87186

Susceptibility test, microdilution or agar dilution, minimum inhibitory concentration (MIC), each drug

The CPT code 87186 describes a laboratory procedure performed to determine the minimum inhibitory concentration (MIC) of a specific antimicrobial agent against an isolated microorganism. This test involves preparing serial dilutions of an antimicrobial drug (e.g., in broth microdilution trays or agar plates) and inoculating them with a standardized suspension of the patient's isolated bacterial or fungal pathogen. After incubation, the lowest concentration of the antimicrobial agent that visibly inhibits the growth of the microorganism is identified as the MIC. This quantitative result provides crucial information for guiding appropriate antimicrobial therapy, particularly for infections where resistance is suspected or when specific dosing regimens are required.

Clinical Indications

  • To guide the selection of appropriate antimicrobial therapy for bacterial or fungal infections.
  • To detect antimicrobial resistance in isolated pathogens.
  • To monitor changes in antimicrobial susceptibility patterns over time or during treatment.
  • For infections caused by organisms with unpredictable susceptibility patterns.
  • In cases of treatment failure despite initial appropriate antimicrobial therapy.
  • For patients with severe or life-threatening infections where precise antimicrobial dosing is critical.
  • To assess the susceptibility of isolates from sterile body sites or those causing complicated infections.

Procedure Steps

  1. Isolation and identification of the pathogenic microorganism from a clinical specimen.
  2. Preparation of a standardized inoculum (e.g., McFarland standard) of the isolated microorganism.
  3. Preparation of serial two-fold dilutions of the specific antimicrobial drug in a suitable medium (broth or agar).
  4. Inoculation of each antimicrobial dilution with the standardized bacterial or fungal inoculum.
  5. Incubation of the test system (e.g., microdilution tray, agar plate) under appropriate conditions (temperature, atmosphere, duration).
  6. Visual examination of each dilution for microbial growth or inhibition.
  7. Determination of the lowest antimicrobial concentration that completely inhibits visible growth (MIC).
  8. Interpretation of the MIC value using established clinical breakpoints (susceptible, intermediate, resistant) for the specific organism and drug.

Coding Guidelines

  • Report CPT code 87186 for each individual antimicrobial agent for which a minimum inhibitory concentration (MIC) is determined for a specific isolate.
  • If multiple drugs are tested against a single isolate, 87186 should be reported for each drug.
  • This code should not be reported for qualitative susceptibility tests (e.g., disk diffusion) or commercial rapid susceptibility systems that provide only a qualitative interpretation (susceptible/resistant) without a specific MIC value.
  • Do not unbundle and report 87186 separately when it is an integral component of a more comprehensive test (e.g., an automated system providing multiple MICs for a panel of drugs, which might be covered by a single panel code if available and appropriate).
  • Ensure documentation clearly specifies the organism tested and each specific antimicrobial agent for which an MIC was determined.
  • The test must be performed by or under the supervision of a physician or other qualified healthcare professional in a CLIA-certified laboratory.

Associated ICD-10 Codes