In which scenario is the calculated oxygen saturation of hemoglobin likely to be in error?

The calculated oxygen saturation of hemoglobin is likely to be in error in the scenario of carbon monoxide poisoning. Hemoglobin has a higher affinity for carbon monoxide than for oxygen, which means that when carbon monoxide is present, it can bind to hemoglobin, forming carboxyhemoglobin. This binding does not provide oxygen to the tissues, yet the standard methods for calculating oxygen saturation often rely on the absorption characteristics of hemoglobin for determining whether it is saturated with oxygen. As a result, the presence of carbon monoxide can falsely elevate the calculated saturation level, making it appear that there is more oxygen bound to hemoglobin than there actually is.

In contrast, diabetic ketoacidosis, while it may present with other complications, does not directly affect the hemoglobin's ability to carry oxygen in the same misleading way as carbon monoxide does. Receiving oxygen therapy typically provides accurate readings of oxygen saturation, as the additional oxygen enhances the actual saturation levels in healthy hemoglobin. Assisted ventilation for respiratory failure also doesn't significantly disrupt the oxygen saturation readings since ventilators typically serve to improve oxygen delivery; they do not interfere with hemoglobin's capacity to accurately reflect saturation levels. Thus, carbon monoxide poisoning stands out as the scenario most likely to introduce errors in the calculated oxygen saturation.