Page 128 - Safety Risk Management for Medical Devices

Risk Analysis Techniques 107

Table 12.7 PFMEA severity rankings
Severity Criteria (Sev)

Rank Severity descriptions—nonsafety Severity description—safety impact
5 Failure to meet Regulatory requirements Catastrophic—Impact of the end-effect at
| Total loss of all functions—primary the System level can be death
and secondary | .70% of the
production has to be scrapped
4 Loss or degradation of primary functions Critical—Impact of the end-effect at the
| Failure to meet product System level can be permanent
specification | Scrapping of 50% impairment or life-threatening injury
70% of the production

3 Loss or degradation of secondary Serious—Impact of the end-effect at the
functions | Reduced reliability but System level can be injury or
still within Spec | Scrapping of 25% impairment that requires professional
50% of the production medical intervention
2 Process Delay | Scrapping of 5% 25% Minor—Impact of the end-effect at the
of the production. | Minor cosmetic System level can be temporary injury
or usability impact but still within or impairment not requiring
Spec professional medical intervention

1 Scrapping of 0% 5% of the production. Negligible—Impact of the end-effect at
Some of the products have to be the System level can be at most an
reworked inconvenience, or temporary
discomfort

In the BXM method there are five classes of Harm severity. One could wonder
how to choose the severity class of the Harm, when it may have a probability of
Harm in all five severity classes. The answer is to choose the most probable severity
class. Example: Let’s say the Failure Mode of the process step that seals a sterile pack-
age for an implantable device, is “incomplete sealing.” This could lead to contamina-
tion of the device, which could lead to infection. In the example that was presented
in Fig. 12.13, Infection had the highest probability of causing a Serious Harm
(72.1%). Therefore in the PFMEA, the Failure Mode of process step to seal the sterile
package would receive a severity ranking of 3, Serious. Remember that in the
PFMEA, this ranking is used only for criticality determination and the prioritization
of resources on mitigating Failure Modes. The risk of Hazards is computed in the
RACT and is a different matter.
The ranking in the Occ column of the PFMEA is indicative of the likelihood of
occurrence of the Failure Mode. Refer to Table 12.8 for the definitions of the
Occurrence rankings. Use quantitative data if available. Otherwise use the qualitative
definitions to determine the Occ rank. If quantitative data is used, ensure the units are