The shortcomings of the risk priority number.
By John C. Strong, AbbVie Inc.; Todd S. McDermott, AbbVie Inc.; Oliver Heinzerling, AbbVie Deutschland GmbH & Co. KG; H. Gregg Claycamp, Office of New Animal Drug Evaluation, FDA Center for Veterinary Medicine
Quality risk management (QRM) is a central concept in quality by design, and the assessment of risk is a specific component of QRM that figures prominently in the development, new drug application submission, and subsequent lifecycle of a marketed drug. Risk is usually described as the combination of the probability of a hazard occurring (O) and the severity (S) of the resulting harm. Often in pharmaceutical risk assessment, the detectability (D) of the hazard prior to the harm occurring is considered as well. A popular approach to account for these components of risk is the calculation of the so-called risk priority number (RPN), where numerical rankings ascribed to each of these risk components (e.g., 1–5 or 1–10 ) are multiplied together, i.e., RPN = S×O×D. The product S×O, coined the “criticality,” is also commonly employed, but this is lumped under the label of RPN for the purposes of this article. RPN is typically used to triage risk mitigation activities or to compare against a threshold value as a trigger for taking mitigation action.