CPT code 88377 – Manual morphometric analysis of a multiplex ISH stain

Key takeaways

CPT code 88377 reports a pathologist’s manual morphometric analysis of a multiplex in situ hybridization stain, per specimen, for each multiplex probe stain procedure.

NCCI policy allows one unit of 88366, 88374 or 88377 per specimen for each multiplex probe stain procedure, however many probes the stain carries.

A single probe stain counted by hand is coded 88368, and a computer-assisted count on a multiplex stain is coded 88374.

The 2026 Medicare Physician Fee Schedule assigns 88377 a work RVU of 1.37, and the final payment varies by MAC locality.

Pabau, the practice management platform we build, sends 837P claims through Claim.MD and posts eligibility responses and remittances to the patient record.

CPT code 88377: Official descriptor and procedure overview

CPT code 88377 reports a pathologist’s manual morphometric analysis of a multiplex in situ hybridization (ISH) stain. The AMA places it in the Surgical Pathology range, 88300-88399, inside the pathology and laboratory section.

The AMA’s CPT code set gives the full descriptor: Morphometric analysis, in situ hybridization (quantitative or semi-quantitative), manual, per specimen; each multiplex probe stain procedure.

Three parts of that descriptor control billing. “In situ hybridization” names the staining technique, and “manual” means the pathologist personally performs the count. “Each multiplex probe stain procedure” sets the unit of service.

The code is reported per specimen, for each multiplex probe stain procedure. A biopsy block stained with two probes in one multiplex procedure takes one unit of 88377 under NCCI, not two. A single probe stain counted by hand is reported with 88368 instead.

What CPT 88377 includes and excludes

CPT code 88377 covers the pathologist’s manual reading of a quantitative or semi-quantitative morphometric result from a multiplex ISH stain. The pathologist performs and attests the count, without an automated image analysis system.

Included under 88377: FISH (fluorescence in situ hybridization), chromogenic ISH and silver ISH, when two or more probes run in one stain. The pathologist counts and reports the signals by hand.

Excluded from 88377:

• Automated or computer-assisted morphometric analysis of a multiplex ISH stain (use 88374 instead)
• A single probe stain counted by hand (use 88368, plus 88369 for each additional single probe)
• Non-ISH special stains and immunohistochemistry (separate code families)
• Gross and microscopic examination of the specimen itself (use 88305 or the appropriate level code)
• ISH read without a morphometric count (use 88365, 88364 or 88366)

A common coding error is billing 88377 when an automated scanner generated the signal count. If the pathologist reviews digitally captured image data instead of counting signals at the microscope, 88374 is the correct code.

How CPT 88377 relates to codes 88366, 88374, and 88360

Several codes in the 88300-88399 range sit close to 88377. They split on the staining method (ISH or immunohistochemistry), on whether a morphometric count was made, and on how it was made. Picking the wrong code from this cluster is a common cause of NCCI bundling denials.

The 88374/88377 split is the one that creates audit exposure. Labs increasingly use whole-slide imaging platforms.

If the instrument produces the signal count and the pathologist only reviews the output, 88374 applies. Billing 88377 in that scenario creates a documentation mismatch that payers flag on post-payment review.

Across the ISH family, two questions decide the code: how the count was done, and how many probes the stain carries. Manual counts use 88368, 88369 and 88377, while computer-assisted counts use 88367, 88373 and 88374. A stain read without a count uses 88365, 88364 and 88366. The grid below lays the nine codes out side by side.

Immunohistochemistry sits outside this grid. A tumor IHC stain scored by hand, such as HER2 or ER/PR, is billed with 88360 for each single antibody stain.

NCCI bundling rules and units-per-specimen restrictions for CPT code 88377

The CMS NCCI Policy Manual (2026 edition) restricts all three multiplex ISH codes. Providers may not report more than one unit of 88366, 88374 or 88377 per specimen for each multiplex probe stain procedure. The limit holds even when the stain tests several probes.

The one-unit cap applies per specimen per multiplex procedure, not per probe. A five-probe multiplex stain on a single lung adenocarcinoma specimen is one unit of 88377, not five. Send it through a medical claims clearinghouse with a report that documents every probe in the stain.

Key NCCI editing points for 88377:

• 88366 + 88377 on the same specimen: One multiplex stain takes one code. A modifier applies only when separate stain procedures ran on distinct specimens.
• 88374 + 88377 on the same specimen: Bundled. Choose one code based on whether the count was manual or computer-assisted.
• Multiple specimens, same encounter: Each distinct specimen, separately identified by the pathologist, may support its own unit, subject to payer policy.
• Modifier 59 or XU: Separates distinct specimens, never separate probes on the same specimen. Record the specimen identifier in the report.

Documentation requirements for CPT code 88377

A claim for CPT code 88377 needs a pathology report that supports each part of the descriptor. That means the ISH technique, the multiplex probe stain, the morphometric analysis and the manual interpretation.

A report that leaves one out hands the payer a reason to deny or recoup payment. Clean claim submission for ISH codes depends on a more specific report than routine surgical pathology needs.

