Title: Folic Acid Intake Assessments
Key words: Tissue biomarkers, serum , red cell folate, isotope dilution method, packed red cell volume (PRCV), Vitamin B12,
Date: Nov 1998
Category: 14. Measurement
Author: Dr van Rhijn
Dietary intake is generally difficult to assess. The use of tissue biomarkers may be a more objective indicator of nutrient intake that can also be used to validate dietary assessment methods. The most cost-effective technique, albeit invasive, for assessing dietary intakes of folic acid in a GP setting would be to measure serum and red blood cell folate levels from a representative sample of the practice. Selecting a reliable laboratory is essential, both to secure valid and quality data and to minimise the potential margin for error.
The measurement of serum folate (collective group name for folic-acid derivatives) is the most widely used method to assess folic acid status. However, measuring whole blood (red-cell) folate provides a more accurate reflection of total body stores because the former measurement is sensitive to recent dietary and metabolic changes. A standard method to determine folate levels is by microbiological assay with Lactobacillus casei. Unfortunately, this is sensitive to the presence of certain drugs, such as methotrexate and antibiotics. A variation technique of competitive binding assay (utilising the high-affinity binding protein derived from milk) is used instead. This isotope dilution method of whole blood haemolysed in 1% fresh ascorbate (folate preservative/antioxidant), requires a high pH in the incubation mixture (to raise the affinity of 5-methyltetrahydrofolate) and boiling of the serum samples to destroy endogenous folate-binding protein.
In order to put the results into clinical perspective, it is important to measure packed red-cell volume (the value is expressed in terms of folate per unit of packed red-cells), as well as Vit B12 (an essential co-factor in the metabolism of folic-acid). This is in order to unmask a secondary folate deficiency due to a potential Vit B12 deficiency in megaloblastic anaemia.
The normal reference values for serum folate are usually between 3 – 20 ng/ml, and for red-cell folate ranging from 145 – 400 ng/ml red cells. UK surveys shows that red-cell folate levels were more closely related to dietary folate intake than plasma folate.
A consistent protocol is essential to minimise the potential adverse effects of uncontrolled external variables that may affect folic acid status. Factors such as time (transit), storage schedules (light, oxygen and heat) or other variables as discussed above (the effect diet, Vit B12, Vit C and drugs) must also be taken into consideration. The laboratory assay procedures used must be sensitive, robust and sufficiently free from fluctuations and interference from other contaminants, and even external quality assurance within and between laboratories may be essential to verify their outcome measures. Establishing validity requires comparing the mean values of test results with external reference values.
Validity implies that the data are true, unbiased measures of the variable, and assumes there are no errors in data collection, analysis and interpretation. Therefore, selecting and adhering to a robust protocol for measuring folic-acid levels and comparing them with reference values is essential, in order to yield scientifically reliable data.
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