RESUMO
Background: Arterial gasometry is considered the gold standard for establishing a diagnosis of respiratory failure of any etiology. However, there are some circumstances in which it loses specificity, making necessary to consider other tests such as pulse oximetry to adequately determine hypoxemia. We report a 67 years old patient with sudden hypoacusia, right hemiparesis and polypnea. His laboratory exams on admission, showed extreme hypoxemia in several readings, without correlation to the patient's clinical condition nor the pulse oximetry, and a leukocytosis of 800.000 cells x ml, with many immature cells. Chronic myeloid leukemia was diagnosed and treatment with hydroxyurea was initiated, achieving normalization in the arterial gases in accordance with the fall of the white cell count. Interpretation of laboratory findings according to the general clinical context of the patient allowed to suspect a spurious hypoxemia, saving the patient from unnecessary and risky interventions.