CPT code 87186 – Antimicrobial susceptibility testing (microdilution)

Key takeaways

CPT code 87186 covers MIC-based susceptibility testing by microdilution or agar dilution, billed as one unit per multi-antimicrobial plate.

The method on the lab report decides the code. Disk diffusion is 87184, enzyme detection is 87185, and a single-agent agar dilution or gradient strip is 87181.

For urine isolates, Medicare coverage follows NCD 190.12. Blood, wound and sputum isolates are judged under MAC policy and the reasonable-and-necessary standard.

The 2026 Medicare CLFS national rate for 87186 is $8.65 per plate, with no locality adjustment.

Practice management software like Pabau checks claims for missing details before they go out and submits them through the Claim.MD clearinghouse.

CPT code 87186: official descriptor and procedure overview

The American Medical Association’s CPT code set gives CPT code 87186 this official descriptor. Susceptibility studies, antimicrobial agent; microdilution or agar dilution (minimum inhibitory concentration [MIC] or breakpoint), each multi-antimicrobial, per plate. The last phrase is the billing rule. Each physical plate tested equals one unit, however many antibiotics sit on it.

The code sits in the CPT microbiology subsection for susceptibility studies (87181-87190). It applies to bacterial isolates when MIC testing is clinically needed. It also covers yeasts and other fungi tested by broth microdilution under CLSI M27, because CPT has no separate antifungal susceptibility code. HIV drug-resistance assays are the exception, with their own codes in the 87900 series.

Microdilution and agar dilution: what the procedure involves

Both microdilution and agar dilution methods measure how much antibiotic is required to inhibit visible bacterial growth, producing an MIC value. The choice affects lab workflow but not the CPT code. Either technique bills as 87186 when the plate tests multiple antimicrobials.

Broth microdilution

The lab prepares a series of wells containing progressively diluted antibiotic concentrations in liquid broth. A standardized bacterial inoculum is added to each well. After incubation, the lowest concentration showing no turbidity is the MIC. Commercial panels (such as those from Becton Dickinson or bioMérieux) usually test 12-20 antibiotics on a single 96-well plate. That plate equals one unit of 87186.

Agar dilution

Agar dilution embeds antibiotic into solid agar at serial concentrations. The organism is spotted or streaked across plates. The lowest concentration with no visible colony growth is the MIC. This method is more labor-intensive but is the reference standard in CLSI (Clinical and Laboratory Standards Institute) guidelines and is required for certain fastidious organisms.

CLSI’s M07 document sets the procedural standard for both methods. CLSI M100, updated annually, publishes the breakpoint tables that sort isolates into susceptible, intermediate or resistant. Labs performing CPT code 87186 testing should record which CLSI version was in effect on the test date. Breakpoints are revised, and payers may audit results against current guidelines.

CPT 87186 vs 87184, 87185 and 87181: choosing the right susceptibility code

All four susceptibility codes describe antimicrobial testing, but each measures something different by a different method. Pick the code from the method written on the lab report.

Common code-selection errors: Labs using an automated panel (VITEK, MicroScan, BD Phoenix) are performing microdilution, so they bill 87186, not 87184. Disk diffusion produces a zone diameter on an agar plate, and that qualitative result maps to 87184. Beta-lactamase strip tests (nitrocefin) are 87185, billed per enzyme. A gradient strip testing one drug is 87181, even though it reports an MIC.

Billing 87184 when the report shows MIC values in mcg/mL underpays each plate by $1.17 at 2026 Medicare rates. It also leaves the claim out of step with the lab record on audit.

Coding rules for CPT code 87186: units, per-plate reporting, and culture pairing

CPT code 87186 is reported per plate, never per antibiotic. Two organisms on two panels are two plates. Holding to that rule prevents both underbilling and overbilling, such as 20 units for a single plate that tests 20 drugs.

