Medicare pays a chiropractor for manual manipulation of the spine only when it corrects a documented subluxation through active treatment, not when it maintains a patient's comfort. The AT modifier is the claim-level attestation that a given visit was active or corrective care, and it is one of the most scrutinized line items on a chiropractic Medicare claim. CMS reported errors in 33.6% of chiropractic claims reviewed under its 2024 CERT sample, a rate that keeps the specialty a recurring audit target. Billing AT on a visit the record shows was maintenance care is not a coding technicality. The finding can support extrapolation across the full claims universe and a recoupment demand far larger than the sample that triggered it.
What the AT Modifier Actually Attests
Medicare requires a chiropractor to append the AT modifier, short for active treatment, to CPT codes 98940, 98941, and 98942 before the claim will pay. A claim submitted without it is treated as maintenance therapy and denied outright. The modifier is only supposed to be used when manipulation is reasonable and necessary under national and local coverage policy, and CMS has been explicit that its presence does not settle the question on its own. Under the Medicare Benefit Policy Manual, Chapter 15, Section 240.1.3, contractors may still deny a claim after medical review if the record does not support active treatment.
Active Treatment Versus Maintenance Therapy
CMS coverage policy divides most spinal complaints into two categories. An acute subluxation is a new injury where manipulation is expected to produce improvement, with coverage extending up to three months as visit frequency tapers. A chronic subluxation is a longer-standing condition where continued care can still be covered as long as it produces measurable functional improvement. Once a patient's clinical status has stabilized and no further objective improvement is expected, continued manipulation becomes maintenance therapy, meaning it now serves to prevent deterioration or promote general health rather than to correct the original subluxation. Medicare does not pay for maintenance therapy under any modifier, and the AT modifier must not be placed on a maintenance claim.
The Documentation That Distinguishes Them
The line between the two categories is drawn by the chart, not the claim form. CMS's chiropractic documentation checklist requires an initial visit record with a diagnosis naming the level of subluxation and a treatment plan stating the recommended level of care, specific treatment goals, and objective measures the chiropractor will use to evaluate progress. Every subsequent visit note then has to show an assessment of the patient's change since the last visit, measured against those same objective measures, not just a note that manipulation was performed. A treatment plan without a documented functional endpoint reads on audit like maintenance care even when the biller has appended AT to every claim. Local coverage determinations set the jurisdiction-specific detail behind these requirements and should be reviewed before a treatment plan is finalized, not after a documentation request arrives.
Why the AT Modifier Draws Audit Scrutiny
The HHS Office of Inspector General has already tested whether the AT modifier works as a control. Its 2015 report on chiropractic payments found that 40 to 47 percent of paid chiropractic claims were for maintenance therapy, and that every claim OIG identified as suggestive of maintenance care carried the AT modifier anyway. That finding is a large part of why chiropractic claims continue to draw RAC reviews and the targeted additional documentation requests described in our SMRC audit overview, and why elevated CERT program error rates tend to invite follow-on review rather than close the matter.
The AT modifier attests that a visit was active treatment. It does not prove it. Under CMS's own coverage rules, only the treatment plan and the visit-by-visit record can answer that question.
Why Early Legal Counsel Is Critical
It is critical that chiropractic providers promptly retain experienced healthcare defense counsel upon receiving a Medicare additional documentation request, a CERT or SMRC audit notice, or any other program integrity inquiry tied to AT modifier billing. Early legal intervention can protect the provider's rights, ensure the response addresses each visit's documentation on its merits, avoid inadvertent admissions, and allow counsel to communicate with the contractor on the provider's behalf. Delaying legal representation can significantly affect the outcome of the audit, particularly once a sample finding raises the possibility of extrapolation.
How Health Law Alliance Can Help
Health Law Alliance defends chiropractors against Medicare audits built on AT modifier findings, including MAC prepayment reviews, CERT and SMRC document requests, and RAC post-payment reviews. Our bench has overseen 2,000+ audits and includes attorneys who have sat on the payor side of program integrity review. If your practice has received a documentation request or audit notice tied to chiropractic billing, contact us for a free, confidential consultation, or read our overview on when to engage a Medicare audit attorney.
