Ideal biochemical test
An ideal biochemical test should be specific, minimally invasive ( performable on blood or urine) and preferably simple and inexpensive. Due to the complex nature of the biochemical machinery, it is difficult to find an ideal test. The criterion of specificity is most difficult to meet. Very often it may be desirable to apply more than one tests.
Biochemical tests for detecting vitamin malnutrition
These are of two type: 1) measurement of the concentration of the vitamin or coenzyme in blood or urine and 2) functional tests.
Urinary and plasma levels often reflect the immediated dietary intake and not necessarily the state of the tissues. Measurement of vitamin levels in 24-hour urine collection over 2-3 days is more reliable than random or timed collections. Load return tests which used to be quite popular for the evaluation of B-vitamin status, have gone out of vogue but deserve a fresh look since they are non-invasive.
Blood levels of vitamin or coenzymes are often difficult to measure accurately. The more sentitive HPLC methods are fast replacing the chemical, microbiological and enzyme coupled assays for measuring vitamins and coenzymes. The choice of the blood compartment (RBC, WBC or serum) is based on empirical experience. Thus while blood cells are preferred for vitamins B1,B2, and folic acid. Vitamin B12 measurements are done in serum. Plasma pyridoxal phosphate is a recommended method for assessing vitamin B6 status. Fat soluble vitamins are always measured in serum because blood cells have negligible amounts of these vitamins.
Functional tests are applicable to B-group of vitamins where the biochemical functions are known. Functional tests are of two types: a) enzyme activation tests and b) metabolite accumulation tests. In the former the activity of an erythrocyte enzyme in the absence and presence of staurating amounts of the coenzyme is measured. Activation coefficient (enzyme activity,+ coenzyme/enzyme activity, -coenzyme) is inversely related to the vitamin nutrition status. Examples of such tests are activation of erythrocyte transketolase (ETK-AC)(thiamin),glutathione reductase (EGA-AC) (riboflavin) and aspartate aminotransferase (EASPAT-AC) and alanine aminotransferase (EalaAT-AC)(pyridoxine).
In the metabolite accumlation test, the level of a metabolite in a timed sample of blood or urine is measured after administering a load of a precursor whose metabolism depends on a vitamin dependant pathway. Examples of such tests are : blood pyruvate and lactate after glucose load (thiamin), urinary xanthunrenic acid after tryptophan load (pyridoxine), urinary fromiminoglutamic acid (FIGLU)after histidine load (folic acid) and urinary methyl malonic acid after valine load (vitamin B12). Detailed discussion of these tests is available in the literature(3,4).