Unit counting in practice

• A single commercial susceptibility panel (one physical plate, multiple antibiotics): one unit of 87186
• Two separate panels tested on the same organism (for example, a routine panel plus an extended drug panel): two units of 87186
• Two different organisms each tested on one panel in the same encounter: two units of 87186
• Repeat testing of the same organism on the same day: documentation of clinical rationale required before billing a second unit

Pairing with culture codes

Susceptibility testing must follow a positive culture. These culture codes are the ones most often billed with CPT code 87186:

• 87086: culture, bacterial; quantitative colony count, urine
• 87088: culture, bacterial; with isolation and presumptive identification of each isolate, urine
• 87070: culture, bacterial; any other source except urine, blood or stool, aerobic, with isolation and presumptive identification of isolates

Culture and susceptibility are separately reportable when both services are performed and documented, so the culture does not bundle into 87186. Check current National Correct Coding Initiative (NCCI) edits before submitting, since they are updated quarterly.

Submitting 87186 without a payable culture on the same claim is a leading denial trigger, unless a documented order shows the organism was already isolated. A pre-release check that pairs every susceptibility line with its culture line stops these claims before the payer sees them.

ICD-10 diagnosis codes that support medical necessity for CPT code 87186

Every susceptibility claim needs a diagnosis code that shows why the test was medically necessary. Payers look for an active infection that needs targeted therapy, rather than a screening scenario.

The diagnosis code must match the condition the provider documented when placing the order. Using a screening or prophylaxis diagnosis (Z codes for routine exams, for example) will trigger a medical necessity denial. Local Coverage Determinations (LCDs) from your Medicare Administrative Contractor (MAC) may extend or restrict this list, so check them before submitting.

Medicare and payer coverage for CPT code 87186

NCD 190.12, titled “Urine Culture, Bacterial,” is the national coverage policy that applies when the isolate comes from urine. The Centers for Medicare and Medicaid Services (CMS) publishes it. It lists the diagnoses that support a bacterial urine culture. Susceptibility testing on that isolate stands or falls with the covered culture.

Blood, wound, sputum and other specimen types fall outside NCD 190.12. For those, coverage rests on your MAC’s local policy and the reasonable-and-necessary standard, so the chart has to document why the culture was ordered.

Medicare generally covers 87186 when a positive culture was ordered for documented signs or symptoms of infection. The MIC also has to guide antibiotic selection. Coverage fails in three common cases:

• The specimen was collected for routine surveillance.
• The culture was negative, so there was no organism to test.
• The same organism was retested in the same encounter with no documented reason for the repeat.

A clearinghouse that runs payer edits before the claim reaches the MAC surfaces diagnosis-to-procedure mismatches while they can still be fixed. Commercial payers set their own lab coverage policies, so check each contract when susceptibility volume is high.

Medicaid coverage varies by state. Some state programs mirror NCD 190.12; others require prior authorization for susceptibility panels beyond a defined count. Check the state billing manual before you set a default workflow for outpatient lab orders.

CPT 87186 fee schedule: 2026 Medicare CLFS rate

Medicare pays CPT code 87186 from the Clinical Laboratory Fee Schedule (CLFS), not the Physician Fee Schedule. Since 2018, the Protecting Access to Medicare Act (PAMA) has set CLFS rates nationally from the weighted median of private-payer rates that labs report. There is no Geographic Practice Cost Index (GPCI) or locality adjustment, so the rate is the same in every MAC jurisdiction.

The CY2026 CLFS lists 87186 at $8.65 per plate. Disk diffusion (87184) pays $7.48, while 87181 and 87185 each pay $4.75. The chart below sets 87186 against those codes and the culture codes billed with it.

CMS publishes the CLFS files each January, with quarterly updates for new and revised codes. Commercial payers often pay a percentage of Medicare or use their own schedule. Compare each remittance against the contracted rate to catch underpayments early.

Common denial reasons for CPT 87186 and how to avoid them

Most CPT code 87186 denials trace back to five root causes, and they show up across lab types and payers.

