Let’s highlight Marianne Thawley’s question that appears in a comment on the March 8 post, Rapid INR Rise in Pneumonia, “What are some causes for decreased PTT results.” In my experience, only technical artifacts shorten the partial thromboplastin time (PTT) to less than the lower limit of the reference interval. Artifacts could include partial activation of the coagulation cascade through a specimen collection error, specimen chilling, excessive incubation with the initial PTT reagent prior to adding CaCl2 solution, incorrect reagent formulation, or wrong incubation temperature. Elevated factor VIII, while it does not shorten the PTT, does render the PTT less sensitive to heparin, thus the assay may underestimate plasma heparin. Does anyone know of another circumstance, in particular, a physiological or pathological circumstance that shortens the PTT? Please add your comment below.