In general, reference values are population specific; accordingly, each major laboratory in homogeneous areas has to derive them from a clinically healthy reference population selected with very specific criteria. These values should preferably be given in percentiles.
In general, cutoff points for an appropriate interpretation of results have been derived statistically from reference values. A current procedure for constructing cutoff point consists of determining the biochemical values that correspond to the earliest determinable physiological, metabolic, functional, and morphological alterations. Such an approach has been followed only in a very few cases, and consequently most available cutoff points should be considered as tentative.
For albumin, the guidelines for interpretation suggested in 1974 are still in use. For children and adults, values <28 gl-1 are indicative of a deficient (high-risk) status, and for pregnant women this value is <30gl-1. A marginal (moderate-risk) status is indicated by the following values: infants, <25gl-1; children 1-5years, <30gl-1; children 6-17years, <35gl-1; adults, 28-34gl-1; pregnant women at first trimester, 30-39 g/l-1; and pregnant women at second and third trimester, 30-34gl-1. All values above the moderate risk are indicative of an acceptable (low-risk) status.
Reference values for transport proteins are provided by plate producers. Tentatively, 0.10gl-1 for prealbumin and 25 mg l-1 for retinol binding protein are considered indicative of protein deficiency. For transferrin, values less than 1 g l-1 are considered indicative of severe protein depletion; marginal status values are between 1 and 2 g l-1.
For the interpretation of serum phospholipid essential fatty acid values, the ratio triene-tetraene (C 20:3 n-9/C 20:4 n-6) above 0.2 was considered by Holman to be the upper limit of normalcy. The ratio C 22:5 n-6/C 22:6 n-3 can be a sensitive index of n-3 fatty acid deficiency.
The cutoff points for the most widely used micro-nutrient tests in adults are reported in Table 2. Cutoff points are different for children, pregnant and lactating women, and the elderly. These values can be found in reference texts. For antioxidant vitamins and provitamins to prevent chronic diseases, the following optimal plasma levels have been proposed: retinol, >2.5 mmoll-1; ^-carotene, >0.40 mmoll-1;
a-tocopherol, >30 mmoll 1; and ascorbic acid, >50 mmol l_1.
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