Falsely low SpO₂ - an educative case study present

Blood gas analysis (BGA) provides the means for the most accurate assessment of patient oxygenation status; the two relevant blood gas parameters being partial pressure of oxygen in arterial blood (pO₂(a)), and % hemoglobin oxygen saturation (sO₂(a)). 

Pulse oximetry, a technology now ubiquitous in all areas of clinical medicine, is an alternative, less accurate method of monitoring oxygenation status that is more convenient, cheaper and safer than BGA. The validity of pulse oximetry depends on SpO₂ (the parameter measured during pulse oximetry) being an acceptable surrogate for sO₂(a). 

Although in health and most clinical contexts SpO₂ is more or less equal to sO₂(a), that is not always the case. The uniquely formatted "Case Records of the Massachusetts General Hospital" series, published in every issue of the New England Journal of Medicine, recently included the case history of a patient in whom marked discordance between SpO₂ and sO₂(a) was attributed to dapsone-induced methemoglobinemia. 

The 52-year-old male patient presented with unremarkable symptoms (sore throat, nasal congestion, non-productive cough) of upper respiratory tract infection, a condition that normally would not affect oxygenation status. SpO₂ was, however, markedly reduced (fluctuating between 85 and 90 %), indicating a degree of hypoxemia consistent with more serious chronic or acute respiratory/cardiovascular illness. 

Without any clinical or radiological evidence to explain the apparent hypoxemia, and the finding of normal arterial blood gases - pO₂(a) 88 mmHg (11.7 kPa), sO₂(a) 96 % - it became clear that SpO₂ was falsely reduced. The cause of the falsely reduced SpO₂ was increased concentration of the dysfunctional hemoglobin, methemoglobin (metHb). In health, metHb is <2 % of total hemoglobin; the patient’s metHb was 9.7 %. 

One of the most common causes of methemoglobinemia is the drug dapsone, which the patient had been taking for some years to treat the chronic skin condition, dermatitis herpetiforms, he was suffering. 

In a detailed and referenced expert discussion that characterizes the "Case Records" series, the authors outline some theory of pulse oximetry and the mechanism that gives rise to falsely low SpO₂ in patients with methemoglobinemia. The clinical significance of methemoglobinemia is discussed along with the many causes; the drug dapsone gets special attention in this regard. 

For this particular patient dapsone use had a secondary effect beyond the spurious pulse oximetry readings. He is a long-standing type 1 diabetic, whose HbA1c has been consistently falsely low due to a hemolytic effect of dapsone. This aspect of the case history is also discussed in some detail. A highly informative case presentation highlighting the limitations of pulse oximetry.