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This study assessed the current practices of heparin dosing and determined the extent of therapeutic activated partial thromboplastin times (aPTT) achieved, utilising a standard heparin nomogram in the coronary care unit and step-down cardiac ward of a health care facility in South Australia. The study also examined the effect of actual body weight (ABW), body mass index (BMI), smoking, concomitant intravenous glycerine trinitrate, age, gender and creatinine levels, individually, on the time taken to attain a therapeutic aPTT in acute coronary syndrome (ACS) patients receiving a heparin infusion. A retrospective correlational research design was utilised to include the collection of quantitative data from 66 men and women of all ages and background consecutively admitted into the coronary care unit and the step-down cardiac ward and receiving a continuous heparin infusion. The quantitative data included demographic details plus all information regarding treatment and results of heparin therapy from the patients' medical records. Descriptive analysis of the data revealed 32% of the participants attained a therapeutic level with the first aPTT taken and that, successively, 35% and 45% of the participants attained a therapeutic level with the second and third aPTT taken. The majority of participants were found to be outside of the therapeutic range at any one time during the study. A generalised linear model (log-binomial model) applied to the data revealed that increased ABW (p=0.002), creatinine levels (p=0.033) and, in particular, BMI (p=0.000) were significant risk factors that contributed to the failure of participants attaining a therapeutic aPTT level. Age (p=0.668), gender (p=0.623), smoking (p=0.993) and the use of concomitant intravenous glycerine trinitrate (p=0.897) did not have a significant effect on the time to reach a therapeutic aPTT. The results provide noteworthy information for the re-evaluation of the use of the standard heparin nomogram. A robust randomised clinical trial is required to further examine BMI as the best predictor for heparin requirements in ACS patients receiving a heparin infusion.