FAA Office of Aerospace Medicine
Civil Aerospace Medical Institute

Report No: DOT/FAA/AM-13/14

Title and Subtitle: Evaluation of a New Equation for Calculating the Maximum Wait Time for Pilots That Have Used an Impairing Medication

Report Date: August 2013

Authors: Canfield DV, Berry M, Whinnery JE, Lewis RK, Dubowski KM

Abstract: Pilots that 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 for pilots involved in fatal aviation accidents have proven that not all pilots wait a sufficient period of time before returning to duty.

Those pilots were found to have impairing concentrations of the drug in the blood at the time of the accident. In the past, somewhat arbitrary wait times were used based on medication half-lives, dosage intervals, class of drug, and other subjective methods to estimate a return-to-duty time. These methods do not take into consideration the time required for the drug to decrease from therapeutic concentrations to a safe sub-therapeutic concentration.

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 equation developed 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. This equation was evaluated for use in determining a safe return-to-duty time for pilots.

Anonymous subjects were recruited according to an approved IRB protocol. All subjects had a preexisting medical condition treated with some type of medication. Blood and plasma were collected at approximately Cmax (2-3hrs) and again after waiting approximately 5 more hours. Subjects were asked to provide information on the drug name, dose, dosing interval, age, height, weight, and gender.

Toxicological analysis was performed on the specimens collected to determine the concentration of the medication at the first and the second collection times. The new equation was evaluated to determine if the wait times calculated by the equation and other methods were sufficient to eliminate the medication to a concentration below the lower therapeutic concentration of the medication.

Key Words: Medications, Wait Time, Aviation, Safety, Equation, Half-Life

No. of Pages: 9