84100

Phosphorus inorganic (phosphate); serum

CPT 84100 describes the laboratory procedure for measuring the level of inorganic phosphorus in a patient's serum or plasma. Phosphorus is an essential mineral that serves as a fundamental building block for DNA, cell membranes, and energy molecules like adenosine triphosphate (ATP). It is also a key component of hydroxyapatite in bone tissue. The homeostasis of phosphorus is tightly regulated by a complex interplay between the parathyroid hormone (PTH), vitamin D, and the kidneys. When a physician orders this test, it is often to investigate abnormalities in calcium metabolism, as phosphorus and calcium levels are inversely related in many physiological processes. This quantitative analysis is typically performed using an automated chemistry analyzer. The standard method involves reacting the phosphate in the serum with ammonium molybdate in an acidic environment to form a phosphomolybdate complex. This complex is then measured directly at a specific ultraviolet wavelength or reduced to molybdenum blue for measurement in the visible spectrum. Interpreting serum phosphorus results requires consideration of the patient's age, as children have significantly higher levels due to active bone growth, as well as renal function and recent dietary intake. Abnormal findings can indicate a wide range of clinical conditions, from renal failure and hypoparathyroidism (which lead to hyperphosphatemia) to malnutrition, alcoholism, and primary hyperparathyroidism (which lead to hypophosphatemia). Measurement of serum phosphorus is also essential for monitoring patients on dialysis, those with metabolic bone diseases, or patients receiving total parenteral nutrition (TPN). The procedure is usually performed on a venous blood sample collected in a standard red-top or tiger-top tube, requiring centrifugation to separate the serum from the cellular components promptly to avoid leakage of intracellular phosphorus into the serum sample.

Clinical Indications

  • Evaluation of suspected renal disease or chronic kidney disease (CKD)
  • Diagnosis of parathyroid disorders (hyperparathyroidism or hypoparathyroidism)
  • Monitoring of patients with vitamin D deficiency or toxicity
  • Investigation of metabolic bone diseases such as rickets or osteomalacia
  • Assessment of electrolyte imbalance in patients with diabetic ketoacidosis
  • Monitoring of patients receiving total parenteral nutrition (TPN)
  • Evaluation of patients with chronic alcoholism or malnutrition
  • Investigation of muscle weakness or bone pain of unknown etiology

Procedure Steps

  1. Collection of a venous blood sample via venipuncture into a serum separator tube (SST) or heparinized plasma tube.
  2. Labeling of the specimen with patient identifiers and collection time.
  3. Prompt delivery of the specimen to the laboratory to prevent hemolysis.
  4. Centrifugation of the sample to separate the serum or plasma from the red blood cells.
  5. Aliquoting the serum for analysis on an automated chemistry analyzer.
  6. Introduction of reagents, specifically ammonium molybdate, into the serum sample.
  7. Incubation of the mixture to allow the formation of the ammonium phosphomolybdate complex.
  8. Measurement of the light absorbance of the complex using spectrophotometry at a specific wavelength (e.g., 340 nm).
  9. Calculation of the inorganic phosphorus concentration by comparing sample absorbance against a known standard curve.
  10. Verification of results against quality control standards and reporting in mg/dL or mmol/L.

Coding Guidelines

  • CPT 84100 is specifically for the measurement of inorganic phosphorus in serum or plasma.
  • For the measurement of inorganic phosphorus in urine, use CPT code 84105.
  • For the measurement of inorganic phosphorus in other body fluids (e.g., peritoneal fluid), use CPT code 84110.
  • Do not report 84100 as part of a Basic Metabolic Panel (80047, 80048) or Comprehensive Metabolic Panel (80053), as phosphorus is not a component of these standardized panels.
  • If multiple samples are collected and analyzed on the same day for a single patient, use appropriate modifiers (e.g., modifier 91) if medically necessary and supported by documentation.
  • Ensure the diagnosis code (ICD-10) supports the medical necessity for phosphorus testing to ensure reimbursement.

Associated ICD-10 Codes