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.