The external quality assurance of phylloquinone (vitamin K₁) analysis in human serum

Abstract

The vitamin K external quality assurance scheme (KEQAS) aims to assist in the harmonization of phylloquinone (vitamin K(1)) analysis in order to improve the comparability of clinical and nutritional studies. Serum samples were despatched to 17 groups from eight countries during 2000-2006. Using pilot data (1996-1999), an analytical performance target of 20% absolute difference from the all-laboratory trimmed mean (ALTM) was assigned and formed the basis for interlaboratory comparison. Assay specificity, analytical bias and assay performance were evaluated. From 21 batches of samples distributed, 414 results were reported of which 2.7% were outliers. The mean interlaboratory absolute difference from the ALTM was 21.7% with 47% of groups consistently meeting the performance target. The mean interlaboratory coefficient of variation was 29.6%. The false positive rate for phylloquinone depleted samples was high at 35%. Bias was found to be independent of HPLC-detector type (fluorescence vs electrochemical). Assay characteristics for the measurement of phylloquinone in human serum compare favourably with methods for analytes at equivalent concentrations. The high proportion of false positive results suggest that poor assay specificity at low phylloquinone concentrations is a common problem, which in the clinical setting could lead to underreporting of vitamin K deficiency.

Circulating Phylloquinone Concentrations of Adults in the United States Differ According to Race and Ethnicity,

“…Serum K1 was measured in thawed samples using reversed-phase HPLC with post-column, solid phase chemical reduction of K1 to its hydroquinone followed by fluorometric detection (19) at the Vitamin K Laboratory at the USDA Human Nutrition Research Center on Aging at Tufts University, where this assay was developed. This laboratory currently participates in the vitamin K external quality assurance scheme (20). The lower limit of detection (LLD) for circulating K1 using this assay with the available sample volume was 0.1 nmol/L; samples with K1 concentrations ,0.1 nmol/L were entered as 0.05 nmol/L (19,21,22).…”

Vitamin K plasma levels determination in human health

“…One external quality assurance service (KEQAS) for circulating phylloquinone analysis is already active [54].…”

Recent trends in the metabolism and cell biology of vitamin K with special reference to vitamin K cycling and MK-4 biosynthesis

“…Within the vitamin K family, much more is known about the absorption and metabolic fate of K 1 than for MKs. This is because K 1 predominates in the diet ( 19 ) and is the only vitamer routinely measured in blood ( 37 ). Concentrations of individual MKs in plasma, such as MK-4, are very low or undetectable ( 38 ).…”