This test should be regarded as 'Research Use Only'. This test has not been cleared or approved by the U.S. Food and Drug Administration. It may not be covered by insurance and providers need to inform all patients of this prior to ordering.
For Medicare patients, an Advanced Beneficiary notice (ABN) is required; for Medicaid patients, a Noncovered Services Waiver is required, and for commercial payers, prior authorization should be obtained.
Retinol-binding protein is a low-molecular-weight protein of 21 kDa that transports retinol (vitamin A alcohol) from the liver to peripheral tissues. Retinol-binding protein is most often found bound to transthyretin, but a small, unbound fraction (<10%) passes freely through glomerular membranes and is reabsorbed by renal proximal tubules cells where it is catabolized. Due to extensive tubular reabsorption, under normal conditions very little of the filtered retinol-binding protein appears in the final excreted urine. Therefore, an increase in the urinary excretion of retinol-binding protein indicates proximal tubule injury and/or impaired proximal tubular function. Measurement of retinol-binding protein in urine is, therefore, a useful aid in the monitoring and/or diagnosis of kidney disease.