Study participants
We retrospectively reviewed patients hospitalized in the COVID-19 wards of Ankara University Faculty of Medicine from 1 September 2020 to 31 January 2021. Among patients with confirmed COVID-19 infection based on a positive real-time reverse transcriptase-polymerase chain reaction (RT-PCR) testing, the patients whose ABG was sampled during their hospitalization and simultaneously SpO2 measurement by pulse oximetry was recorded were enrolled in the study. The following patients were excluded from the study: patients with hypotension (mean arterial pressure< 65 mmHg); patients with hypothermia (body temperature < 35 0C); patients with low hemoglobin level (Hb<10 gr/dL); patients with hematological malignancy; patients with peripheral vascular disease; patients who had been under long-term oxygen therapy due to chronic respiratory failure, patients with connective tissue disease that can affect skin thickness and peripheral vascular structures, patients with methemoglobinemia, and patients with missing laboratory data. The study was approved by the Human Research Ethics Committee of Ankara University (16.02.21/12-129-21).