Pitfalls in the interpretation of blood tests used to assess and monitor micronutrient nutrition status

Abstract

Assessment of micronutrient (MN) status is of particular importance in patients who require medical nutrition therapy, especially those requiring parenteral nutrition. Blood testing is generally the only tool available in clinical settings to assess MN status. However, using plasma or serum concentration faces pitfalls mainly because of the impact of inflammation that diverts the MNs from the circulating compartment. This review aims to review the blood tests that are useful and provide information about how to integrate functional markers of status to reach a clinically relevant diagnosis. Most impacted, with a significant and proportional decrease in plasma concentrations, are iron, selenium, zinc, thiamin, folic acid, cobalamin, and vitamins A, C, and D; copper is the only MN for which the plasma concentration increases. Therefore, a surrogate marker of inflammation, C‐reactive protein, must always be determined simultaneously. Validated intracellular and functional tests are proposed to improve status assessment. A protocol is suggested for tests required both on commencing and during nutrition support. A timely turnaround of analysis is essential for results to be clinically useful. In some cases, the appropriate provision of MNs should be commenced before results have been obtained to confirm the clinical assessment. Laboratory tests of MN status are an area prone to misuse and misinterpretation. The appropriate use and interpretation of such tests are essential to ensure the correct management of nutrition problems.

An update on essential micronutrients in critical illness

“…Reliable biomarkers have been identified. In inflammation, selenium levels decrease rapidly due to redistribution, but glutathione peroxidase activity (erythrocyte GPX-1 and plasma GPX-3) reflects status during inflammation, as does Selenoprotein P [2 ▪ ]. Broman et al [28 ▪ ] conducted a repletion study in 100 critically ill patients: values normalized after 5 days of 1000 μg/day, with an improvement of GPX-3, but it took 10 days for selenoprotein P to revert to optimal activity.…”

“…Among the important progress during the last period, the 2022 ESPEN micronutrient guideline has grouped clinically relevant information in one large document [1 ▪▪ ], while a review article has focused on the analytical caveats linked to the status assessment [2 ▪ ]. The text hereafter will focus mainly on research published in 2021 and 2022, mostly on observational cohorts and RCTs.…”

Select micronutrients for the preterm neonate

“…Although the PN and EN sources for preterm neonates are well documented in the NICU setting, knowing the exact nutrition composition of the primary EN source, human breast milk, can be challenging as human milk reflects the nutrition status of another individual whose nutrition intake is less known to the preterm infant's clinician. To further complicate assessment in preterm infants, micronutrient status is often determined by plasma and serum concentrations, which can be impacted by other processes such as inflammation, necessitating assessment of inflammatory markers, such as C‐reactive protein, as well 1 . Premature infants at risk for micronutrient deficiencies and toxicities often experience conditions related to inflammation (eg, infection or surgery), which can impact the accurate interpretation of these micronutrient measurements 1 .…”

Nutrition association with skin integrity and pressure injury in critically ill pediatric patients

“…However, the inflammatory processes, acute phase reaction, redistribution of binding proteins, and altered tissue uptake that occur in critical illness can alter plasma levels of several micronutrients, making interpretation difficult. The amount of a nutrient measured may not reflect its function because of differences in serum and intracellular levels affected by critical illness 60 . Table 3 provides guidance for initial micronutrient laboratory assessment and initial supplementation dosing 23 .…”

“…The amount of a nutrient measured may not reflect its function because of differences in serum and intracellular levels affected by critical illness. 60 Table 3 provides guidance for initial micronutrient laboratory assessment and initial supplementation dosing. 23 There are no established intervals for obtaining follow-up laboratory monitoring of micronutrient levels.…”