Lack of evidence for a percent saturation gap in cyanide poisoning

To determine if toxic concentrations of cyanide in blood result in a difference between calculated and measured percent hemoglobin oxygen saturation (percent saturation gap).

An in vitro laboratory study.

Hospital laboratory.

Arterial blood from five stable patients residing in a tertiary hospital ICU. Venous blood samples obtained from five healthy volunteers.

Two-mL aliquots from each blood sample were placed into four test tubes and mixed with phosphate buffer. Cyanide was added to three of the tubes so that the four tubes contained 0, 6, 12, and 25 mg/L cyanide.

The percent saturation gap was calculated by subtracting the percent oxyhemoglobin measured on an oximeter from the percent saturation calculated by a blood gas analyzer. Using two-tailed, paired t tests, we could not demonstrate a difference in mean percent saturation gaps between arterial samples or venous samples with and without cyanide. All blood samples had a normal percent saturation gap (P greater than .99 by Fisher's exact test). We had greater than a 95% chance of demonstrating a difference in mean percent saturation gaps of only 0.46% in arterial blood and of 3.3% in venous blood if such a difference existed.

Our results, as well as a review of the literature, indicate that there are no data supporting the suggestion that a percent saturation gap should imply the diagnosis of poisoning by inorganic cyanide.