A Urinary Phosphorus Test measures the amount of phosphorus excreted in urine over a specific period, usually 24 hours. It helps evaluate kidney function, phosphorus metabolism, and conditions affecting calcium and phosphate balance.

Why is the test done?

Doctors may order this test to:

• Assess kidney function (since kidneys regulate phosphorus levels).
• Diagnose or monitor bone disorders, including osteoporosis.
• Evaluate parathyroid gland function (parathyroid hormone regulates phosphorus).
• Check for electrolyte imbalances in conditions like chronic kidney disease (CKD) or hyperparathyroidism.
• Determine phosphate wasting disorders (e.g., Fanconi syndrome).

How is the test performed?

1. 24-hour urine collection:
   • You will collect all urine produced over 24 hours in a special container.
   • The sample must be stored properly, usually refrigerated.
2. Random urine sample: Sometimes, a single urine sample may be tested for phosphorus levels.

Normal Range

• Typically 400–1,300 mg per 24 hours (may vary slightly depending on the lab).

Causes of High Urinary Phosphorus (Hyperphosphaturia)

• Hyperparathyroidism (overactive parathyroid glands)
• Vitamin D toxicity
• Chronic kidney disease (CKD)
• Excessive dietary phosphorus intake
• Certain diuretics
• Diabetic ketoacidosis

Causes of Low Urinary Phosphorus (Hypophosphaturia)

• Hypoparathyroidism (underactive parathyroid glands)
• Malnutrition or low phosphorus intake
• Vitamin D deficiency
• Acute kidney injury or advanced CKD

Preparation for the Test

• Your doctor may advise dietary restrictions before the test.
• Certain medications (e.g., diuretics, antacids, phosphate binders) may need to be adjusted.