Quote from Dr. Garrett Smith on November 8, 2018, 12:33 pm

https://emedicine.medscape.com/article/126004-overview
"Vitamin A can be mobilized from the liver to peripheral tissue by a process of deesterification of the retinyl esters. In blood, vitamin A is bound to RBP, which transports it as a complex with transthyretin. The hepatic synthesis of RBP is dependent on the presence of zinc and amino acids to maintain its narrow serum range of 40-50 mcg/dL."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5490644/
"Serum retinol concentrations reflect liver vitamin A stores only when liver stores are severely depleted (< 0.07 μmol/g liver) or extremely high (>1.05 μmol/g liver) (4). However, the population distribution of serum retinol concentrations or its carrier retinol-binding protein (RBP) or the prevalence of individuals with serum retinol or RBP concentrations <0.7 μmol/L can provide important information about the vitamin A status of a population and may reflect the severity of VAD as a public-health problem (5). Studies have shown an ∼1:1 relation between serum retinol and RBP, and therefore, RBP is often substituted as an indicator of vitamin A status (6) with the advantage of RBP being more robust for sample collection and handling processes and less expensive to measure (7)."

https://emedicine.medscape.com/article/126004-workup
"Laboratory Studies

See the list below:

• A serum retinol study is a costly but direct measure using high-performance liquid chromatography. A value of less than 0.7 mg/L in children younger than 12 years is considered low. ^[21]
• A serum RBP study is easier to perform and less expensive than a serum retinol study, because RBP is a protein and can be detected by an immunologic assay. RBP is also a more stable compound than retinol with respect to light and temperature. However, RBP levels are less accurate, because they are affected by serum protein concentrations and because types of RBP cannot be differentiated. ^{[22, 23, 24]}
• A zinc level is useful because zinc deficiency interferes with RBP production."