• Missing or non-covered diagnosis code. The claim submits with a Z-code screening diagnosis or no diagnosis at all. Prevention: require a billable infection-specific ICD-10 code on every susceptibility order before it reaches the billing queue.
• Incorrect unit count. Billing one unit for a two-plate encounter, or billing per antibiotic rather than per plate. Prevention: have coders enter the plate count from the lab report, never the antibiotic count.
• No supporting culture on the claim. The susceptibility is submitted without the upstream culture code, or the culture was denied on a prior claim. Prevention: always submit culture and susceptibility on the same claim when both were performed on the same date.
• Routine/surveillance claim. The specimen was collected for infection control monitoring rather than clinical management. Prevention: document the physician order with a specific clinical indication, not a standing protocol.
• Missing physician order. The lab ran the test as routine, without a documented provider order. Prevention: require a requisition or electronic order with a clinical indication for every susceptibility panel before it runs.

Sort denials by root cause across all your lab codes, not only 87186, to see patterns a claim-by-claim review hides. Grouping the medical billing denial codes on your remittances by CPT code shows whether a problem is practice-wide or tied to one ordering provider. A practice-wide problem usually sits in the documentation workflow.

Documentation requirements for CPT code 87186: what labs and providers must record

A CPT code 87186 claim that holds up needs documentation in two places: the provider’s chart, which shows medical necessity, and the lab report, which shows what was performed. An audit that finds one without the other will recoup the payment.

Provider-side documentation

• Physician order or electronic requisition specifying susceptibility testing with a clinical indication (not a blanket “culture and sensitivity” standing order)
• Documented signs and symptoms supporting the active infection diagnosis
• Supporting diagnosis code that maps to the clinical presentation

Lab-side documentation

• Specimen source and collection date
• Organism identified (genus and species), since susceptibility results mean little without an identified isolate
• Method documented: “broth microdilution” or “agar dilution” explicitly stated, not just “MIC panel”
• Number of plates tested (supports unit count on the claim)
• MIC values in mcg/mL for each antibiotic tested, or breakpoint interpretations (S/I/R) against the current CLSI table version
• Final report signed by the lab director or designee

The lab report must connect back to the physician order and the clinical record. When those three documents align, the claim holds up. Capture the method and plate count when the result is reported, so coders never rebuild them from a partial report.

How claims management software reduces 87186 denials

The errors behind most 87186 denials start as data entry. A unit count gets typed from memory, a screening diagnosis slips through, or the susceptibility line goes out without its culture. Catching them by hand means a coder rechecks every lab claim against the report.

Practice management software like Pabau pulls the patient, treatment and insurer details from the patient record into the claim. Its claims management software runs validation checks each time you send, flagging missing details such as membership numbers and authorization codes. US claims go to payers through the Claim.MD clearinghouse, which also runs real-time eligibility checks before the order.

Claim status updates and ERA remittances post back to the same dashboard. When an 87186 line is denied, your team sees the reason code next to the claim. They can fix the plate count or diagnosis without switching systems.

Catch lab claim errors before submission

Pabau checks each claim for missing details before it goes out, submits through the Claim.MD clearinghouse and tracks every denial. Your microbiology claims need less manual rework.

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Conclusion

A clean 87186 claim depends on three records agreeing: the order, the lab report and the claim. When the report says broth microdilution on three plates, the claim bills three units and the diagnosis matches the specimen.

Recurring denials are usually fixed upstream of billing. Tighten how orders capture the clinical indication and how the lab records plate count, and fewer claims need rework later. Book a demo to see how Pabau’s claim checks and Claim.MD submission keep your lab claims moving.

Continue your research

Need help navigating medical billing from end to end? What is medical billing walks through the full claim lifecycle from patient encounter to payment posting.

Want to reduce claim rejections across all lab codes? Submitting a clean claim covers the pre-submission checklist that prevents the most common microbiology billing errors.

Managing denials across a high-volume lab? Denial management in healthcare explains how to categorize, track, and reverse claim denials by root cause.

Billing the urine culture behind the susceptibility test? CPT code 87088 covers isolation and presumptive identification of urine isolates, with its 2026 fee.

Culturing a wound or sputum specimen? CPT code 87070 explains the aerobic culture code for sources other than urine, blood or stool.

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