Page 119 - Safety Risk Management for Medical Devices

98 Safety Risk Management for Medical Devices

ratings. Any End Effect that traces up to a Hazard must have a Y in the Safety Impact
column.

Tip Failure Modes whose Occurrence is indistinguishable from zero, are deemed not credi-
ble and thus are excluded from the DFMEA/PFMEA. However, to help the reviewers of the
DFMEA/PFMEA know that the analyst has considered a particular Failure Mode, even though it
is not credible, at the discretion of the analyst, it’s permissible to cite such Failure Modes in
the DFMEA. Failure Modes that are not credible should be clearly delineated and need not be
further analyzed.

Assign ratings. Table 12.3, Table 12.4, and Table 12.5 offer suggestions for rankings
of Severity, Occurrence, and Detectability, respectively. Let’s examine each rating.
Severity is the significance of the worst reasonable consequence of the End Effect
at the boundary of analysis. Severity ranking definitions are different depending on
whether the End Effect has a safety impact or not. For End Effects that do not have a
safety impact, use the left column in Table 12.3, and for those with a safety impact
use the right column.

Table 12.3 Definitions of DFMEA severity ratings
Severity Criteria (Sev)
Rank Qualitative criteria—no safety impact Qualitative criteria—safety impact

5 Catastrophic: Described failure mode will Catastrophic—Impact of the end-effect at
cause immediate failure of the the System level can be death
Subject. (Total loss of all functions—
primary and secondary)
4 Critical: Described failure mode will Critical—Impact of the end-effect at the
severely impact Subject functionality | System level can be permanent
Complete loss of primary functions impairment or life-threatening injury
3 Serious: Described failure mode will Serious—Impact of the end-effect at the
reduce Subject functionality. (Partial System level can be injury or
loss of primary functions | Complete impairment that requires professional
loss of secondary functions) medical intervention

2 Minor: Described failure mode will have Minor—Impact of the end-effect at the
temporal or self-restoring impact on System level can be temporary injury
functionality | partial loss of or impairment that does not require
secondary functions professional medical intervention
1 None: Described component failure will Negligible—Impact of the end-effect at
have no impact on functionality the System level can be at most an
inconvenience, or temporary
discomfort