Cross-susceptibility Between Ampicillin-Sulbactam and Amoxicillin-Clavulanate
In vitro susceptibility to ampicillin-sulbactam (Unasyn) does not reliably imply susceptibility to amoxicillin-clavulanate (Augmentin). [1][2]
Mechanistic Basis for Possible Discordance
Both agents combine an aminopenicillin with a β-lactamase inhibitor. [1]
Clavulanate and sulbactam can differ in spectrum and strength against specific β-lactamase enzymes. [1]
Even when the β-lactamase target overlaps, pharmacokinetic and pharmacodynamic differences between the two drugs can affect achievable drug exposures at the site of infection. [2]
Evidence on Correlation and Interchangeability
Correlation studies in β-lactamase–producing Enterobacteriaceae have reported overlap in susceptibility results between amoxicillin-clavulanate and ampicillin-sulbactam. [1]
Discordance between the two combinations has been described, indicating they are not interchangeable in all organisms and all settings. [2]
Recent analyses have emphasized that treating amoxicillin-clavulanate and ampicillin-sulbactam as equivalent can misclassify susceptibility when only one is tested. [2]
Clinical Interpretation of a Reported “Sensitive” Result
A reported susceptibility to ampicillin-sulbactam should be interpreted as evidence for ampicillin-sulbactam activity under the conditions used by the laboratory test method. [1]
A reported susceptibility to amoxicillin-clavulanate requires amoxicillin-clavulanate testing (or validated inference criteria specific to the organism and testing approach). [2]
Circumstances Increasing Likelihood of Discordance
Organisms with resistance mechanisms affecting inhibitor activity beyond the β-lactamase targeted by sulbactam or clavulanate can produce discordant results. [1][2]
Inhibitor effect differences across Enterobacterales β-lactamase types can lead to non-equivalent in vitro activity for the two combinations. [1]
Practical Approach to Susceptibility-Based Selection
Antibiotic choice should follow the organism-specific AST result for the intended drug (amoxicillin-clavulanate vs ampicillin-sulbactam). [2]
If amoxicillin-clavulanate susceptibility is not reported, laboratory guidance should be sought on whether AST inference from ampicillin-sulbactam susceptibility is acceptable for the specific organism and method. [2]
Bottom-Line Implication for the Specific Question
Sensitivity to Unasyn does not guarantee sensitivity to Augmentin. [1][2]