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Background: Few published data exist to guide interpretation of coagulation times in extremely premature infants., Objective: To determine coagulation reference ranges on day 1 of life in extremely premature infants., Methods: A retrospective review of day 1 coagulation tests was performed in 144 infants <27 weeks' gestation between 2004 and 2010 in a tertiary neonatal unit. Samples were drawn through a non-heparinized umbilical or peripheral venous catheter as part of routine clinical care., Results: Mean (SD) and median (range) prothrombin times (PT) of 21.5 (5.3) and 20.2 (13.3-39) s, respectively, activated partial thromboplastin times (APTT) of 75.2 (27.8) and 67.4 (34.9-191.6) s, respectively, and plasma fibrinogen levels of 1.9 (1.1) and 1.4 (0.5-4.8) g/l, respectively, were reported. Using reference intervals derived from the 2.5th to 97.5th centiles, ranges of 14.4-36.7 s, 40.5-158.5 s and 0.7-4.8 g/l were determined for PT, APTT and plasma fibrinogen levels, respectively. In a subcohort with grade 0-2 intraventricular haemorrhage (n = 92), mean PT and APTT were 20.9 and 71.3 s, respectively, versus mean PT and APTT of 23.1 and 88.4 s (p = 0.06 and p = 0.03), respectively for those with grade 3-4 intraventricular haemorrhage. Mean PT and APTT in a cohort of infants defined to be small for gestational age were 22 and 76.8 s. These results did not differ significantly from non-small for gestational age infants, with a mean PT and APTT of 19.5 and 73.4 s (p = 0.09 and p = 0.7)., Conclusions: Reference ranges based on retrospective data were determined for PT, APTT and fibrinogen in a large cohort of extremely preterm infants., (© 2013 S. Karger AG, Basel.)