MIC Where does that number come from? Stacey Hamilton

Purpose: Refresher for the Micro crew. Level set the team before we go live with Sensititre Either a Refresher or tutorial for our non-Micro friends

Refresher on terminology: MIC stands for Minimum Inhibitory Concentration This is the lowest concentration of an antimicrobial that will inhibit the visible growth of an organism It is reported in mcg/mL Can be measured in many ways (microbroth dilution, E-test, Agar dilution) Breakpoint stands for a chosen concentration of antibiotic which defines whether a bacteria is susceptible or resistant to the antibiotic. If the MIC is less than or equal to the susceptibility breakpoint the bacteria is considered susceptible to the antibiotic

Breakpoint determinations are decided by CLSI (United States) EUCAST (everyone else) These regulatory agencies also determine all of the factors that must be followed to make the testing valid. *Conditions (CO² or non- CO²) *QC (specific organism strain) *Media types for testing (contains blood or not)

Also keep in mind that the MIC we report correlates to a level that can be achieved in the blood. You may have to increase or decrease dosing if you are treating infections in tissue, CSF or urine. The only exception is that we do have different breakpoints for Strep pnuemo in Blood and CSF for penicillin and cefotaxime/ceftriaxone.

System for the Microscan and Sensititre. Microbroth dilution System for the Microscan and Sensititre. 96 well plate that contains several antibiotics. Antibiotics can have several wells in a serial dilution or as a single or double well around the breakpoint. A panel type is meant to accommodate several organism types that may have different breakpoints for the same drug. For example PC33 panel is for Staph and Enterococcus. The breakpoint for vancomycin for Staph aureus is ≤ 2 mcg/mL, for Staph coag negative is ≤4 mcg/mL and Enterococcus is ≤4 mcg/mL

How is an MIC read? First look at your growth well. Organisms grow differently on the MIC panels. You need to know how the organism grows in the microtiter plate to determine the MIC Button Hazy

Is the drug bactericidal or bacteriostatic? Bactericidal-kill bacteria If the drug is bactericidal, you read the MIC at complete inhibition of growth. Bacteriostatic stop growth If the drug is bacteriostatic, you read the MIC at about 80% inhibition compared to the growth well. This can sometimes be difficult

8 mcg/mL This drug is bactericidal. Growth well What if the drug is bacteriostatic? What is the MIC? 8 mcg/mL

This drug is bactericidal Growth well What is the MIC? ≤ 0.5 mcg/mL

This drug is bacteriocidal Growth well What is the MIC? >16 mcg/mL

What is the vancomycin MIC for Staph aureus on PC33? 1 mcg/mL

What is the Levofloxacin MIC Pseudo aeruginosa on NM42? ≤ 0.5 mcg/mL mg/L

What is the penicillin MIC for Strep pneumo on Microstrep? 0.25 mcg/mL

What is the ampicillin MIC for Haemophilus influenzae on HAE panel? 0 0.25mcg/mL

What is the ampicillin MIC for Haemophilus influenzae on HAE panel? >4 mcg/mL

Questions?