Conclusion: Despite a very high negative predictive value of a negative PCN skin test to the major and minor determinants and reports that no life-threatening false-negative reactions have been reported when PCN is administered after a negative PCN skin test, physicians need to be very cautious in administering piperacillin/tazobactam and other b-lactams in patients with a history of severe reactions such as anaphylaxis to past PCN administrations.