The APTT is elevated in the newborn compared to the adult reference range. At least in part this is due to reduction of 30%-50% in activity of factors XI, XII, HMWK, and Fleher Factor. Activity is even lower in premature infants. Adult values are reached in 3-6 months. Vitamin K–dependent factors are reduced to 20%-60% of adult reference range at birth. Antithrombin III and Protein C are also reduced. If the infant’s hematocrit level is very high (over 65%), the specimen plasma volume is reduced relative to the amount of anticoagulant in the tube and elevated APTT or PT may result.