Modifier ‑90 (Reference/Outside Laboratory) indicates that a laboratory test was performed by an entity other than the treating or reporting provider. The American Medical Association defines modifier ‑90 as “Reference (Outside) Laboratory,” and it is appended to the usual CPT procedure code when the performing laboratory is not the billing or referring provider. For Medicare and other payers that follow CMS guidance, when a clinical diagnostic laboratory (POS 81) refers a specimen to an outside lab, the referring lab may bill using modifier ‑90; the claim must clearly identify the outside lab, include both CLIA numbers, reflect the purchase price in charges, mark item 20 appropriately, and include the performing lab’s NPI and address in item 32. These requirements ensure Medicare reimburses the performing (reference) laboratory rather than the referring lab. (Source: AMA terminology; Noridian/CMS guidance.)

Not all services are appropriate to report with modifier ‑90. The modifier must not be used for anatomic or surgical pathology services (for example, CPT codes 88300‑88309), specimen drawing services (such as CPT 36415), or non‑laboratory services including evaluation and management or specimen handling codes (99000, 99010). CMS guidance emphasizes that only one laboratory may bill for a referred lab service — the lab that actually performs the testing. (Source: Providence health plan summary of CMS instructions.)

Modifier ‑90’s effect on payment varies by payer. For Medicare, the performing reference laboratory is typically reimbursed when claims are submitted in accordance with CMS instructions and properly identify the outside lab and purchase price. Commercial payers set their own policies: some may reimburse reference lab services, while others, like Asuris, state modifier ‑90 services will not be reimbursed unless specific contractual exceptions apply. Providers should check individual payer policies to determine whether modifier ‑90 services are payable. (Sources: Noridian Medicare; Asuris policy.)

Practical billing examples illustrate these rules: a Medicare example shows an independent lab billing CPT 80074‑90 when referring the test to an external lab, with item 20 marked and item 32 populated so Medicare reimburses the performing lab. A commercial example shows a physician office appending 88150‑90 for a cytopathology specimen sent to an outside lab, where the payer’s policy (Asuris) disallows reimbursement and the claim may be denied. These examples underscore the importance of following payer‑specific instructions and CMS claim formatting when using modifier ‑90. (Sources: Noridian Medicare; Asuris; Providence policy.)