Introduction: An arterial blood gas (ABG) is a blood test that is performed using blood from an artery. When obtaining an arterial blood sample via percutaneous puncture, there is a risk of accidentally obtaining venous blood. Conventional methods of confirming arterial blood at the bedside such as blood colour and pulsatile return can be misleading in patients with low blood pressure or hypoxaemia. The blood of patients with hypoxaemia can show a dark colour similar to venous blood and patients with low blood pressure may have very low pulsatile action. Aim: The purpose of this study was to determine if the arterial sampler filling time can be an accurate predictor of obtaining arterial blood sample in adults. Methodology: Forty patients were enrolled prospectively who required arterial blood sample or venous sample in medical and surgical intensive care unit. During the arterial and venous puncture procedures the amount of time it took to fill the sample tube was measured with the help of stop watch in s/ml was measured. Results: Twenty patients were in the arterial group and 20 patients in venous group. The mean ± SD filling time was 12 ± 3 s/ml for the arterial group and 112±21 s/ml for the venous group. Conclusion: There is a statistically significant difference between arterial and venous filling times using an arterial blood sampler in human subjects. There is no relationship between mean arterial pressure (MAP) and arterial sampler filling times.