Required documentation elements:

• Probe identity: Full commercial or validated probe name (e.g. DAKO HER2 FISH pharmDx), target gene, and clone/catalog number where applicable
• Multiplex statement: A note that the probes ran together in one stain procedure, so one unit per specimen matches the claim
• Technique description: ISH method (FISH, CISH, SISH) with hybridization conditions sufficient to confirm manual signal counting
• Quantitative or semi-quantitative result: Actual signal counts, ratios, or percentage positive cells as required by the clinical indication (e.g. HER2/CEP17 ratio per ASCO/CAP guidelines)
• Manual interpretation attestation: Pathologist signature affirming personal interpretation of ISH morphometric analysis
• Specimen identifier: Unique accession number linking the staining result to the specific tissue block
• CLIA certification: The performing laboratory must hold a valid CLIA certificate of compliance or accreditation covering cytogenetics or molecular pathology, as applicable

Payer audits for 88377 tend to center on two documentation failures. The first is a report that names the probe only generically (“HER2 FISH performed”) with no quantitative signal data. The second is a missing statement that the pathologist personally interpreted the count instead of relying on automated output.

Medicare reimbursement rate and 88377 fee schedule 2026

The 2026 Medicare Physician Fee Schedule assigns CPT code 88377 a work RVU (wRVU) of approximately 1.37, based on published CMS RVU data files.

Total RVUs are higher when practice expense and malpractice components are added, and the final reimbursement amount varies by Medicare Administrative Contractor (MAC) locality. Use the CMS Physician Fee Schedule lookup tool to retrieve the non-facility and facility rates for your specific geographic location.

Use FastRVU’s 2026 RVU lookup to retrieve the full component breakdown and apply your locality’s geographic practice cost index (GPCI). The 1.37 wRVU figure comes from published CMS RVU data. Confirm it against the final 2026 MPFS file before year-end reconciliation.

Common denial reasons for CPT 88377 and how to avoid them

Most 88377 denials fall into one of five categories. Each has a corrective action that prevents a repeat through a process change, without an individual appeal.

Denial management workflows for pathology labs treat these as systemic edits instead of one-off rework. The CARC denial codes reference lists the adjustment reason codes payers attach to each denial type.

Clinical scenarios where CPT code 88377 is correctly applied

Four common pathology lab encounters show which ISH workflows support a valid 88377 claim and which belong under a different code.

HER2 FISH on breast core biopsy: A core biopsy from a breast mass returns an equivocal HER2 IHC result (2+). The lab runs a dual-probe HER2/CEP17 FISH stain, and the pathologist counts signals in 20 cells by hand to calculate the ratio. Both probes share one hybridization, so the stain is one multiplex procedure and takes one unit of 88377. If image analysis produced the count, the code is 88374.

ALK FISH on lung adenocarcinoma: After an IHC screen, a pathologist runs ALK FISH on a lung biopsy to confirm a rearrangement. The signals are counted by hand. 88377 applies only when the lab reports the probe set as a multiplex stain. A stain the lab reports as a single probe procedure is coded 88368. Name the probe kit in the report so the payer can see which code fits.

Kappa/lambda light chain ISH for small lymphocytic lymphoma: This scenario is a frequent source of NCCI confusion for 88377. When kappa and lambda probes run together as one multiplex ISH stain on a specimen, NCCI allows one unit of 88377, not one per probe. Bill one unit, with documentation covering both probe results.

B-cell lymphoma FISH panel: A panel tests three probes (CCND1, BCL2 and MYC) on a single lymph node specimen. When the probes are hybridized together in one stain, the panel is one multiplex procedure and takes one unit of 88377 under NCCI. Break-apart and fusion probes in that stain do not create separate units. Probes run as separate stains are separate procedures, coded with 88368 and 88369.

How claims management software supports 88377 billing

Anatomic pathology billing carries more NCCI edits than most specialties. Code 88377 shares its one-unit rule with 88366 and 88374, so probe count, stain type and place of service all affect the claim.

Labs running ISH panels every day cannot review each claim by hand, so accuracy depends on the medical billing workflow itself.

Pabau keeps the pathology report and the claim in one patient record. The biller works from the signed report, not a re-keyed summary.

Its claims management software sends CMS-1500 and 837P claims through the Claim.MD clearinghouse, which reaches thousands of US payers. Eligibility responses and electronic remittance advice come back against that same record.

A unit rejection on a multiplex stain then shows up as soon as the payer returns it. The slide and the report are still easy to pull. Keeping reports to medical billing compliance standards also leaves the lab ready if a MAC requests records after payment.

Keep pathology reports and claims in one record

Pabau’s claims management software sends 837P claims through Claim.MD, checks patient eligibility, and posts remittances against the patient record. See how it handles pathology code combinations like 88377.

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Conclusion

Before you bill 88377, check two facts against the report. Each unit should map to one multiplex stain on one specimen, and a pathologist should have made the count by hand. If either fact fails, another code in the ISH family fits better, and the claim is safer under that code.

The report carries the rest of the claim. Signal counts, the probe kit and a manual interpretation statement are what a payer looks for when it reviews a multiplex panel. Book a demo to see how Pabau keeps pathology reports and claim submission in one patient record.

Continue your research

Confused by denial adjustment reason codes on your 88377 remittance? Understanding electronic remittance advice (ERA) walks through how to read 835 files and act on CARC codes.

Billing the IHC stain that often comes before FISH? CPT code 88342 covers immunohistochemistry per specimen for the initial single antibody stain.

Looking for a broader reference on surgical pathology billing? Revenue cycle management in healthcare covers the end-to-end billing workflow from charge capture through final payment posting.

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