Conjunctival impression cytology fails to detect subclinical vitamin A deficiency in young children

Systematic Review and Meta-Analysis of the Relative Dose-Response Tests to Assess Vitamin A Status

Vitamin A (VA) is an essential nutrient often lacking in the diets of people in developing countries. Accurate biomarkers of VA status are vital to inform public health policy and monitor interventions. The relative dose-response (RDR) and modified-RDR (MRDR) tests are semi-quantitative screening tests for VA deficiency that have been used in Demographic and Health Surveys and VA intervention studies. A systematic review and meta-analysis of sensitivity and specificity were conducted to summarize the physiological evidence to support the RDR tests as methods to assess VA status and investigate the impact of different pathological and physiological states on the tests. A total of 190 studies were screened for inclusion, with 21 studies comparing the RDR tests with the gold-standard biomarker, liver VA concentration (68% and 80% sensitivity and 85% and 69% specificity for the RDR and MRDR, respectively). Nearly all studies with VA interventions in VA-deficient populations demonstrated a response of the tests to VA intake that would be expected to improve VA status. The impacts of chronic liver disease, protein malnutrition, age, pregnancy and lactation, infection and inflammation, and various other conditions were examined in 51 studies. The RDR and MRDR tests were reported to have been used in 39 observational studies, and the MRDR has been used in at least 6 national micronutrient surveys. The RDR and MRDR are sensitive tests for determining population VA status and assessing VA interventions. Although they are robust to most physiological and pathological states, caution may be warranted when using the tests in neonates, individuals with chronic liver disease, and those with protein or iron malnutrition. Research on further improvements to the tests to increase accessibility, such as sampling breast milk instead of blood or using intramuscular doses in subjects with malabsorption, will allow wider adoption. This review was registered with PROSPERO as CRD42019124180.

Vitamin a status in children with diarrhoea

Vitamin A status was measured in 50 pre-school children with acute and persistent diarrhoea. It was measured by (a) Fluorometric micromethod and (b) Conjunctival impression cytology (CIC). The results were compared with 25 normal children. Vitamin A status was lower in children with persistent diarrhoea whereas the results were comparable between the children with acute diarrhoea and control subjects.

Vitamin A: biomarkers of nutrition for development

Vitamin A is essential for multiple functions in mammals. Without vitamin A, mammals cannot grow, reproduce, or fight off disease. Because of its numerous functions in humans, biomarkers of vitamin A status are quite diverse. Assessment of liver reserves of vitamin A is considered the gold standard because the liver is the major storage organ. However, this measure is not feasible in human studies. Alternative biomarkers of status can be classified as biological, functional, histologic, and biochemical. Historically, signs of xerophthalmia were used to determine vitamin A deficiency. Before overt clinical damage to the eye, individuals who suffer from vitamin A deficiency are plagued by night blindness and longer vision-restoration times. These types of assessments require large population-based evaluations. Therefore, surrogate biochemical measures of vitamin A status, as defined by liver reserves, have been developed. Serum retinol concentrations are a common method used to evaluate vitamin A deficiency. Serum retinol concentrations are homeostatically controlled until liver reserves are dangerously low. Therefore, other biochemical methods that respond to liver reserves in the marginal category were developed. These included dose-response tests and isotope dilution assays. Dose-response tests work on the principle that apo-retinol-binding protein builds up in the liver as liver reserves become depleted. A challenge dose of vitamin A binds to this protein, and serum concentrations increase within a few hours if liver vitamin A concentrations are low. Isotope dilution assays use stable isotopes as tracers of total body reserves of vitamin A and evaluate a wide range of liver reserves. Resources available and study objectives often dictate the choice of a biomarker.

Physiologic indicators of vitamin A status

Physiologic indicators reflect the functional consequences of vitamin A deficiency and may be particularly useful for detecting early perturbations in vitamin A status. In conjunctival impression cytology (CIC), epithelial morphology and the presence or absence of mucin spots and goblet cells allow samples, obtained by applying filter paper to the temporal conjunctiva, to be characterized as normal or typical of vitamin A-deficient keratinizing metaplasia. The validity of CIC has been established with reference to other indicators of vitamin A status, and a prevalence of > or =20% abnormal results has been suggested as indicative of a public health problem. However, interpretation of specimens requires considerable training, and nonresponsiveness to supplementation is a frequent problem, which limits the utility of CIC as a method for evaluating the impact of intervention programs. Several simplified field protocols for dark adaptation have been developed, including one in which dark adaptation is assessed by the responsiveness of the pupil to light. Night blind subjects have consistently shown abnormal results on this test, and a significant response to placebo-controlled dosing has been demonstrated for children and pregnant women. Scores have correlated significantly with serum retinol and relative dose response. Pupillary dark adaptation testing is acceptable to most children as young as 2 y old. Limitations of this technique include a time course for recovery after dosing as long as 4-6 wk, a testing time of 20 min, and the need for 1-3 d of training. Given its low cost, noninvasive nature, and lack of the need to transport samples, pupillary dark adaptation offers advantages over other techniques for assessing a population's vitamin A status.