Summary & Overview
CPT 84100: Serum Inorganic Phosphate Measurement
CPT code 84100 represents a common clinical laboratory assay measuring inorganic phosphate in patient specimens such as serum. Nationally, measurement of phosphate is an essential diagnostic and monitoring tool for conditions affecting mineral metabolism, renal function, and electrolyte balance, making this test routinely ordered across inpatient and outpatient settings. Key payers covered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise explanation of the clinical purpose and typical settings for CPT code 84100, an overview of payer coverage patterns and allowed charge benchmarks where available, and notes on coding context relevant to laboratory services. The publication also summarizes common modifier usage, service line placement, and areas where policy updates or prior authorization requirements may affect billing workflows. This summary is intended for administrators, billing professionals, and clinical laboratory managers seeking a clear national overview of CPT code 84100 and its relevance to routine laboratory practice.
Billing Code Overview
CPT code 84100 describes a laboratory test that measures the level of inorganic phosphate (phosphorus not contained in biological compounds) in a patient specimen, commonly serum. This test is performed by a laboratory analyst and is used to evaluate phosphate balance, which has clinical relevance for bone metabolism, kidney function, and metabolic disorders.
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Service type: Clinical laboratory quantitative chemistry test
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Typical site of service: Hospital laboratory, independent clinical laboratory, or outpatient phlebotomy/collection site
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Clinical & Coding Specifications
Clinical Context
A 62-year-old male with chronic kidney disease stage 3 presents to the outpatient nephrology clinic for routine metabolic monitoring. The clinician orders a serum inorganic phosphate level to assess mineral and bone disorder risk and guide adjustment of phosphate binders. A phlebotomist collects a serum specimen during the office visit; the specimen is sent to the clinical laboratory. A medical laboratory scientist performs the quantitative inorganic phosphate assay (84100) using a validated automated chemistry analyzer. Results are reported to the ordering nephrologist and documented in the electronic health record for medication and dietary counseling.
Coding Specifications
| Modifier | Description | When to Use |
|---|---|---|
26 | Professional component | Use when only the professional component of a lab test is billed separately by a physician who performs or interprets the test. |
90 | Reference (outside) lab | Use when testing is performed by an outside reference laboratory and billed accordingly. |