Platelet Clumping

Friend and Colleague Dennis Ernst, Center for Phlebotomy Education, shared this January 8, 2021 question with George:

Hello Phlebotomy Today, We’ve come across a patient whose platelets clumped in EDTA and citrate tubes. We understand that ammonium oxalate may be used to provide an accurate platelet count. Do you know of a manufacturer of these tubes? Thanks very much. Cynthia.Gerger-Salakar, MPM, MT [ASCP], Quality Coordinator.

George responded, Hello, Dennis and Cynthia, and thanks for your question. For assistance, I conferred with hemostasis experts Dave McGlasson and Larry Brace, PhD. We’ve seen platelet satellitosis [but not clumping] that persisted in 3.2% sodium citrate tubes. In this case we used heparin tubes to get an accurate platelet count, although we didn’t report the remaining heme parameters from that tube. With heparin you have to do the count right away, as heparin itself can cause platelet clumping.
We know of no company that markets an ammonium oxalate tube, so you will have to make your own. I found several companies that markets an ammonium oxalate solution for blood collection, they appear to recommend a 3.5% solution.

My friends and I have been around long enough to recall when we collected capillary blood and mixed it with 1% ammonium oxalate for manual platelet counts on hemocytometers, but that may be a lost art. We would then make blood films from the finger sticks and perform platelet estimates from the film to roughly confirm the platelet count. Neither approach was extremely accurate. I also recall using sodium oxalate tubes with black stoppers early in my career, but I don’t see those in any venipuncture equipment catalogue. They worked well for coag testing, but didn’t preserve the specimen as well as citrate and were gradually replaced in the 1980s.

Participants, please help out if you know of any sources for ammonium oxalate tubes or another way to manage platelet clumping.