CPT® Code 87186

Quick Reference: CPT 87186

• Official Descriptor: Susceptibility studies, antimicrobial agent; microdilution or agar dilution (minimum inhibitory concentration [MIC] or breakpoint), each multi-antimicrobial, per plate.
• Code Family: Antimicrobial Susceptibility Studies (CPT range 87181–87190).
• Billing Unit: Per plate, per isolate. Bill one unit of 87186 for each organism isolated that undergoes MIC/breakpoint panel testing.
• Medicare CLFS Rate (2025–2026): $8.65 per plate (national limitation amount, effective 04/01/2025).
• CLIA Requirement: High complexity certificate required. This is not a CLIA-waived test. No QW modifier applies.
• Must Follow Positive Culture: Bill 87186 only after an organism has been isolated and identified. Do not bill without a preceding positive culture result.
• Key Add-On: CPT 87187 (minimum lethal concentration, MLC) may be billed in addition to 87186 when MLC is also determined.
• Common Modifier: Use modifier 59 when testing two distinct isolates from the same specimen on the same date. Use modifier 91 when the same test is repeated on a new isolate requiring a new result.

CPT 87186 is the workhorse code of clinical microbiology laboratory billing for antimicrobial susceptibility testing (AST). It describes the determination of how effectively antibiotics inhibit bacterial growth — reported as a minimum inhibitory concentration (MIC) value or a breakpoint category (Susceptible / Intermediate / Resistant) — using a single plate that simultaneously tests multiple antibiotics against a single organism. This method is now overwhelmingly performed by automated platforms such as VITEK® 2, BD Phoenix™, and MicroScan WalkAway®, all of which map to this code when performing broth microdilution panels. The results directly guide clinicians in selecting the most effective antibiotic therapy — particularly critical in an era of escalating antimicrobial resistance (AMR) and antimicrobial stewardship program (ASP) mandates.

Code Definition & Methodology Explained

The full AMA CPT descriptor for 87186 is: Susceptibility studies, antimicrobial agent; microdilution or agar dilution (minimum inhibitory concentration [MIC] or breakpoint), each multi-antimicrobial, per plate.

This code covers two closely related laboratory methodologies applied to a single multi-drug panel plate:

Broth Microdilution (the most common method): The organism is suspended in liquid growth medium at a standardized concentration per the Clinical and Laboratory Standards Institute (CLSI) M07 standard. Serial dilutions of each antibiotic are prepared across a multi-well microtiter plate (typically 96 wells). After 16–20 hours of incubation at 35°C, the lowest concentration of each antibiotic that visibly inhibits growth is recorded as the MIC. This is the format used by automated systems (VITEK® 2, BD Phoenix™, MicroScan®).

Agar Dilution: Antibiotic is incorporated into agar at a series of concentrations. A standardized inoculum of the organism is spotted onto each plate and incubated. The lowest agar concentration with no visible growth is the MIC. This method is used for certain fastidious or slow-growing organisms and for reference laboratory testing.

Both methods yield the same output — MIC values expressed in micrograms per milliliter (mcg/mL) — which are then categorized against established clinical breakpoints published by CLSI (M100) or the U.S. FDA. Results are reported as Susceptible (S), Susceptible-Dose Dependent (SDD), Intermediate (I), or Resistant (R). These categories directly inform antibiotic prescribing decisions at the bedside.

Important: CPT 87186 captures the testing of multiple antibiotics simultaneously on a single plate. It does NOT apply to single-agent testing (use 87181 or 87188) or to the disk diffusion (Kirby-Bauer) method (use 87184). The code requires that the isolate has already been obtained and identified through a prior culture procedure.

