Hemoglobin Variant With Discordance Between SpO2 and SaO2

Abstract Introduction This is a unique case of a patient with a pulse oximetry (SpO2) and calculated arterial oxygen saturation (SacO2) discrepancy, and found to have an unreported hemoglobin (Hb) variant. Case report A 60-year-old patient was seen in a sleep clinic due to concerns of obstructive sleep apnea (OSA). Patient was otherwise asymptomatic, non-cyanotic and had no cardiopulmonary conditions. Significant background included the patient and patient's 3 adult children baseline SpO2 being 85-90%. Apart from SpO2 of 88%, vital signs and cardiopulmonary examination were normal. Complete Blood Counts revealed: decreased Hb (11.4g/dL), MCH (20.3pg) and MCV (65.7fL), normal total white cells and platelets. On polysomnography, whilst OSA was absent, baseline and average SpO2 was at 85%. However, arterial blood gas on room air showed pH 7.46, PaCO2 31mmHg, PaO2 109mmHg, SaO2 99%. As there was dissociation between SpO2 and SaO2, hemoglobinopathy was considered. Co-oximetry found SaO2 85%, MethHb 0%, CarboxyHb 0%. High p50 of 50.06mmHg (normal:24-28mmHg) was calculated with Doyle's equation, supporting a right shift of the oxygen (O2)-dissociation curve. Hemoglobin electrophoresis and DNA analysis showed alpha 1- thalassemia minor, and an alpha globin variant caused by a mutation at codon 33 (TTC>ATC) that replaces phenylalanine with isoleucine. The latter has not been reported in international databases. Discussion The pulse oximeter measures different wavelengths of oxygenated and deoxygenated Hb to calculate the proportion of Hb saturated with O2 to obtain SpO2. Blood gas analysis of SaO2 (SacO2) calculates arterial blood O2 saturation from measured PaO2 based on O2-dissociation curve of normal Hb. This difference in obtaining oxygenated Hb saturation can account for discrepancy between SpO2 and SacO2 if the: 1)Hb variant has a wavelength absorbance different from normal Hb or 2)Hb variant has low O2-affinity that shifts the O2-dissociation curve to the right. However, a co-oximeter directly measures the SaO2. Hence, a reduced level indicates a true reduction in SpO2 consistent with the presence of low O2-affinity Hb variant. The patient was informed 1) of their double hemoglobinopathies: the thalassemia trait that is not known to affect Hb-O2 affinity and a previously unreported alpha-globin variant with possible low O2-affinity, 2) literature review revealed that other low O2-affinity Hb were not associated with adverse outcomes that required management, 3) as the variant is novel, further evaluation with a direct p50 testing and Hb structural function studies may be considered and 4) genetic testing and counselling for patient's affected children was advised.