HK/XA ratio

Method(s): LC-MS/MS (1).

What is measured on the same platform, click here.
Platform D: PLP, PL, PA, (PAr), PN, Thi, TMP, Ribo, FMN, Cysta, Neopt, Trp, Kyn, (KTR), KA, AA, HK, XA, (HK/XA), HAA, Pic, QA, NA, NAM, mNAM, Cot, OHCot, Trig

What is the HK/XA ratio?

HK/XA is the ratio in serum or plasma of 3-hydroxykynurenine (HK) divided by xanthurenic acid (XA), which are substrate-product pair of the vitamin B6-dependent enzyme, kynurenine aminotransferase (KAT). In general, metabolite ratios cancel out the influence from potential confounders affecting the components (metabolites) in the nominator and denominator. This explains why the HK/XA ratio is not associated with renal function and inflammation (2). HK/XA ratio shows a steep inverse association with plasma pyridoxal 5´-phosphate at concentrations less than 20 nmol/L, and stands out as a specific and sensitive functional marker of vitamin B6 status (3).

Performance of the assay (LC-MS/MS)

Lower limit of detection (LOD) (HK/XA): 2/0.5 nmol/L.
Within-day CV: na; between-day CV: na.


Assessment of vitamin B6 status.

Specimen, collection and processing

Matrix: EDTA plasma and serum.
Volume: Minimum volume is 60 µL, but 200 µL is optimal and allows reanalysis.
Preparation and stability: HK decreases while XA is stable during storage of serum/plasma samples. The blood sample must be centrifuged and the plasma/serum fraction put on ice, and frozen.

Transportation; for general instruction on transportation, click here.

Frozen, on dry ice.

Reported values, interpretation

Reported values: 1.5-2.7.
Intraclass correlation coefficients (ICC): na.


1. Midttun, O., Hustad, S., and Ueland, P.M. (2009). Quantitative profiling of biomarkers related to B-vitamin status, tryptophan metabolism and inflammation in human plasma by liquid chromatography/tandem mass spectrometry. Rapid Commun Mass Sp 23, 1371-79.
2. Ueland, P.M., Ulvik, A., Rios-Avila, L., Midttun, Ø., and Gregory, J.F. (2015). Direct and Functional Biomarkers of Vitamin B6 Status. Annu Rev Nutr 35, 33-70.
3. Ulvik, A., Theofylaktopoulou, D., Midttun, O., Nygård, O., Eussen, S.J., and Ueland, P.M. (2013). Substrate product ratios of enzymes in the kynurenine pathway measured in plasma as indicators of functional vitamin B-6 status. Am J Clin Nutr 98, 934-940.