When to Use 87186 vs. Related Susceptibility Codes

The key to choosing the correct susceptibility code is understanding the method and the number of antibiotics tested per plate. 87186 applies specifically when a multi-antimicrobial plate (testing many drugs simultaneously) uses microdilution or agar dilution. Below are the key triggers for 87186 over other codes:

Use 87186 when: The lab uses an automated susceptibility panel (e.g., VITEK® 2 card, BD Phoenix™ panel, MicroScan® Breakpoint Combo panel), a manual broth microdilution 96-well plate with multiple antibiotics, or agar dilution plates testing multiple agents simultaneously. Results are MIC values or S/I/R breakpoints.

Use 87184 instead when: The lab uses disk diffusion (Kirby-Bauer), where antibiotic-impregnated disks are placed on an agar plate seeded with the organism and inhibition zones are measured. Up to 12 or fewer agents can be tested per plate.

Use 87181 instead when: A single antibiotic is tested using agar dilution (e.g., E-test/gradient strip). Bill one unit per antibiotic agent, per isolate.

Use 87187 in addition to 87186 when: The minimum lethal concentration (MLC) is also determined from the same test. 87187 is always an add-on to 87186 and is never billed alone.

Use 87188 instead when: Macrobroth dilution (large test tube format) is used to test a single agent per tube. Bill per agent.

flowchart TD
    A["Antimicrobial Susceptibility Testing"] --> B{"Method used?"}
    B -->|"Disk diffusion (Kirby-Bauer)"| C["CPT 87184"]
    B -->|"Microdilution / Agar dilution"| D{"Number of agents?"}
    B -->|"Macrobroth dilution"| E["CPT 87188 (per agent)"]
    B -->|"Gradient strip / E-test"| F["CPT 87181 (per agent)"]
    D -->|"Multi-antimicrobial plate"| G["CPT 87186 (per plate)"]
    D -->|"Single agent"| F
    G --> H{"MLC also determined?"}
    H -->|"Yes"| I["Add CPT 87187"]
    H -->|"No"| J["87186 only"]

Billing Units: Per Plate, Per Isolate

This is the most common billing error associated with CPT 87186. The code is billed per plate, and a single plate tests one organism against many antibiotics simultaneously. Therefore, the correct billing unit is one unit of 87186 per isolate (not per antibiotic).

For example: A urine culture grows two organisms — Escherichia coli and Klebsiella pneumoniae — and susceptibility testing is performed on each. The correct billing is 87186 × 2 units (one per isolate, or use modifier 59 on the second unit to distinguish separate testing events). Do NOT bill one unit of 87186 per antibiotic; that would constitute unbundling and is a significant audit risk.

CPT Assistant Clarification (December 2024): The AMA’s CPT Assistant confirmed that when antimicrobial susceptibility testing using microdilution or agar dilution is performed on two isolates — each on its own plate — it is appropriate to report CPT 87186 twice (one unit per plate/isolate). The billing unit is the plate, not the antibiotic agent tested.

Polymicrobial Infections

Urinary tract infections (UTIs) are often polymicrobial. When multiple significant organisms are isolated from the same specimen and each undergoes susceptibility testing on its own panel, 87186 is billable for each isolate independently. CMS guidelines specifically state that codes 87088, 87184, and 87186 may be used multiple times in association with or independent of 87086, because UTIs may be polymicrobial.

CLIA Certification Requirements

Antimicrobial susceptibility testing by microdilution or agar dilution is classified as a high-complexity laboratory test under CLIA (Clinical Laboratory Improvement Amendments of 1988). Laboratories must hold a valid CLIA Certificate of Compliance or Accreditation to perform and bill CPT 87186. A Certificate of Waiver is insufficient.

The CLIA certificate number must appear on all claims: in Loop 2300 or 2400 (REF/X4, qualifier 02) for electronic claims, or in Item 23 of the CMS-1500 for paper claims. Claims without a valid CLIA number will be rejected or denied. Modifier QW (CLIA-waived) does NOT apply to 87186 and should never be appended to this code.

Accreditation through the College of American Pathologists (CAP), The Joint Commission (TJC), AABB, or state-approved accreditation bodies is accepted in lieu of biennial CLIA inspections for qualifying laboratories. Quality control (QC) procedures per CLSI guidelines must be maintained for each antimicrobial panel or plate used, documented in the laboratory’s standard operating procedures (SOPs).

