Decoding Medication Errors: The NCC MERP Severity Index

Medication errors are preventable events that may lead to inappropriate medication use or patient harm, occurring during prescribing, dispensing, administration, or monitoring. While many errors are intercepted and cause no harm, others result in temporary or permanent injury or, in worst cases, death. Inconsistent definitions and severity classifications across institutions have long impeded comparisons of incident rates, trend identification, and systematic safety improvements.

In response, the National Coordinating Council for Medication Error Reporting and Prevention (NCC MERP) was founded in 1995 through a public–private coalition including professional, regulatory, and safety organizations.¹ Its charge: to promote a uniform definition of medication errors, standardize reporting, and advocate system-based prevention. One of NCC MERP’s most significant contributions is the “Medication Error Index” (also known as the Severity Index), which classifies errors by their actual or potential patient impact. This tool has become central to advancing medication safety in the United States and abroad.

The NCC MERP Severity Index

The Index categorizes medication errors along a spectrum from Category A (circumstances that could lead to error but no error occurred) to Category I (errors that contributed to or resulted in patient death). Categories B–D capture events in which errors reached the patient but did not cause harm, while Categories E–H encompass errors that caused harm or required interventions. ¹ By providing such structured gradations, the Index replaces vague descriptors (“minor,” “serious,” etc.) with consistency that supports comparison, communication, and prioritized corrective action.

In practice, a delay in delivery of a high-risk anticoagulant that does not harm a patient may be classified as Category B. An incorrect antibiotic that triggers a mild allergic reaction requiring treatment would fall under Category F. A serious insulin dosing error causing hypoglycemia that requires intensive care support would be Category H. These classifications allow pharmacy teams and interdisciplinary health professionals to gauge severity, discern risk areas, and tailor interventions.

Insights from the 25-Year Anniversary Report

Published in 2020, the NCC MERP 25-Year Anniversary Report reflects on progress made in medication safety, identifies ongoing challenges, and sets future priorities. It emphasizes that many healthcare organizations have adopted error reporting systems and classification frameworks; however, underreporting and inconsistent adoption remain problems.²

The report highlights goals moving forward: addressing medication safety in non-hospital settings, improving error detection through predictive risk modelling, and enhancing error data quality and reporting incentives. It also notes the importance of open communication and a non-punitive approach to errors, principles congruent with what is often called a "just culture," even if that specific term is not used.

Application in Pharmacy Practice

Pharmacists are on the front lines when medication errors happen. The Severity Index equips them with a shared language and framework, enabling them to classify errors in a way that is precise, consistent, and informative. Rather than loose descriptors, teams can say “Category E due to minor harm,” or “Category H requiring intensive care,” which helps with internal reporting, root cause analysis, and communicating with leadership about needed changes.

Using the Index also facilitates trend analysis. For example, repeated near misses or errors in high-alert drug classes (like anticoagulants or insulin) may signal underlying process problems, such as verification gaps or poor order communication. Once trends are identified, pharmacists can lead interventions: workflow changes, better system checks (e.g. barcode scanning), education, and technology integration.

System-Based Strategies to Reduce Errors

Even the best classification tools cannot prevent errors alone. System-based strategies are essential. These include implementation of electronic prescribing, barcode or scanning verification, double-check systems, and standardized safety checks. Interdisciplinary communication among pharmacists, nurses, and physicians is critical, especially for identifying points of failure before harm occurs.³ Ongoing professional development keeps pharmacy staff current with evolving drug therapies, high-alert medications, and safety evidence.

Furthermore, fostering an environment that encourages reporting of errors or near misses without fear of blame, aligns with safety-culture ideals and helps uncover “hidden” risks. Organizations that commit to such culture are more likely to detect issues early, learn from incidents, and deploy preventive action.⁴

Implications for Healthcare Systems

As healthcare continues to evolve, with increased use of tele pharmacy, expanded pharmacist roles, and sophisticated health information technology, the NCC MERP Severity Index remains foundational. It provides a metric for evaluating safety initiatives, guiding policy and protocol development, and supporting regulatory or accreditation requirements. Consistent use of the Index across different healthcare settings (inpatient, outpatient, community) enhances comparability and the ability to generalize findings.

Moreover, the global relevance of NCC MERP’s work is underscored by its incorporation in other countries’ safety frameworks and guidelines. This breadth enhances the capacity to share data, learn best practices, and accumulate evidence that supports patient safety advancement worldwide.⁵

Conclusion

Medication errors are a complex and persistent challenge. The NCC MERP Severity Index offers a robust, standardized classification system that helps quantify and describe errors in a way that informs prevention, policy, and practice. Its value has been affirmed across 25 years of NCC MERP’s work, particularly through its influence on error reporting, data consistency, and safety culture. While the 25-year report does not specifically label the approach a “just culture,” many of its recommendations are aligned with that philosophy, emphasizing non-punitive reporting, transparency, and system improvements.

For pharmacists, the Index is more than a taxonomy; it is a tool for learning, advocacy, and system redesign. As medication use systems grow more complex, the principles embodied in NCC MERP’s Severity Index are increasingly vital to protect patients, improve health outcomes, and strengthen organizational safety.

RESOURCES

1. National Coordinating Council for Medication Error Reporting and Prevention. About Medication Errors. NCC MERP. Updated February 2019. Accessed September 25, 2025. http://www.nccmerp.org/about-medication-errors.

2. Munley RG, Melnyk D, eds. 25 Years of Building Medication Safety. National Coordinating Council for Medication Error Reporting and Prevention; 2020. Accessed September 25, 2025. https://www.nccmerp.org/sites/default/files/nccmerp-25-year-report.pdf

3. Institute of Medicine. To Err Is Human: Building a Safer Health System. Washington, DC: The National Academies Press; 2000. Accessed September 25, 2025. doi:10.17226/9728

4. World Health Organization. Medication Errors: Technical Series on Safer Primary Care. Geneva, Switzerland: World Health Organization; 2016. Accessed September 25, 2025. https://www.who.int/publications/i/item/medication-errors-technical-series-on-safer-primary-care

5. Smetzer JL, Vaida AJ, Cohen MR, Tranum D, Pittman MA, Armstrong CW. Findings from the ISMP Medication Safety Self-Assessment for Hospitals. Jt Comm J Qual Saf. 2003;29(11):586-597.