An HPLC Ultraviolet Method Using Low Sample Volume and Protein Precipitation for the Measurement of Retinol in Human Serum Suitable for Laboratories in Low- and Middle-Income Countries.

BACKGROUND:Assessing vitamin A status in populations remains a high public health priority for low- and middle-income countries. However, analytical difficulties with serum retinol measurements persist in international laboratories. Nearly all participants in a Centers for Disease Control and Prevention external quality assessment program use HPLC to measure serum retinol, but round-to-round results failing to meet acceptable criteria suggest the need to provide a straightforward stable HPLC ultraviolet (UV) method that can be adopted by these laboratories to improve performance. We present a protein precipitation HPLC-UV method that measures serum retinol below the deficiency cutoff value (<0.7 μmol/L or 20 μg/dL) that is suitable for low- and middle-income countries and uses commercially available materials. METHODS:Serum (25 μL) added to retinyl acetate was precipitated with acetonitrile (125 μL) to extract retinol. Solvent-based calibration solutions required no extraction. Calibration used either single-point (50 μg/dL) or multipoint solutions (0.52-100 μg/dL). C18 column (4.6 × 100 mm) and acetonitrile with 0.1% triethylamine/water (83/17, v/v) as isocratic mobile phase (1.1 mL/min), achieved baseline separation (7 minutes). RESULTS:With only 25 μL of serum, the limit of detection was 0.52 μg/dL. Single- and multipoint calibration generated equivalent results. Over several years, between-run imprecision was ≤7.1% in multiple quality-control materials. Overall mean (CV) method bias for NIST-certified reference materials (e-series) was -0.2% (5.8%). Maximally, 180 samples were processed within 24 h. CONCLUSIONS:This method was robust and stable over years and accurately measured serum retinol with low-volume samples. Thus, it may be of interest to low- and middle-income countries and to pediatric and finger stick applications.