Medicare Coverage, CLFS Rate & Coverage Policies

Clinical Laboratory Fee Schedule (CLFS) Rate

CPT 87186 is paid under the Medicare Clinical Laboratory Fee Schedule (CLFS), not the Physician Fee Schedule (PFS). There is no professional (TC/26) component split — the CLFS rate is all-inclusive for the technical and interpretive work performed by the laboratory.

Rates effective April 1, 2025 through March 31, 2026. Source: CY 2026 CLFS Update, CMS Transmittal 13514 (CR 14312, December 5, 2025). Always verify current rates using the CMS CLFS lookup or your MAC’s fee schedule publication.

National Coverage Determination (NCD): Urine Culture

CMS maintains a National Coverage Determination for urine culture that directly governs 87186 billing in that context. Under this NCD, susceptibility testing is covered when an organism is isolated from a urine culture and clinically significant. Covered indications include: abnormal urinalysis results, signs and symptoms of lower or upper UTI, suspected urosepsis, and monitoring in renal transplant recipients on immunosuppressives.

Importantly, the NCD specifies that the number of units for 87184 or 87186 is determined by the number of significant isolates, and that testing for asymptomatic bacteriuria in prenatal care is considered screening and is therefore not covered by Medicare.

“Reflex” Testing and Medical Necessity

Susceptibility testing (87186) is a reflex test — it is ordered and performed only after a culture is positive and the isolate is identified. Billing 87186 without a corresponding positive culture code (e.g., 87086, 87070, 87075, or 87077) on the same claim or in the patient’s record will raise a medical necessity flag. Some payers use automated edits to deny 87186 when no parent culture code appears. Ensure your lab’s laboratory information system (LIS) reflexes susceptibility orders appropriately and that the triggering culture code is captured on the claim.

ICD-10 Diagnosis Codes That Justify Medical Necessity

Although CPT 87186 is a laboratory code and does not itself require complex MDM documentation, the diagnosis code paired with the claim must support the medical necessity of susceptibility testing. The following ICD-10 codes are among the most common and well-supported diagnoses for 87186:

Payers may maintain Local Coverage Determinations (LCDs) listing approved ICD-10 codes for susceptibility testing. Always verify with your Medicare Administrative Contractor (MAC) for your jurisdiction’s specific covered diagnosis code lists.

Modifier Usage

Modifier 59 – Distinct Procedural Service

When susceptibility testing (87186) is performed on two or more distinct isolates from the same specimen or from different specimens on the same date, modifier 59 should be appended to the second (and subsequent) unit of 87186. This signals to the payer that the additional testing represents a separate, clinically distinct service — not a duplicate or unbundled charge.

Example: Blood culture grows both Staphylococcus aureus and Candida albicans. Bacterial susceptibility is tested on the S. aureus isolate (87186 × 1). A separate antifungal susceptibility panel is run on the Candida isolate (87186-59 × 1). Use modifier 59 on the second unit to override any bundling edit. (Note: For fungal susceptibility, also consider whether code 87186 or a mycology-specific code applies per your MAC.)

Modifier 91 – Repeat Clinical Diagnostic Laboratory Test

Modifier 91 is used when the same test (87186) is legitimately repeated on the same date of service to obtain a new result from a new isolate or a new specimen collected later that day. It is NOT used for repeat testing due to laboratory error, equipment malfunction, or quality control purposes.

Example: A hospitalized patient has a morning blood culture and an afternoon urine culture, both collected the same day. Both cultures grow organisms requiring susceptibility testing. Bill 87186 for the blood isolate and 87186-91 for the urine isolate to indicate the test was repeated for a new, medically necessary reason — not due to lab error.

