CPT code 84100: Phosphorus test, blood phosphate level2026 Medicare lab fee · Clinical Laboratory Fee Schedule
Measures inorganic phosphate in blood for evaluation of mineral balance, kidney function, and related metabolic or bone disorders.
Medicare pays $4.74 for 84100 under the 2026 Clinical Laboratory Fee Schedule: one national amount, the same in every state and setting.
Medicare lab fee · CLFS 2026 Q4
National CLFS amount
$4.74
In effect since Jan 1, 2026. The same amount in every state, payment locality and setting.
- Geographic adjustment
- None
- Office vs facility
- Same
- Since 2020
- Unchanged
- CMS updates
- Quarterly
Medicare pays the lesser of the lab’s charge and this amount. Patients usually owe no Part B deductible or coinsurance for covered clinical lab tests.
On this page 8 sections
What 84100 covers
This chemistry assay measures inorganic phosphate in a blood specimen, typically serum or plasma. Clinicians may order it when evaluating kidney function, disturbances in mineral balance, or bone and parathyroid disorders. Clinical laboratories perform the assay, and clinicians use the result alongside the patient’s other chemistry findings and clinical presentation.
Report 84100 for the blood phosphorus assay; urine phosphorus is a different test. Medicare pays the service only through the Clinical Laboratory Fee Schedule (CLFS), at one national amount that is the same across states and localities, with no office-versus-facility difference. The 2026 national CLFS amount is $4.74, unchanged since 2020. The physician fee schedule excludes this test by statute, so payment is through the CLFS.
This summary was written with AI assistance from CMS Clinical Laboratory Fee Schedule data. Amounts on this page come directly from CMS files.
How Medicare pays 84100
Clinical lab tests aren’t priced like physician services. They’re paid from their own fee schedule, with its own rules. See the whole lab fee schedule.
One national amount
CMS sets a single Clinical Laboratory Fee Schedule amount for 84100. There’s no geographic (GPCI) adjustment, so it pays the same in every state and payment locality.
No office or facility rate
Lab tests aren’t built from relative value units, so there’s no office and facility split: the place of service doesn’t change the CLFS amount.
Updated every quarter
CMS republishes the CLFS each quarter. New tests, often proprietary laboratory analyses (PLA codes), join during the year; existing amounts are updated each January.
Excluded from the physician fee schedule
The physician fee schedule lists 84100 with status X (excluded by statute), so the test itself is paid only from the CLFS.
84100 on the lab fee schedule since 2020
84100 · CLFS 2026 Q4 (current)
$4.74
Unchanged since Jan 1, 2020
27 quarterly CLFS releases on file, first CLFS 2020 Q1. A code missing from a quarter wasn’t on that release.
84100 compared with similar tests
- 84105Urine phosphorusUrine specimen$5.78
- Choose 84100 for blood phosphorus and 84105 for urine phosphorus; the specimen determines the code.
- 82310Total calciumTotal, serum or plasma$5.16
- Code 84100 measures phosphorus, while 82310 measures calcium. The tests may be ordered together but report different analytes.
- 84132PotassiumSerum, plasma, or whole blood$4.76
- Code 84100 measures blood phosphorus; 84132 measures serum potassium. Select the code for the analyte actually tested.
84100 billing questions
When should 84100 be used instead of 84105?
Use 84100 for a blood phosphorus assay. Use 84105 when the assay is performed on urine.
What specimen does 84100 represent?
It represents a blood phosphate measurement, typically performed on serum or plasma. Document the specimen tested in the laboratory record.
Does Medicare pay 84100 under the physician fee schedule?
No. Medicare pays 84100 through the CLFS; the physician fee schedule excludes it by statute.
Does the CLFS amount vary by location or setting?
No. CMS sets one national amount for 84100, without a geographic or office-versus-facility adjustment.
Is 84100 the same test as a calcium assay?
No. 84100 measures phosphorus, while 82310 measures calcium; clinicians may order both when evaluating mineral balance.
Where this amount comes from
FeeBase reads lab amounts directly from the CMS Clinical Laboratory Fee Schedule file for the quarter: one row per code, with its payment indicator and national amount. Contractor-priced tests are labeled, never given a made-up amount. Amounts are Medicare payment limits, not a patient’s bill or a commercial rate.
CMS CLFS 2026 Q4 · effective Oct 1, 2026 through the day before Jan 1, 2027
Show the CMS file line behind this amount
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