Laboratory professionals are comfortable with blood; after all, without it we wouldn’t be able to perform many of the tests necessary for delivering quality healthcare to the people in our communities. We work with blood every day, often taking it from tubes lined with various kinds of anticoagulants to keep it from clotting, depending on the testing that we perform. But some of our patients already have trouble with clotting, due to bleeding and coagulation disorders. March is recognized as Bleeding Disorders Awareness Month, and the Hemophilia Federation of America is raising awareness on social media and educating the general public about the impact of these disorders.
Diagnosis and treatment of bleeding disorders, as with so many other disorders, begins in the lab. A patient who bleeds profusely from small injuries, or who is at familial risk for bleeding or clotting disorders, is sent for a blood draw and a coagulation workup. Basic coagulation testing usually starts with the collection of a blue-top tube, which is then spun down in a centrifuge to get clear plasma. The spun plasma is then put through the PT and APTT tests: protime and activated partial thromboplastin time, which are used as an initial assessment of a patient’s risk for bleeding. Results from these tests can lead to more complex coagulation factor testing or genetic testing, or to help guide anticoagulant therapy for patients who are at risk of blood clots.
Blue top tubes contain a small amount of sodium citrate anticoagulant, usually in a concentration of 3.2%. The sodium citrate keeps the blood from clotting by binding to (chelating) the blood’s calcium ions, preventing them from triggering the chemical cascades of the clotting process. The PT and APTT tests measure clotting time after adding calcium ions back into the plasma and allowing the cascades to resume. An underfilled tube means that there’s excess citrate in the sample, which will tie up some of that new calcium coming in: that results in extended PT and APTT times. Any variation in the ratio of blood to anticoagulant will impair the accuracy of the testing.
Most laboratory tests can be run on an underfilled tube in less-than-ideal situations, even if it means pouring off the tiny amount of available plasma into an aliquot tube. But blue tops need to be filled all the way, much to the dismay of the phlebotomist sitting across from a difficult vein. It’s critical that they be filled to within 10% of the recommended fill line in order to avoid erroneous results and risk to patient safety.
What measures can be taken to ensure that your lab’s coagulation results come from accurately collected specimens?
- Avoid using winged blood collection sets (also called “butterfly needles”) for coagulation draws if possible. The dead space in the tubing will mean less blood is drawn into the collection tube. If a winged set is necessary, a plain discard tube should always be drawn first in order to “prime” the line, and then the sodium citrate tube can be drawn.
- Check expiration dates carefully: over time, the vacuum in collection tubes can become weaker, resulting in underfilling. Many manufacturers have shortened the shelf-life of their sodium citrate tubes as a result, since the fill is so critical. Make sure to only order a small amount of these tubes at a time, so that you can be sure to use them up long before their expiration date approaches.
- Keep a comparison tube handy, so that you can check a freshly-filled tube against the standard and see how it measures up. Pop the top off an empty tube, add water to the faintly-etched fill line, and then add some ink or food coloring to make it easier to see. Both the blood draw area and coagulation testing area should have a comparison tube available to check the fill and assess specimen quality. Reject any that don’t make the grade.
Most importantly, if you’re having a hard time getting a full blue-top tube from a patient, never try to cover up the problem by opening up the stoppers and adding blood from one short sodium citrate tube to another. While that may bring the blood up to the required fill level, you’ll be adding in excess anticoagulant from two different tubes, making the situation that much worse. There are smaller tubes available for pediatric and geriatric patients, and these may be a good option for labs whose patient population includes many “difficult sticks.”