Test ID: BLACT    

Useful For Suggests clinical disorders or settings where the test may be helpful

Predicting the resistance of beta-lactamase producing isolates to hydrolysis-susceptible beta-lactam antimicrobials

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Various bacteria produce a class of enzymes called beta-lactamases, which may be mediated by genes on plasmids or chromosomes. Production of beta-lactamase may be constitutive or induced by exposure to antimicrobials. Beta-lactamases hydrolyze (and thereby inactivate) the beta-lactam rings of a variety of susceptible penicillins and cephalosporins. Beta-lactamases are classified by their preferred antimicrobial substrate and the effect of various inhibitors (such as clavulanic acid) on them.


Some antimicrobials, such as cefazolin and cloxacillin are resistant to such hydrolysis (at least for staphylococcal beta-lactamases).


Beta-lactamase producing strains of the following are resistant to many types of penicillin: Staphylococcus species, Hemophilus influenzae, Neisseria gonorrhoeae, Bacteroides species, Enterococcus species, and Moraxella catarrhalis.


The above organisms, when isolated from critical specimens such as blood or spinal fluid, should always be tested for beta-lactamase production.


Addition of a beta-lactamase inhibitor to a beta-lactam (such as sulbactam plus ampicillin) restores the activity of the antimicrobials.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Negative (reported as positive or negative)

Interpretation Provides information to assist in interpretation of the test results

A positive test indicates production of beta-lactamase.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Some beta-lactamase tests (iodometric and acidometric methods) may not detect certain beta-lactamases.


Many bacteria that do not produce beta-lactamase will be resistant to beta-lactams by other mechanisms.


This test should not be used to detect extended-spectrum beta-lactamases.

Clinical Reference Recommendations for in-depth reading of a clinical nature

Livermore DM, Williams JD: Beta-lactams: mode of action and mechanisms of bacterial resistance. In Antibiotics in Laboratory Medicine. Fourth edition. Edited by V Lorian. Baltimore, MD, Williams and Wilkins, 1996, pp 502-578

Special Instructions Library of PDFs including pertinent information and forms related to the test