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).

Quality assurance

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.