Modifier 90 – Reference (Outside) Laboratory

When the ordering provider’s lab sends the specimen to a reference laboratory for susceptibility testing, and the ordering lab is billing on behalf of the reference lab, modifier 90 must be appended to 87186. This alerts the payer that the service was performed outside the billing laboratory. The billing laboratory may not mark up the charge beyond the actual cost paid to the reference lab in most Medicare Part B scenarios.

No QW Modifier

As a high-complexity test, CPT 87186 is never reported with modifier QW. QW is reserved exclusively for CLIA-waived test systems. Appending QW to 87186 is an error that will trigger payer denials and potential compliance inquiries.

Audit-Proof Documentation Standards

While susceptibility testing is a technical laboratory procedure — not an E/M service — the laboratory’s records must clearly support medical necessity, correct methodology, and accurate unit billing to withstand payer audits.

Your laboratory report and internal documentation should clearly reflect the following elements:

1. Source and Date of Specimen Collection: The laboratory report must show the specimen type (e.g., urine, blood, wound swab), date and time of collection, and patient identifiers. This establishes the chain of testing and confirms the culture preceded the susceptibility study.

2. Organism Identification: The isolate must be identified (e.g., by MALDI-TOF, biochemical panel, or molecular method — often coded separately under 87077, 87140, 87143, 87147, or 87149) before susceptibility testing is billed. The species name should appear on the report.

3. Methodology Statement: The report or SOP should state the specific method used: “Broth microdilution per CLSI M07” or “Automated microdilution via VITEK® 2” — not just “susceptibility testing.” This supports 87186 over 87184 (disk) or 87181 (single-agent agar dilution) in the event of a methodology audit.

4. MIC Values or Breakpoint Results Reported: Results must be documented as MIC values (in mcg/mL) and/or S/I/R/SDD interpretations for each antibiotic tested, per CLSI M100 or FDA breakpoints. Documenting only “susceptible” or “resistant” without the supporting MIC is insufficient for high-quality practice and a red flag in CAP/TJC inspections.

5. Number of Isolates: If multiple isolates are tested, each isolate must be documented separately with its own susceptibility profile. This supports billing multiple units of 87186 and the use of modifier 59 or 91.

6. Ordering Physician and Indication: The order or laboratory requisition should include the ordering provider’s name, NPI, and a diagnosis or clinical indication. “R/O UTI” or “sepsis workup” linked to a covered ICD-10 code satisfies medical necessity documentation requirements.

Code Comparison: Antimicrobial Susceptibility Study Codes

CLSI & Stewardship Standards (2024–2026 Updates)

Clinical laboratories performing MIC testing are expected to follow the current edition of CLSI standards, which are updated annually. Staying current with these standards is essential not only for patient safety, but also for compliance with CAP accreditation requirements and CMS Conditions of Participation for hospital laboratories.

CLSI M07 (12th Edition, 2024): The foundational reference for broth microdilution and agar dilution methods for aerobic bacteria. The 2024 edition provides updated inoculum preparation, incubation conditions, and quality control (QC) ranges for reference strains. Laboratories billing 87186 must follow M07 methodology to justify the code — deviation from this standard (e.g., using non-standardized inocula or incubation temperatures) could invalidate results and expose the lab to billing fraud concerns.

CLSI M100 (34th Edition, 2024): The performance standards document that contains the clinical breakpoints (MIC and disk diffusion) used to categorize organisms as S/SDD/I/R. Key changes in the 2024 edition included addition of sulbactam-durlobactam breakpoints for Acinetobacter spp., a new Salmonella/Shigella breakpoint table (Table 2A-2), and reorganization of dosage recommendations. In 2025, CLSI updated aminoglycoside breakpoints for Acinetobacter spp. and removed doxycycline and tetracycline breakpoints for that organism due to insufficient PK/PD data. Labs billing 87186 should implement current M100 breakpoints promptly upon each annual publication.

Antimicrobial Stewardship Program (ASP) Integration: CMS has elevated the importance of ASP for hospitals through the Promoting Interoperability (PI) Program. Beginning in 2025, the AUR Surveillance measure was split into two separate measures — Antimicrobial Use (AU) Surveillance and Antimicrobial Resistance (AR) Surveillance — both required for eligible hospitals. Susceptibility data generated under CPT 87186 feeds directly into NHSN AR reporting, making accurate, methodology-appropriate testing and billing integral to ASP compliance.

