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Here鈥檚 how to avoid duplicate billing.

Clinical labs frequently reach for modifiers 90 (Reference (outside) laboratory) and 91 (Repeat clinical diagnostic laboratory test) to support claims coverage. But each modifier comes with its own pitfalls that could prevent proper pay.  

Check out these tips to make sure your lab鈥檚 鈥渞epeat test鈥� or 鈥渞eference lab鈥� bills are clean.  

Navigate Repeat Services With 91

To keep payers from denying a legitimate claim as duplicate billing when your lab repeats the same diagnostic test on the same date of service, you can deploy modifier 91 to make your case.

Study the following guidance to learn when you should 鈥� or shouldn鈥檛 鈥� append modifier 91 to a test code:

Show medical necessity: When the ordering physician indicates that repeating the same test is medically necessary to obtain subsequent diagnostic values, you should append modifier 91 to each repeated test after the first. For instance, if the physician monitors a post-surgical patient for low sodium levels (E87.1, Hypo-osmolality and hyponatremia), they may order repeat sodium tests such as 84295 (Sodium; serum, plasma or whole blood) at intervals throughout a single day.  

Nix 91 for quality control: If you repeat a lab test for an inadequate sample or to resolve equipment errors, don鈥檛 bill the test code twice with modifier 91. Anytime a normal, one-time reportable result is all that the physician requests for patient treatment, you should not bill a repeat test that the lab conducts for quality control purposes.

Be specific: CPT^庐 provides specific codes for some assays that involve repeating the same test. For instance, a physician may order a glucose tolerance test to help diagnose diabetes. The test involves the lab processing three glucose specimens from a patient at specific intervals before and after consuming a glucose dose. You should report this test using 82951 (Glucose; tolerance test [GTT], three specimens [includes glucose]), not 82947 (Glucose; quantitative, blood (except reagent strip)) and 2 units of 82947-91.

Tip: Codes in the 鈥淓vocative/Suppression Testing鈥� CPT^庐 section describe unique assays that require repeating the same lab measurement. For example, 80400 (ACTH stimulation panel; for adrenal insufficiency. This panel must include the following: Cortisol (82533 x 2)) requires repeat cortisol testing. Modifier 91 does not apply when reporting an evocative/suppression test such as the one described by this code.

Beware panel pitfall: Sometimes a physician orders a test from the 鈥淥rgan or Disease-Oriented Panel鈥� CPT^庐 section, then based on findings, asks the lab to repeat just one analyte in the panel for medically necessary reasons. In those cases, you will be reporting two separate codes (a panel code and an individual analyte code), but you should still append modifier 91 to the repeat analyte test.

Here鈥檚 why: The National Correct Coding Initiative (NCCI) lists each component analyte test of a panel code as a column 2 code, meaning that you should not report the codes together. However, you鈥檙e allowed to override the edit under appropriate circumstances using an appropriate NCCI-associated modifier.

For instance: If the physician orders an electrolyte panel, then later in the day orders a repeat potassium test for a patient on diuresis, you should report 80051 (Electrolyte panel) and 84132 (Potassium; serum, plasma or whole blood), appending modifier 91 to 84132 to specify that you repeated the potassium for medical reasons.

Shine This Spotlight on Modifier 90

When a provider sends out a test to an external lab, modifier 90 may 鈥� or may not 鈥� be the solution to billing for the test. In fact, this modifier comes with lots of rules about who, when, and how to bill a lab service using modifier 90.

The following advice can help you make sure your claims get paid when your lab participates in referred billing, also called pass-through billing.

Who bills: In some circumstances, a referring lab that sends out a test may bill the code with modifier 90, while the outside reference lab that performs the test does not bill for it. Only one lab can bill for the test. If the reference lab performs and bills for the test, the referring lab cannot bill, and modifier 90 will not be involved in the claim.

69 only: Specifically, only an independent clinical laboratory (specialty code 69) can refer a test to an outside (reference) lab and bill for that test. If a physician office lab or other provider sends out a test, they cannot bill for it 鈥� the reference lab must bill for the test that it performs. This single rule significantly narrows the field of legitimate modifier 90 use.

When 69 is not enough: Even if you are an independent clinical lab with specialty code 69, you may not legitimately be able to use modifier 90. To bill for a test that you send out to a reference lab, you must meet one of the following conditions:

• The referring laboratory is located in, or is part of, a rural hospital.
• The referring laboratory is wholly owned by, or wholly owns, the reference lab that performs the test; or a third party owns both the referring and reference lab.
• The referring laboratory does not refer more than 30 percent of the clinical laboratory tests for which it receives requests for testing during the year (not counting referrals made under the wholly owned condition described above).

Identification: The name, address, and Clinical Laboratory Improvement Amendments CLIA number of both the referring laboratory and the reference laboratory must be on the claim or the payer will reject it.

Unprocessable: Be aware that if you bill for a referred test and your lab does not qualify for specialty code 69, or if you bill a referred test without modifier 90, Medicare will return the claim as unprocessable.

Source: You can find modifier 90 guidance in the .

Defeat Duplicate Billing Denials

If a payer receives the same CPT^庐 code for the same patient on the same date of service with no further explanation, someone is not getting paid for a 鈥渄uplicate bill.鈥�

For labs, the explanation to avoid the appearance of duplicate billing can be as simple as the appropriate modifier 鈥� 91 to show a medically necessary repeat test, or 90 to show that the referring lab is the appropriate entity to bill for the test.

The process: When a claim comes into the system, Medicare compares the following elements to try to identify an exact duplicate:

• Medicare Beneficiary Identifier (MBI) or provider number
• Date of service (from and through)
• Type of service
• Procedure code
• Place of service
• Billed amount

If the system already has a claim with identical elements in process or processed, the claim will likely be suspended or denied.

Results: A denial will result in no payment or delayed payment, in addition to adding to your administrative costs. If you get too many denials, Medicare may identify you as an abusive biller, landing you in oversight or investigation for fraud.  

Ellen Garver, BS, BA, Contributing Writer