Swirling is an attempt to assess donated platelets for the presence of a moving, varied color appearance when backlit. The principle behind swirling is that viable platelets in an “unactivated” state have a discoid appearance, and that shape causes light to be scattered in multiple different directions (this is really pretty cool-looking; check out this cool YouTube video). Platelets that are activated or are in a low pH environment, on the other hand, lose their discoid shape, and lose their light-scattering abilities. Swirling was initially touted as a simple, non-invasive method for assessing platelet viability due to its correlation with pH values (Bertolini F and Murphy S, A multicenter inspection of the swirling phenomenon in platelet concentrates prepared in routine practice. Transfusion 1996;36:128-132). However, it isn’t a great method (especially in inexperienced hands). Swirling is subjective by definition, lacks sensitivity for anything other than pH (it certainly is not a reliable method to detect bacterial contamination of platelet products), and we aren’t really sure that in-vitro assessments really correlate with transfusion efficacy. At this point, no one test can answer or predict the clinical response to a platelet transfusion, but since swirling is cheap and simple to understand, most facilities still evaluate platelet products in this way.

Written by Dr. Tuan Le in 2012; updated by Joe Chaffin in 2016