Clinical Billing Scenarios

Scenario 1: Routine UTI with Single Isolate (Straightforward)

Situation: A 68-year-old Medicare patient presents with dysuria and frequency. A urine culture is ordered. The culture grows Escherichia coli at >100,000 CFU/mL. Susceptibility testing is performed on an automated VITEK® 2 panel covering 20 antibiotics.

Correct Coding:

• 87086 – Urine culture, quantitative colony count
• 87088 – Presumptive identification of isolate
• 87147 – MALDI-TOF organism identification (if performed)
• 87186 × 1 – MIC panel for E. coli isolate

ICD-10: N39.0 (UTI, unspecified)

Key Point: Bill 87186 only once — for the single isolate. Do not bill per antibiotic. Medicare CLFS reimburses $8.65 for 87186.

Scenario 2: Polymicrobial UTI with Two Isolates

Situation: A catheterized inpatient’s urine culture grows both Klebsiella pneumoniae and Enterococcus faecalis. Both organisms are deemed clinically significant. Separate susceptibility panels are performed on each.

Correct Coding:

• 87086 – Urine culture
• 87088 × 2 – Presumptive identification of each isolate (modifier 59 on second)
• 87186 × 1 – MIC panel for K. pneumoniae
• 87186-59 × 1 – MIC panel for E. faecalis (modifier 59 signals distinct testing)

ICD-10: N39.0; also consider B96.1 (Klebsiella) or B95.2 (Enterococcus) as additional codes.

Key Point: CMS explicitly permits billing 87186 multiple times for polymicrobial infections. Use modifier 59 on additional units to prevent bundling denial.

Scenario 3: Bacteremia with Vancomycin MIC & MLC Testing

Situation: A blood culture in a hospital patient grows Staphylococcus aureus (MRSA confirmed). The infectious disease team requests a vancomycin MIC and MLC to guide AUC/MIC-targeted dosing.

Correct Coding:

• 87040 – Blood culture with isolation
• 87077 or 87147 – MRSA identification
• 87186 × 1 – Full MRSA susceptibility panel (MIC)
• 87187 × 1 – MLC determination (add-on to 87186)

ICD-10: A41.01 (Sepsis due to MRSA), Z16.11 (Resistance to penicillin)

Key Point: 87187 ($40.17) may only be billed when MLC is actually determined — not when only MIC is reported. The MLC add-on is the most valuable pairing with 87186 and is appropriate in serious MRSA infections requiring precision dosing.

Scenario 4: Reference Lab Referral with Modifier 90

Situation: A physician’s office lab isolates Pseudomonas aeruginosa from a wound culture but lacks the capability to perform MIC testing. The specimen is sent to a reference laboratory, which performs the susceptibility panel. The office lab bills the patient’s insurer.

Correct Coding:

• 87070 – Wound culture (performed in-house)
• 87186-90 × 1 – MIC susceptibility testing performed at reference lab

Key Point: Modifier 90 is required when billing for services performed by a reference laboratory. The billing lab must not mark up the reference lab charge for Medicare Part B patients and must document the referral relationship.

Scenario 5: Repeat Testing Same Day (Modifier 91)

Situation: A hospitalized patient has a central line blood culture drawn at 0600 and a separately ordered urine culture collected at 1400. Both cultures become positive the same day. Each grows a different organism requiring susceptibility testing.

Correct Coding:

• 87040 – Blood culture
• 87086 – Urine culture
• 87186 × 1 – MIC panel for blood culture isolate
• 87186-91 × 1 – MIC panel for urine isolate (same test, same day, new result from different specimen)

Key Point: Modifier 91 indicates the repeat testing was for a new, medically necessary clinical reason — not a quality control re-run or an instrument retest.