RETINOL BINDING PROTEIN

CLINICAL UTILITY:

Retinol binding protein may be sensitive to changes in dietary protein and energy intake and, because of its short half-life (0.5 day), high levels of RBP reportedly result from impaired renal filtration. Preliminary studies have shown that RBP is highly correlated with serum vitamin A over a broad range of the vitamin concentration. As RBP is catabolized in the tubule, renal tubular poisoning by chronic heavy metal poisoning (e.g., cadmium) or impairerd tubular function of other causes is associated with increased urinary RBP.

Retinol binding protein functions to bind and transport retinol (vitamin A). Because of its low molecular weight (21 Kd), RBP must bind to transthyretin within the plasma to prevent passage through the glomerular membrane. After delivery of retinol to the tissue, RBP is released from transthyretin.

Low serum RBP may be found in nutritional disorders such as protein-calorie malnutrition and vitamin A deficiency. Low RBP levels have been observed in 80% of retinitis pigmentosa patients. Decreased levels of RBP has also been associated with chronic liver disease. Increased levels of RBP can be seen in chronic renal disease reflecting the failure of the kidney to filter RBP from the circulation after delivery of vitamin A to the tissues. Renal tubule poisoning is reflected in elevated urinary RBP, as tubule cells normally metabolize RBP to amino acids.

METHOD DESCRIPTION:

Retinol binding protein in this laboratory is measured in a nephelometric method.

REFERENCE RANGE:

2.1 - 6.4 mg/dl (SI Units: 0.21-0.63; conversion: mg/dL *0.01 = SI Units).

SPECIMEN REQUIREMENTS:

Serum is the specimen of choice, plasma is acceptable; 0.5 mL minimum, 1.0 mL optimum.