The Saturation Gap as a Diagnostic Clue for Methemoglobinemia in a Child with Persistent Low Oxygen Saturation

Abstract Background Persistent low peripheral oxygen saturation in children commonly prompts investigation for pulmonary or cardiovascular disease. However, when oxygen desaturation persists despite an unrevealing cardiopulmonary evaluation, dyshemoglobinemia should be considered. Case presentation We report the case of a 7-year-old girl with persistent low peripheral oxygen saturation for approximately five years. Her peripheral oxygen saturation remained between 85% and 89% on room air despite an otherwise normal clinical condition. Extensive investigations, including echocardiography, contrast echocardiography, cardiac computed tomography angiography, chest computed tomography, pulmonary function testing, and neurologic evaluation, did not identify a cardiopulmonary cause for the low oxygen saturation. Arterial blood gas analysis demonstrated a preserved partial pressure of arterial oxygen (PaO₂) of 115 mmHg on room air, increasing to 165 mmHg during a hyperoxia test. The calculated alveolar–arterial oxygen gradient was 2.3 mmHg. The discrepancy between the low pulse oximetry readings and preserved PaO₂ prompted evaluation for a dyshemoglobinemia. Co-oximetry demonstrated a methemoglobin level of 15%, supporting the diagnosis of methemoglobinemia. Conclusions Methemoglobinemia should be considered in children with persistent low peripheral oxygen saturation that is disproportionate to their clinical condition and cardiopulmonary findings. Recognition of a discrepancy between pulse oximetry and arterial oxygenation can provide an important diagnostic clue and may help prevent prolonged and unnecessary investigations.