A new equation for calculating the maximum wait time for pilots who use an impairing medication

Pilots who use an impairing medication to treat a medical condition are required to wait an appropriate amount of time after completing the treatment before returning to duty. However, toxicology findings from fatal aviation accidents indicate not all pilots wait a sufficient period of time. Methods used today do not take into consideration the time required for the drug to reach subtherapeutic concentrations.

An equation was developed based on the therapeutic range and the maximum expected half-life of the medication to objectively calculate a safe return-to-duty time for pilots. The new equation assumes the treating physician will not dose the patient beyond the upper therapeutic range of the medication and the person taking the medication has the maximum half-life reported in the literature. The equation N ln(0.5*Cmin/Cmax)/ln(0.5) was developed to determine the number of half-lives (n) required to reach one-half of Cmin, where Cmin = lower therapeutic concentration and Cmax = upper therapeutic concentration. Anonymous subjects were recruited under an approved IRB protocol. Blood and plasma were collected at approximately Cmax (2-3 h) and again after waiting approximately another 5 h. Toxicological analysis was performed on the specimens collected.

One subject taking a 25-mg dose had a 0.033 ug x ml(-1) blood concentration after waiting 8 h, well above the 0.025 ug x ml(-1) reported as impairing concentration. The new equation estimated a mean wait time for the 18 medications composed of 4 half-lives.

The new CAMI equation takes into consideration safety without grossly over estimating pilot wait times.