An E-test strip for ciprofloxacin is placed on a Mueller-Hinton agar plate inoculated with a urinary E. coli isolate. The elliptical inhibition zone intersects the strip at a value of 0.5 mg/L. According to CLSI breakpoints (susceptible ≤0.25 mg/L, resistant ≥1 mg/L), how should this isolate be reported?

• A Susceptible
• B Resistant
• C Indeterminate — repeat testing required
• D Intermediate (susceptible-dose dependent) ✓

Explanation

An MIC of 0.5 mg/L falls between the susceptible breakpoint (≤0.25) and the resistant breakpoint (≥1), placing the isolate in the intermediate category (now termed susceptible-dose dependent, SDD, under newer CLSI guidance). SDD indicates that optimal clinical response requires maximized dosing. Reporting as susceptible without this qualifier would be incorrect and could lead to treatment failure, particularly in severe infections.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.