bigger tube leaves more room to write out the patient’s information.

 

Light Blue – Sodium Citrate

Sodium Citrate tubes – from bd.com

Blue top tubes are used primarily in coagulation studies, like monitoring heparin or warfarin therapy, or looking for clotting disorders before a patient goes to surgery. Sodium citrate, like EDTA, also prevents clotting by tying up calcium ions, but it’s better than EDTA in preserving the rest of the blood’s clotting factors. The tubes are always the same size, and contain a set amount of sodium citrate. Most coagulation tests start by adding some calcium back in and seeing how long it takes for the blood to clot, so it’s extremely important to fill the tubes all the way. An underfilled tube will have an excess of sodium citrate, which will tie up some of the calcium the instrument is adding in, which will make it look like the blood’s taking a very long time to clot. If you aren’t on blood thinners, and you’re not showing signs of a clotting disorder, you aren’t likely to see the phlebotomist pull out a blue tube.

 

Light Green – Heparin

Heparin tube with gel separator – from bd.com

These were the most commonly used tubes in the chemistry section of the hospital lab. They’re used for glucose (blood sugar) testing, electrolytes like sodium and potassium, and other important analytes like cholesterol, liver enzymes, and cardiac markers that can indicate a heart attack. This anticoagulant is usually a Lithium-Heparin salt, instead of a potassium or sodium salt, because most basic metabolic profiles (you’ll sometimes hear them called a Chem-7 or Chem-some-other-number on TV medical shows) will measure potassium and sodium, and we don’t want to falsely increase those numbers with our anticoagulant. We also can’t use EDTA, because the calcium would look too low.

Because chemistry testing is focused on the plasma, the blood is centrifuged to get the cells out of the way. Some tubes go an extra step with a built-in gel barrier, which keeps the cell portion trapped below so that even if you invert the tube, the cells stay put. This makes it easier to aliquot the sample (take small volumes from the main tube for other testing) without disturbing the cells and making it necessary to spin the tube again.

 

Gold – Gel Separator, No Additives

Gold tube, no additive – from bd.com

In situations where it’s okay for the blood to be clotted, a gold top tube can be used. It has no anticoagulant, so after the blood has been in the tube for a few minutes, it will form a nearly solid clot. Once spun, the tube will have cells and serum separated by a gel barrier, which makes it easy to pour the serum off into other tubes for separate tests. These tubes are often used when the serum is to be sent to an outside lab for special testing (anything not done at the lab where it’s drawn), because it’s easier to pour the serum into transport tubes for refrigeration or freezing, and they’re a little cheaper because they have no additives.

Many labs use these as their workhorse tubes, accepting them instead of heparin tubes for chemistry workups. I don’t actually know why the lab I was working in used the green tubes for most of the chemistry – a lot depends on the instruments being used in the lab and whether they have specifications for the test material. If your test’s instructions say it needs to be a heparin tube, then that’s what you should use, because the results may not be reliable if you use something else. Also, it’s better for the patient if we can run several tests off of one tube, and not just to keep costs down. Repeated blood draws can lead to bruising, and if excessive, to phlebotomy-induced anemia. So, if you can choose to use either a green or a gold tube for a certain test, because both are allowed by the instrument’s specifications, but another important test in the lab needs to be on a green top, it may make sense to bundle those tests together onto one tube.

Other – Special Cases

Some other colors are out there, but they’re used infrequently and you’re not likely to see them unless you’re having fairly rare tests done. In our lab, dark blue tubes were used detection of heavy metals like copper or lead, because the tubes and interior of the caps were free of trace metals. Red top tubes had no additives, like the gold tubes, but contained no gel separator, so they could be used for some therapeutic drug levels – the gel has a tendency to absorb some drugs over time, so a red top would be more accurate in those cases. Gray tubes were used for lactic acid levels, but some places use them for glucose, because the potassium oxalate anticoagulant in the tubes stops the red cells from using it all up.

While different labs will use the same tubes for different tests, depending on their methodologies, a lavender top tube in one hospital will contain the same additives as one in any doctor’s office. It’s common sense, really – you don’t want a part-time employee who works at two different facilities to get confused and use the wrong tube for a specimen collection, because it’s what he’s used to at the other job. Yes, it gets looked at in the lab, but we’re all human and sometimes a wrong tube can go on an instrument, and since all the instrument does is read a bar code and perform the tests it’s instructed to, you’ll still get a result, and it may be very wrong. Consistency is key to medical and laboratory safety.