Objectives: The goal of this research was to determine the role of the alveolar-arterial gradient, which can be easily calculated with the help of acid-base status, in the diagnosis and assessment of the severity of pulmonary embolism. Materials and methods: A total of 168 patients at the University Hospital of Split, who underwent CT angiography of the pulmonary artery due to suspected pulmonary embolism in the period from 2019 to 2023 were included in the research. An arterial blood sample was taken from the subjects for gas analysis at the emergency department and, as a rule, it was immediately sent for analysis to a radiometer called ABL90 FLEX. Results: The results show that the A-value ratio was significantly higher in patients with PE (median 2.14) compared to those without PE (median 1.18), and the P-value was less than 0.001. The obtained results show that the ratio of expected and observed A values can become a useful diagnostic tool for identifying patients with PE, with a sensitivity of 75.27% and a specificity of 78.38%, Conclusions: It has been proven that the Aa gradient is elevated in patients with CT-confirmed pulmonary embolism, in 92% of patients. The ratio between obtained and normal A gradient was significantly higher in patients with PE compared to those without PE, with high statistical significance. Our results showed a significant correlation between the ratio of expected and observed A values and the presence of PE, with a high level of statistical significance. Further research should be conducted to examine the value of the A-a gradient in patients with substrate on lungs or lung diseases.