Introduction: Arterial blood gas analysis is the gold standard procedure performed in the emergency room that guides the clinicians in establishing diagnosis and treatment plan and ultimately mortality and morbidity. However, venous sampling has lesser complications and is easier to collect. Objective: To determine the correlation between the variables (pH, bicarbonate, pCO2, pO2) of arterial and venous blood gas analysis and to ascertain whether arterial blood gas analysis can be replaced by venous blood gas analysis in acutely ill patients. Materials and Methods: This is an observational study of variables of blood gas analysis of acutely ill patients presented to emergency department of a tertiary care centre. From eligible patients, arterial and venous samples were collected, analysed and the results were correlated using Bland Altman statistical method and plotted. Results: The study revealed a good statistical correlation of pH and bicarbonate with reference to the physician’s acceptable range. There was good clinical correlation between arterial and venous blood gas analysis in determining the condition and management of the patients. Conclusion: Venous sampling can be used as an alternative to arterial sampling in acutely ill patients presenting to emergency department as there is good clinical correlation.