On November 1 I replied to a question from Joe Lamb, Columbus, GA about room temperature storage of specimens for partial thromboplastin time (PTT) assays that are sent to reference laboratories. Joe was concerned that his reference lab claimed PTT results were valid on specimens less than 24 hours old, whereas the CLSI Guideline H21-A5 sets the limit at four hours. In follow-up, H21-A5 permits for room-temperature whole-blood storage beyond four hours provided the local institution provides in-house data. This qualifying statement is based upon O’Neill EM, Rowley J, Hansson-Wicher M, et al. Effect on 24-hour whole-blood storage on plasma clotting factors. Transfusion 1999;39:488–91. This reference provides the justification Joe is looking for. Geo.