In PEN-FAST scoring for penicillin allergy, what is the negative predictive value when the score is 0?

• A. 95.0%
• B. 99.4%
• C. 85.0%
• D. 100.0%

Answer: (B)

Negative predictive value tells you how often a negative result (in this case, a PEN-FAST score of 0) correctly indicates absence of the condition (true penicillin allergy). If the score is 0, there are no history features that raise suspicion for an allergy, so the chance that the patient actually does not have a true penicillin allergy is extremely high.

In PEN-FAST validation, a score of zero corresponds to a negative predictive value of about 99.4%. That means 99.4% of people with a zero score are not truly allergic, while ~0.6% may still be allergic despite a zero score. This makes a zero score a very strong rule-out tool.

Remember that NPVs depend on how common the allergy is in the population studied; in other populations with higher or lower prevalence, the exact NPV can shift. But the reported figure for a zero PEN-FAST score is 99.4%, which is why that option is correct.