Modifier 91: Repeat Clinical Lab Test
Modifier 91 reports a lab test repeated the same day to get a new result for treatment. Medicare rules, when 91 is not allowed, and 91 vs 59, 76 and 77.
Modifier 91 is a billing modifier for a clinical laboratory test repeated on the same patient on the same day because treatment requires a new result, such as serial glucose or potassium levels during a hospital stay. Medicare limits it to tests paid under the Clinical Laboratory Fee Schedule (CLFS) and never allows it for reruns to confirm a result or fix a specimen or equipment problem (Pub. 100-04, ch. 16, §100.5.1).
Key takeaways
- 91 is only for lab tests paid under the Clinical Laboratory Fee Schedule, not for physician fee schedule services. A repeat X-ray or ECG uses 76 or 77.
- The repeat must be needed to get multiple results in the course of treatment.
- Not allowed for confirmation reruns, specimen or equipment problems, or when one result is all that's needed.
- Don't use 91 when a code already describes the series, such as a glucose tolerance test.
- 91 doesn't change the per-test rate. Each test is paid its CLFS amount.
Modifier 91 · payment effect
With and without the modifier
82947 without 91 · national facility
$0.00
82947-91 · No change to the rate
$0.00
Repeat clinical diagnostic test: paid at its full rate; not for re-running a test to confirm a result.
When to use modifier 91
Use 91 on the second and later lines of the same lab code on the same day when:
- The test is paid under the Clinical Laboratory Fee Schedule. CMS says 91 may be used only for those tests (ch. 16, §100.5.1).
- The patient needed multiple results in the course of treatment: a value had to be rechecked after an intervention or over time.
- No single code describes the series. Glucose tolerance and evocative/suppression testing have their own codes.
Example. A hospitalized patient with diabetic ketoacidosis has blood glucose (82947) checked at 8 a.m., noon and 4 p.m. to adjust insulin. Bill 82947 once without a modifier and the repeats with 91. For CLIA-waived tests, Noridian says to append QW along with 91.
Panels. If a test that's part of a panel already billed that day (for example a basic metabolic panel, 80048) is repeated later, NCCI edits pair the panel with its component tests. The NCCI Policy Manual allows 59 or 91 on the repeated component when the repeat is medically necessary (ch. X).
When not to use modifier 91
- Physician fee schedule services, including imaging, ECGs and pathology interpretations. Use 76 or 77.
- Different tests that NCCI bundles. That's 59 or an X modifier.
- Tests with a code for the series, such as glucose tolerance tests.
- Bilateral or site-specific services already described by RT, LT, 50, or finger, toe and eyelid modifiers (Noridian).
How modifier 91 affects payment
Modifier 91 carries no percentage. Each repeat is paid at the test's CLFS rate, and medically unlikely edits still cap units per day. These tests aren't priced on the physician fee schedule: the PFS files list them with status X, a statutory exclusion from physician fee schedule payment, which is why FeeBase shows no PFS rate for them.
Modifier 91 vs 59, 76 and 77
| Modifier | Use it when | Applies to |
|---|---|---|
| 91 | Same lab test repeated for a new result during treatment | CLFS lab tests |
| 59 | A component test repeated after its panel, or two different bundled services | Lab and other services |
| 76 | Same procedure repeated by the same physician | PFS services |
| 77 | Same procedure repeated by a different physician | PFS services |
FAQ
What is modifier 91 used for?
To report a clinical lab test performed more than once on the same patient on the same day because treatment required a new result each time.
What is the difference between modifiers 91 and 59?
91 reports the same lab test repeated for new results. 59 reports two different services that are normally bundled but were separate. For a component test repeated after its panel, the NCCI manual allows either 59 or 91.
Can I use modifier 91 for a repeat ECG or X-ray?
No. 91 is limited to tests paid under the Clinical Laboratory Fee Schedule. Repeat ECGs and imaging use 76 when the same physician repeats them and 77 when a different physician does.
Can modifier 91 be used to confirm an abnormal result?
No. CMS doesn't allow 91 for reruns to confirm initial results or because of specimen or equipment problems.
Does modifier 91 bypass NCCI edits?
Yes, it is one of the NCCI procedure-to-procedure-associated modifiers, but only when the repeat test is medically necessary and meets CMS's conditions.
Keep reading
- Modifier 76 and modifier 77 for repeat procedures on the physician fee schedule.
- Modifier 59 for separate, normally bundled services.
- Codes on this page: 82947 80048 93000
Sources: CMS Medicare Claims Processing Manual, Pub. 100-04, ch. 16, §100.5.1; Medicare NCCI Policy Manual, ch. I and ch. X (2026); Noridian JF Part B, Modifier 91 (updated May 12, 2025). Verified October 6, 2026.
