Quality control and technical assistance
Collection and storage of samples
Specific recommendations on sample collection for serum, plasma, or blood-spot assays are given under “Assay Specific Queries” (page 1839S of the pdf download), of the BOND Review on Vitamin A.
In general blood samples should be protected from light and kept cool but not frozen until they are delivered to the lab for processing, analysis or storage; DBS should be thoroughly dried (in humid climates) before being stored.
Standard Reference Materials
Standard reference materials are available from the National Institute of Standards and Technology NIST (SRM 1950 Metabolites in Human Plasma).
Confounders of vitamin A deficiency assessment
Zinc or protein deficiency Concurrent zinc and/or protein deficiency can lower hepatic synthesis of retinol binding protein, resulting in lower plasma/serum retinol concentrations (5)
Iron deficiency Concurrent iron deficiency can reduce hepatic mobilisation of retinol, resulting in lower plasma/serum retinol concentration (45).
Pregnancy During pregnancy, haemodilution affects serum retinol concentrations, resulting in lower plasma/serum retinol concentrations (5)
Concomitant infections As mentioned above, the acute phase response has a transient negative effect on serum retinol concentrations, thus inflammatory markers (C-reactive protein and α1-acid glycoprotein) should be measured to interpret serum retinol data. Regression analysis can be used to adjust serum retinol concentrations in populations with high rates of inflammation (19).
Obesity Serum RBP concentration may not reflect vitamin A status in obese individuals (5).
US CDC Vitamin A assessment quality assurance scheme: VITAL-EQA
The US Centers for Disease Control and Prevention (CDC) Vitamin A Laboratory – External Quality Assurance (VITAL-EQA) program is a standardization program designed to provide laboratories measuring nutritional markers in serum with an independent assessment of their analytical performance. The program assists in monitoring the degree of variability and bias in laboratory assays. Information received from the program can be used to eliminate bias or precision problems in the assay system, and confirm the quality of analysis and increase the confidence level of the lab.
Participation in VITAL-EQA is voluntary and free of charge. Results are not used for accreditation or certification.
See the VITAL-EQA webpage for more information and details on how to sign up.