The choice of a venous access system to provide safe blood collection and reliable analytical results for that sample is of paramount importance in any accident and emergency department. The objective of this study was to identify the factors associated with haemolysis in venous blood samples, where the variables studied were: type of venipuncture (needle and catheter), type of catheter (3 catheters of 3 different materials) and diameter of the catheter. The sample was obtained from all patients who required a blood test in the accident and emergencies department of the Virgen del Camino Hospital over 34 days, collected in 3 different periods (September-November), involving a total of 1.933 procedures. Positive haemolysis determined by laboratory technicians was found in 2% (7/348) of samples obtained by needle compared to 14% (222/1585) obtained by catheter. We observe an 8% (39/475) of haemolysis in the samples taken by protective Teflon catheter, 18% (77/426) by Protectiv plus polyurethane and 15% (106/684) by BD-Nexiva Vialone. The haemolysis index fell with an increase in the size of the catheter, those of 18G showing 13% (115/867) and those of 20G showing 15% (107/708). The combination of catheter type and size maintains the smallest percentages of haemolysis in Teflon catheters and high diameters of 18G with 6% (19/301), less than half the haemolysis of the polyurethane catheters and a third of that for Vialone catheters respectively.