Biological variability is an important aspect for appropriate interpretation of hemostasis testing, but unreliable evidence has been provided to date regarding the effect of different postures on first-line clotting tests such as activated partial thromboplastin time (APTT), prothrombin time (PT) and fibrinogen. Variation of APTT, PT and fibrinogen was therefore investigated in 19 healthy volunteers after 25 min in supine position, 20 min in sitting position and 20 min stationary standing in upright position. The change from supine to sitting position caused a significant decrease of PT and a significant increase of fibrinogen. The variation of fibrinogen exceeded the quality specifications for bias. The change from supine to standing position caused a significant bias in all the parameters tested, with a decrease of PT and an increase of fibrinogen that exceeded the quality specifications for bias. The change from sitting to standing position also caused a significant bias in all the parameters tested, but none of these variations was greater than the quality specifications for bias. The results of this investigation show that patient posture may have a significant impact on results of routine hemostasis testing, especially when patient position is changed from supine to standing.