A method of disinfecting a medical device comprises three steps. First, a disinfectant is contacted with a process indicator and the medical device. The disinfectant comprises at least one aldehyde. The process indicator contains a synthetic amine-containing compound disposed on a substrate. The synthetic amine-containing compound comprises at least one of primary amino groups or secondary amino groups, and is reactive with the disinfectant to form at least one adduct. The synthetic amine-containing compound and the medical device are in fluid communication through the disinfectant. A predetermined disinfectant exposure criterion exists for contacting the disinfectant with the medical device. Second, the process indicator is spectrally observed and at least one parameter is obtained therefrom that is predictive of the predetermined disinfectant exposure criterion. The third step is determining that the predetermined disinfectant exposure criterion has been achieved.