Selected laboratory-based biomarkers for assessing vitamin A deficiency in at-risk individuals

Vitamin A deficiency is a highly detrimental micronutrient deficiency associated with poor growth, impaired immune responses, increased incidence of disease, ocular impairments, and maternal and child mortality. Reliable diagnostic assessment of vitamin A status is crucial to inform its clinical management. Currently, direct index measures and dose response biomarkers have been developed to provide assessments of vitamin A status.

To determine the accuracy of index tests routinely used as markers of subclinical vitamin A deficiency in individuals at risk for vitamin A deficiency. Secondary objectives are to assess covariates as sources of heterogeneity for the accuracy of index tests routinely used as markers of subclinical vitamin A deficiency.

We searched CENTRAL, MEDLINE, Embase, and six other databases up to 18 August 2022, without restriction (any sex, age, pregnancy status, breastfeeding status, physiological condition, living in any country).

We included any studies implementing concurrent measurement of at least one reference standard and one index test to measure vitamin A status. Eligible studies included cross-sectional or cohort-accuracy studies; longitudinal studies; and direct, indirect, and random comparison studies, in which multiple index tests with a reference standard were evaluated. Interventional studies measuring vitamin A status following supplementation or intervention were also included, while case-control studies defining cases by vitamin A status were excluded.

Two review authors independently screened studies and extracted data. We evaluated the methodological quality, that is, risk of bias of included studies and their applicability by using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) tool. When meta-analysis was appropriate, we used random-effects bivariate models to obtain pooled estimates of sensitivity and specificity. We stratified all analyses by the reference standard and cutoff used, and assessed the certainty of the evidence using GRADE.

We included 40 studies reporting 65 records. None of the studies was designed as a diagnostic test accuracy (DTA) study, limiting our analyses and assessments. Index test performance was described by 25 studies for serum or plasma retinol (SR) versus retinol isotope dilution (RID), 16 studies for SR versus liver vitamin A, eight studies for retinol-binding protein (RBP) versus retinol isotope dilution (RID), three studies for RBP versus liver vitamin A, one study for breast milk vitamin A versus RID, three studies for relative dose response (RDR) versus RID, and four studies for RDR versus liver vitamin A. No studies evaluating modified RDR were eligible for inclusion. Specificity data were available from all studies; sensitivity was often estimable from only a portion of studies due to some studies having no condition-positive cases according to the reference standard (zero true positive and false negative cases). One comparison, RDR versus RID, yielded no sensitivity data, therefore we could evaluate only pooled specificity data. We generally judged risk of bias as 'unclear' across studies. Serum or plasma retinol for diagnosing vitamin A deficiency SR pooled sensitivity against RID at the 0.1 μmol/g cutoff was 10% (95% confidence interval (CI) 2 to 38; 23 studies, 385 participants; very low-certainty evidence), and specificity was 92% (95% CI 85 to 96; 23 studies, 1110 participants; low-certainty evidence). SR pooled sensitivity against RID at the 0.07 μmol/g cutoff was 13% (95% CI 4 to 34; 24 studies, 246 participants; very low-certainty evidence), and specificity was 94% (95% CI 87 to 97, 24 studies, 1295 participants; low-certainty evidence). SR pooled sensitivity against liver vitamin A at the 0.1 μmol/g cutoff was 53% (95% CI 30 to 75; 16 studies, 192 participants; very low-certainty evidence) and specificity was 83% (95% CI 63 to 93; 16 studies, 370 participants; moderate-certainty evidence). SR pooled sensitivity against liver vitamin A at the 0.07 μmol/g cutoff was 54% (95% CI 33 to 74; 16 studies, 137 participants; very low-certainty evidence) and specificity was 79% (95% CI 57 to 91; 16 studies, 348 participants; moderate-certainty evidence). Retinol-binding protein for diagnosing vitamin A deficiency RBP pooled sensitivity against RID at the 0.1 μmol/g cutoff was 50% (95% CI 33 to 67; 8 studies, 30 participants; low-certainty evidence) and specificity was 76% (95% CI 72 to 81; 8 studies, 730 participants; moderate-certainty evidence). RBP pooled sensitivity against RID at the 0.07 μmol/g cutoff was 45% (95% CI 31 to 59; 8 studies, 47 participants; low-certainty evidence) and specificity was 77% (95% CI 71 to 82; 8 studies, 711 participants; moderate-certainty evidence). RBP pooled sensitivity against liver vitamin A at the 0.1 μmol/g cutoff was 0% (95% CI 0 to 100; 3 studies, 12 participants; very low-certainty evidence) and specificity was 98% (95% CI 84 to 100; 3 studies, 40 participants; very low-certainty evidence). RBP pooled sensitivity against liver vitamin A at the 0.07 μmol/g cutoff was 0% (95% CI 0 to 100; 3 studies, 9 participants; very low-certainty evidence) and specificity was 98% (95% CI 85 to 100; 3 studies, 43 participants; very low-certainty evidence) Relative dose response for diagnosing vitamin A deficiency RDR pooled sensitivity against RID at the 0.1 μmol/g and 0.07 μmol/g cutoffs were not estimable due to lack of true-positive and false-negative cases from three studies. RDR pooled specificity against RID at the 0.1 μmol/g cutoff was 89% (95% CI 26 to 99; 3 studies, 34 participants; low-certainty evidence), and RDR pooled specificity against RID at the 0.07 μmol/g cutoff was 91% (95% CI 54 to 99; 3 studies, 34 participants; low-certainty evidence).

Available data indicate that methods to determine vitamin A deficiency had generally low sensitivity, when estimable (0% to 54%), and generally high specificity (74% to 94%) in individuals at risk for vitamin A deficiency. Estimates should be interpreted with caution because no included studies were designed or conducted as DTA studies. Data assessing the accuracy of the breast milk vitamin A, RDR, and MRDR compared to reference standards, particularly in patients with vitamin A deficiency, are limited.