Urea Clearance

Test Code

7329

CPT Codes
84545

Includes

Urea Nitrogen (BUN)
Urea Nitrogen, 24 Hour Urine
Urea Clearance (calculation)

CPT code 81050 may be added at an additional charge for volume measurement

Preferred Specimen

10 mL urine from a 24-hour collection, collected in a plastic urine container, no preservative AND
1 mL serum collected in a serum separator tube (SST®)

Minimum Volume

2 mL urine • 0.5 mL serum or plasma

Other Acceptable Specimens

Urine preserved with 6N HCl to maintain a pH of <6.0, urine collected in a urinalysis (yellow-top, blue fill line, preservative) tube • Plasma collected in: lithium heparin or sodium heparin (green-top) tube

Instructions

Aliquot from a well-mixed 24-hour collection. Record total volume on both the transport container and the test requisition.

Transport Temperature

Urine: Refrigerated (cold packs)
Serum or plasma: Room temperature

Specimen Stability

Urine
Room temperature: 24 hours
Refrigerated: 7 days
Frozen: Indefinitely

Serum and plasma
Room temperature: 7 days
Reffigerated: 5 days
Frozen: 28 days

Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)

Anticoagulants other than heparin

Methodology
Spectrophotometry (SP)

Setup Schedule

A.M. Sets up 6 days a week.

Clinical Significance

This test estimates urea clearance based on the measurements of urea nitrogen levels in a blood specimen and a 24-hour urine specimen [1]. Urea clearance may be helpful in screening for chronic kidney disease and predicting glomerular filtration rate (GFR) in patients with advanced kidney dysfunction [1].

Urea is the main nitrogen-containing product of protein breakdown, and approximately 90% of the urea excretion is through the kidneys. In individuals with normal kidney function, urea clearance is roughly half of creatinine clearance owing to tubular back-diffusion of urea to blood and making it a poor alternative to assess kidney function [1]. Averaging creatinine clearance and urea clearance is also not recommended for this purpose owing to its inferior performance [2]. However, in patients with advanced kidney dysfunction, as the urea back-diffusion decreases urea clearance may better reflect GFR [1].

Blood urea levels, commonly expressed by the concentration of blood urea nitrogen (BUN), may be increased in individuals on a high-protein diet or with increased protein breakdown, gastrointestinal hemorrhage, chronic heart failure, or treatment with cortisol [2]. BUN level may be decreased in individuals with low-protein diet and end-stage liver disease [2].

Urinary urea levels, commonly expressed by the concentration of urinary urea nitrogen, may be increased in individuals with hyperthyroidism or excess protein intake or breakdown [3]. Urinary urea nitrogen level may be decreased in individuals with malnutrition, kidney damage or insufficiency, low-protein and high-carbohydrate diet, or liver disease. Pregnant persons and healthy children may also have low urinary nitrogen levels [3].

The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.

References
1. Oh MS, et al. Evaluation of renal function, water, electrolytes, and acid-base balance. In: McPherson RA, et al, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021
2. Lamb EJ, et al. Kidney function tests. Rifai N, et al. eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elservier Inc; 2022
3. Rao LV, et al. Laboratory tests. In: Rao LV, eds. Wallach's Interpretation of Diagnostic Tests. Pathways to Arriving at a Clinical Diagnosis. 11th ed. Wolters Kluwer; 2020.

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.