Do sickle cell and other hemoglobinopathies impact pulse oximeter accuracy?

Sickle cell disease (SCD) is a blood disorder that affects the structure of hemoglobin. It impairs the delivery of oxygen to the tissues. Due to this alteration in hemoglobin structure, patients with SCD’s oxygen dissociation curve is often shifted to the right, as shown in the figure below. This leads to lower SaO2 values for a given PaO2, particularly as patients become more hypoxemic. 

The oxygen hemoglobin dissociation curve shows the values for hemoglobin (Hb) A (solid line) and a patient with Hb S (dashed line).

This figure is taken from a Free Access paper: Wagner, M. H., & Berry, R. B. (2007). A patient with sickle cell disease and a low baseline sleeping oxygen saturation. Journal of Clinical Sleep Medicine, 3(3), 313-315.

The change in structure of hemoglobin has been thought to impact pulse oximeter accuracy. Several small studies over the last 30 years have been performed and references are listed below. These studies showed mixed results, ranging from “no clinically significant bias” (Fitzgerald et al, 2001) to a difference between SpO2 and SaO2 of more than 2% in one-quarter of study participants (Ariglani et al, 2020). In the majority of studies, SpO2 underestimated SaO2 by 1-3%. 

The findings of these studies are summarized below:

F Ortiz et al. : Small study in adults with SCD in acute vaso-occlusive crisis. 22 paired SaO2 and SpO2 samples. Found a mean 1.1% underestimate of SaO2 using SpO2. Concluded that “as long as strong and regular photoplethysmographic waves are present, pulse oximeters can be relied upon not to misdiagnose either hypoxemia or normoxemia in SCD.”

Ariglani et al.: Small study in pediatric patients with SCD. Compared 39 simultaneous and paired SaO2 and SpO2 samples. SaO2 samples were taken from arterialized earlobe blood gasses. Findings: “Pulse oximetry was inaccurate in almost a quarter of measurements in ambulatory pediatric patients with SCD, especially at SpO2 values ≤93%.” “In 9/39 (23%) cases, the difference in SpO2–SaO2 was greater than the expected error range ±2%, with SaO2 more often underestimated by SpO2 (6/9), especially at SpO2 values ≤93%”

Blaisdell et al. Pediatric population with SCD. Assessing rate of missed hypoxemia (Assumes SpO2<93% equates to PaO2<70mmHg). Compared 22 pairs of SaO2 and SpO2. Mean difference of 5%, with SD of 5.3%. 

Fitzgerald et al. : Evaluating accuracy of pulse oximetry via correlation with SaO2 in a pediatric population with SCD. 24 patients, total of 21 simultaneous and paired SpO2 and SaO2 samples. Findings: The pulse oximeter correlated well with the cooximeter-measured arterial saturation (r2 = .74). The pulse oximeter significantly underestimated saturation by a mean of −1.6% (95% confidence interval, −0.3 to −3;p = .03). “Pulse oximetry significantly underestimates true arterial saturation, but the bias is clinically insignificant”

Figure from: Wagner, M. H., & Berry, R. B. (2007). A patient with sickle cell disease and a low baseline sleeping oxygen saturation. Journal of Clinical Sleep Medicine, 3(3), 313-315. 

References: 

Ortiz, F. O., Aldrich, T. K., Nagel, R. L., & Benjamin, L. J. (1999). Accuracy of pulse oximetry in sickle cell disease. American journal of respiratory and critical care medicine, 159(2), 447-451.

Arigliani, M., Zheng, S., Ruiz, G., Chakravorty, S., Bossley, C. J., Rees, D., & Gupta, A. (2020). Comparison of pulse oximetry and earlobe blood gas with CO-oximetry in children with sickle cell disease: a retrospective review. BMJ Paediatrics Open, 4(1).

Blaisdell, C. J., Goodman, S., Clark, K., Casella, J. F., & Loughlin, G. M. (2000). Pulse oximetry is a poor predictor of hypoxemia in stable children with sickle cell disease. Archives of pediatrics & adolescent medicine, 154(9), 900-903.

FitzGerald, R. K., & Johnson, A. (2001). Pulse oximetry in sickle cell anemia. Critical care medicine, 29(9), 1803-1806.

Keywords: sickle cell disease, HbS