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July 2023 Quick Question Summary: Frozen Specimens

Our July 2023 Quick Question, How long may platelet-poor plasma be stored at –20°C? drew 52 respondents. Here are our results:

  1. 4 hours: 4 [8%]
  2. 24 hours: 4 [8%]
  3. 2 weeks: 21 [40%]
  4. 2 months: 15 [29%]
  5. 12 months: 8 [15%]

This question arose in response to our lengthy June 15, 2023 post, Freezing Coagulation Specimens. Using the 2008 CLSI H21-A5, the first answer, “4 hours” is the time limit for non-heparin PTT citrated specimens, centrifuged or uncentrifuged, stored at ambient temperature, 18–25ºC, and not chilled.

Specimens for the PTT assay suspected of containing therapeutic unfractionated heparin [UFH] should be centrifuged immediately after collection and the plasma tested within four hours. This is necessary since platelet factor 4 is secreted ex vivo. rapidly neutralizing UFH.  Neutralization interferes with both PTT and chromogenic anti-Xa UFH measurement. If the therapeutic UFH status of a specimen is unknown, laboratory scientists follow the second guideline.

Answer 2, “24 hours” is the storage limit for centrifuged or uncentrifuged citrated specimens designated for PT assays only. For both PT and PTT specimens, the tube should be upright and held at ambient temperature.

Answer 3, “2 weeks” is the current H21-A5 limit, so it is the correct answer to the question for now. As noted in the June 15 post, we anticipate an imminent update to a new H21-A6 guideline, which may extend the limit to 2 or 3 months. As stipulated in the June 15 post, the freezer must not be self-defrosting, as temperatue fluctuations will affect results.

Answer 4, “2 months” could be the new limit based on recently published data plus the Woodhams article cited in the June 15 entry, Woodhams B, Girardot O, Blanco MJ, Colesse G, Gourmelin Y. Stability of coagulation proteins in frozen plasma. Blood Coagul Fibrinolysis. 2001;12:229–36..

Answer 5, “12 months” is the H21-A5 limit for citrated specimen storage at -70°C. This, too, may be expanded by the new guideline. Watch for an announcement here in Fritsma Factor as soon as the new guideline is released, perhaps in September 2023.

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